history of the earth essay

Formation of Earth

Our planet began as part of a cloud of dust and gas. It has evolved into our home, which has an abundance of rocky landscapes, an atmosphere that supports life, and oceans filled with mysteries.

Chemistry, Earth Science, Astronomy, Geology

Manicouagan Crater

Asteroids were not only important in Earth's early formation, but have continued to shape our planet. A five-kilometer (three-mile) diameter asteroid is theorized to have formed the Manicouagan Crater about 215.5 million years ago.

We live on Earth’s hard, rocky surface, breathe the air that surrounds the planet , drink the water that falls from the sky, and eat the food that grows in the soil. But Earth did not always exist within this expansive universe, and it was not always a hospitable haven for life. Billions of years ago, Earth, along with the rest of our solar system, was entirely unrecognizable, existing only as an enormous cloud of dust and gas. Eventually, a mysterious occurrence—one that even the world’s foremost scientists have yet been unable to determine—created a disturbance in that dust cloud, setting forth a string of events that would lead to the formation of life as we know it. One common belief among scientists is that a distant star collapsed, creating a supernova explosion, which disrupted the dust cloud and caused it to pull together. This formed a spinning disc of gas and dust, known as a solar nebula . The faster the cloud spun, the more the dust and gas became concentrated at the center, further fueling the speed of the nebula . Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. The hydrogen protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the star that is the center point of our solar system—the sun—roughly 4.6 billion years ago. Planet Formation The formation of the sun consumed more than 99 percent of the matter in the nebula . The remaining material began to coalesce into various masses. The cloud was still spinning, and clumps of matter continued to collide with others. Eventually, some of those clusters of matter grew large enough to maintain their own gravitational pull, which shaped them into the planets and dwarf planets that make up our solar system today. Earth is one of the four inner, terrestrial planets in our solar system. Just like the other inner planets —Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist so close to the Sun and withstand its heat. In Earth's Beginning At its beginning, Earth was unrecognizable from its modern form. At first, it was extremely hot, to the point that the planet likely consisted almost entirely of molten magma . Over the course of a few hundred million years, the planet began to cool and oceans of liquid water formed. Heavy elements began sinking past the oceans and magma toward the center of the planet . As this occurred, Earth became differentiated into layers, with the outermost layer being a solid covering of relatively lighter material while the denser, molten material sunk to the center. Scientists believe that Earth, like the other inner planets , came to its current state in three different stages. The first stage, described above, is known as accretion, or the formation of a planet from the existing particles within the solar system as they collided with each other to form larger and larger bodies. Scientists believe the next stage involved the collision of a proto planet with a very young planet Earth. This is thought to have occurred more than 4.5 billion years ago and may have resulted in the formation of Earth’s moon. The final stage of development saw the bombardment of the planet with asteroids . Earth’s early atmosphere was most likely composed of hydrogen and helium . As the planet changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the atmosphere around Earth. Slowly, the oceans began to take shape, and eventually, primitive life evolved in those oceans. Contributions from Asteroids Other events were occurring on our young planet at this time as well. It is believed that during the early formation of Earth, asteroids were continuously bombarding the planet , and could have been carrying with them an important source of water. Scientists believe the asteroids that slammed into Earth, the moon, and other inner planets contained a significant amount of water in their minerals, needed for the creation of life. It seems the asteroids , when they hit the surface of Earth at a great speed, shattered, leaving behind fragments of rock. Some suggest that nearly 30 percent of the water contained initially in the asteroids would have remained in the fragmented sections of rock on Earth, even after impact. A few hundred million years after this process—around 2.2 billion to 2.7 billion years ago—photosynthesizing bacteria evolved . They released oxygen into the atmosphere via photosynthesis and, in a few hundred million years, were able to change the composition of the atmosphere into what we have today. Our modern atmosphere is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.

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Related Resources

Historical Geology

A free online textbook for Historical Geology courses

A Brief History of Earth

After reading this chapter, students should be able to:

• Describe the origins the of the Universe in the context of Big Bang theory
• Describe the origins of our solar system in the context of nebular theory
• Identify major tectonic and biologic events that occurred in various geologic eons

Based on An Introduction to Geology, Chapter 8: Earth History

Entire courses and careers have been based on the wide-ranging topics covering Earth’s history. Throughout the long history of Earth, change has been the norm. Looking back in time, an untrained eye would see many unfamiliar life forms and terrains. The main topics studied in Earth history are paleogeography, paleontology, and paleoecology and paleoclimatology —respectively, past landscapes, past organisms, past ecosystems, and past environments. This chapter will cover (briefly) the origin of the universe and the 4.6 billion year history of Earth. It will act as a guide, linking out to other chapters, case studies, and sections in this book.

Origin of the Universe

The universe appears to have an infinite number of galaxies and solar systems and our solar system occupies a small section of this vast entirety. The origins of the universe and solar system set the context for conceptualizing the Earth’s origin and early history.

Big-Bang Theory

The big-bang theory proposes the universe was formed from an infinitely dense and hot core of material. The bang in the title suggests there was an explosive, outward expansion of all matter and space that created atoms. Spectroscopy confirms that hydrogen makes up about 74% of all matter in the universe. Since its creation, the universe has been expanding for 13.8 billion years and recent observations suggest the rate of this expansion is increasing . 

Spectroscopy

Cosmic Microwave Background Radiation

Stellar Evolution

Birth of a star.

Death of a Star

The death of a star can range from spectacular to other-worldly (see figure). Stars like the Sun form a planetary nebula, which comes from the collapse of the star’s outer layers in an event like the implosion of a building. In the tug-of-war between gravity’s inward pull and fusion’s outward push, gravity instantly takes over when fusion ends, with the outer gasses puffing away to form a nebula. More massive stars do this as well but with a more energetic collapse, which starts another type of energy release mixed with element creation known as a supernova. In a supernova , the collapse of the core suddenly halts, creating a massive outward-propagating shock wave. A supernova is the most energetic explosion in the universe short of the big bang. The energy release is so significant the ensuing fusion can make every element up through uranium .

The death of the star can result in the creation of white dwarfs, neutron stars, or black holes. Following their deaths, stars like the Sun turn into white dwarfs .

White dwarfs are hot star embers, formed by packing most of a dying star’s mass into a small and dense object about the size of Earth. Larger stars may explode in a supernova that packs their mass even tighter to become neutron stars. Neutron stars are so dense that protons combine with electrons to form neutrons. The largest stars collapse their mass even further, becoming objects so dense that light cannot escape their gravitational grasp. These are the infamous black holes and the details of the physics of what occurs in them are still up for debate.

Origin of the Solar System: The Nebular Hypothesis

Planet Arrangement and Segregation

Pluto and planet definition

Geoscientists use the geological time scale to assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes as recorded in rocks and fossils. This section summarizes the most notable events of each major time interval. For a breakdown on how these time intervals are chosen and organized, see chapter 7 .

The Hadean Eon, named after the Greek god and ruler of the underworld Hades, is the oldest eon and dates from 4.5–4.0 billion years ago .  

Origin of Earth’s Crust

Origin of the Moon

Computer simulation of the evolution of the Moon (2 minutes).

Origin of Earth’s Water

Archean Eon

Late Heavy Bombardment

Objects were chaotically flying around at the start of the solar system, building the planets and moons. There is evidence that after the planets formed, about 4.1–3.8 billion years ago, a second large spike of asteroid and comet impacted the Earth and Moon in an event called late heavy bombardment . Meteorites and comets in stable or semi-stable orbits became unstable and started impacting objects throughout the solar system. In addition, this event is called the lunar cataclysm because most of the Moons craters are from this event. During late heavy bombardment, the Earth, Moon, and all planets in the solar system were pummeled by material from the asteroid and Kuiper belts. Evidence of this bombardment was found within samples collected from the Moon.

Origin of the Continents

The first solid evidence of modern plate tectonics is found at the end of the Archean, indicating at least some continental lithosphere must have been in place. This evidence does not necessarily mark the starting point of plate tectonics; remnants of earlier tectonic activity could have been erased by the rock cycle .

The stable interiors of the current continents are called cratons and were mostly formed in the Archean Eon. A craton has two main parts: the shield , which is crystalline basement rock near the surface, and the platform made of sedimentary rocks covering the shield. Most cratons have remained relatively unchanged with most tectonic activity having occurred around cratons instead of within them. Whether they were created by plate tectonics or another process, Archean continents gave rise to the Proterozoic continents that now dominate our planet.

First Life on Earth

Although the origin of life on Earth is unknown, hypotheses include a chemical origin in the early atmosphere and ocean, deep-sea hydrothermal vents, and delivery to Earth by comets or other objects. One hypothesis is that life arose from the chemical environment of the Earth’s early atmosphere and oceans, which was very different than today. The oxygen-free atmosphere produced a reducing environment with abundant methane, carbon dioxide, sulfur, and nitrogen compounds. This is what the atmosphere is like on other bodies in the solar system. In the famous Miller-Urey experiment, researchers simulated early Earth’s atmosphere and lightning within a sealed vessel. After igniting sparks within the vessel, they discovered the formation of amino acids, the fundamental building blocks of proteins.  In 1977, when scientists discovered an isolated ecosystem around hydrothermal vents on a deep-sea mid-ocean ridge (see Chapter 4 ), it opened the door for another explanation of the origin of life. The hydrothermal vents have a unique ecosystem of critters with chemosynthesis as the foundation of the food chain instead of photosynthesis. The ecosystem is deriving its energy from hot chemical-rich waters pouring out of underground towers. This suggests that life could have started on the deep ocean floor and derived energy from the heat from the Earth’s interior via chemosynthesis. Scientists have since expanded the search for life to more unconventional places, like Jupiter’s icy moon Europa.

Animation of the original Miller-Urey 1959 experiment that simulated the early atmosphere and created amino acids from simple elements and compounds.

Another possibility is that life or its building blocks came to Earth from space, carried aboard comets or other objects. Amino acids, for example, have been found within comets and meteorites. This intriguing possibility also implies a high likelihood of life existing elsewhere in the cosmos.

Proterozoic Eon

An oxygenated world also changed the chemistry of the planet in significant ways. For example, iron remained in solution in the non-oxygenated environment of the earlier Archean Eon. In chemistry, this is known as a reducing environment. Once the environment was oxygenated, iron combined with free oxygen to form solid precipitates of iron oxide, such as the mineral hematite or magnetite. These precipitates accumulated into large mineral deposits with red chert known as banded-iron formations, which are dated at about 2 billion years .

The disagreements over these complex reconstructions is exemplified by geologists proposing at least six different models for the breakup of Rodinia to create Australia , Antarctica , parts of China , the Tarim craton north of the Himalaya , Siberia , or the Kalahari craton of eastern Africa . This breakup created lots of shallow-water, biologically favorable environments that fostered the evolutionary breakthroughs marking the start of the next eon, the Phanerozoic.

Life Evolves

Another important event in Earth’s biological history occurred about 1.2 billion years ago when eukaryotes invented sexual reproduction. Sharing genetic material from two reproducing individuals, male and female, greatly increased genetic variability in their offspring. This genetic mixing accelerated evolutionary change, contributing to more complexity among individual organisms and within ecosystems (see Chapter 7 ).

Proterozoic land surfaces were barren of plants and animals and geologic processes actively shaped the environment differently because land surfaces were not protected by leafy and woody vegetation. For example, rain and rivers would have caused erosion at much higher rates on land surfaces devoid of plants. This resulted in thick accumulations of pure quartz sandstone from the Proterozoic Eon such as the extensive quartzite formations in the core of the Uinta Mountains in Utah.

Phanerozoic Eon: Paleozoic Era

Life in the early Paleozoic Era was dominated by marine organisms but by the middle of the era plants and animals evolved to live and reproduce on land . Fish evolved jaws and fins evolved into jointed limbs. The development of lungs allowed animals to emerge from the sea and become the first air-breathing tetrapods (four-legged animals) such as amphibians. From amphibians evolved reptiles with the amniotic egg. From reptiles evolved an early ancestor to birds and mammals  and their scales became feathers and fur. Near the end of the Paleozoic Era, the Carboniferous Period had some of the most extensive forests in Earth’s history. Their fossilized remains became the coal that powered the industrial revolution

Paleozoic Tectonics and Paleogeography

During the Paleozoic Era, sea-levels rose and fell four times. With each sea-level rise, the majority of North America was covered by a shallow tropical ocean. Evidence of these submersions are the abundant marine sedimentary rocks such as limestone with fossils corals and ooids. Extensive sea-level falls are documented by widespread unconformities. Today, the midcontinent has extensive marine sedimentary rocks from the Paleozoic and western North America has thick layers of marine limestone on block faulted mountain ranges such as Mt. Timpanogos near Provo, Utah . 

Animation of plate movement the last 3.3 billion years. Pangea occurs at the 4:40 mark.

While the east coast of North America was tectonically active during the Paleozoic Era, the west coast remained mostly inactive as a passive margin during the early Paleozoic. The western edge of North American continent was near the present-day Nevada-Utah border and was an expansive shallow continental shelf near the paleoequator. However, by the Devonian Period, the Antler orogeny started on the west coast and lasted until the Pennsylvanian Period. The Antler orogeny was a volcanic island arc that was accreted onto western North America with the subduction direction away from North America. This created a mountain range on the west coast of North American called the Antler highlands and was the first part of building the land in the west that would eventually make most of California, Oregon, and Washington states. By the late Paleozoic, the Sonoma orogeny began on the west coast and was another collision of an island arc. The Sonoma orogeny marks the change in subduction direction to be toward North America with a volcanic arc along the entire west coast of North America by late Paleozoic to early Mesozoic Eras .

By the end of the Paleozoic Era, the east coast of North America had a very high mountain range due to continental collision and the creation of Pangea. The west coast of North America had smaller and isolated volcanic highlands associated with island arc accretion. During the Mesozoic Era, the size of the mountains on either side of North America would flip, with the west coast being a more tectonically active plate boundary and the east coast changing into a passive margin after the breakup of Pangea.

Paleozoic Evolution

According to evidence from glacial deposits, a small ice age caused sea-levels to drop and led to a major mass extinction by the end of the Ordovician. This is the earliest of five mass extinction events documented in the fossil record. During this mass extinction, an unusually large number of species abruptly disappear in the fossil record (see video). 

4-minute video describing mass extinctions.

 Land plants had also evolved into the first trees and forests . Toward the end of the Devonian, another mass extinction event occurred. This extinction, while severe, is the least temporally defined, with wide variations in the timing of the event or events. Reef building organisms were the hardest hit, leading to dramatic changes in marine ecosystems .

Permian Mass Extinction

Phanerozoic Eon: Mesozoic Era

8.7.1 Mesozoic Tectonics and Paleogeography

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means the Atlantic ocean started opening and was first formed here. The southern Atlantic opened next, with South America separating from central and southern Africa. Last (happening after the Mesozoic ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways.  The breaking points of each rifted plate margin eventually turned into the passive plate boundaries of the east coast of the Americas today.

Video of Pangea breaking apart and plates moving to their present locations. By Tanya Atwater.

Tectonics had an influence in one more important geographic feature in North America: the Cretaceous Western Interior Foreland Basin, which flooded during high sea levels forming the Cretaceous Interior Seaway . Subduction from the west was the Farallon Plate, an oceanic plate connected to the Pacific Plate (seen today as remnants such as the Juan de Fuca Plate, off the coast of the Pacific Northwest). Subduction was shallow at this time because a very young, hot and less dense portion of the Farallon plate was subducted. This shallow subduction caused a downwarping in the central part of North America . High sea levels due to shallow subduction, and increasing rates of seafloor spreading and subduction, high temperatures, and melted ice also contributed to the high sea levels . These factors allowed a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean to divide North America into two separate land masses , Laramidia to the west and Appalachia to the east, for 25 million years . Many of the coal deposits in Utah and Wyoming formed from swamps along the shores of this seaway . By the end of the Cretaceous, cooling temperatures caused the seaway to regress .

8.7.2 Mesozoic Evolution

K-T Extinction

Similar to the end of the Paleozoic era, the Mesozoic Era ended with the K-Pg Mass Extinction (previously known as the K-T Extinction ) 66 million years ago . This extinction event was likely caused by a large bolide ( an extraterrestrial impactor such as an asteroid, meteoroid, or comet) that collided with earth . Ninety percent of plankton species, 75% of plant species, and all the dinosaurs went extinct at this time.

Phanerozoic Eon: Cenozoic Era

8.8.1 Cenozoic Tectonics and Paleogeography

Animation of the last 38 million years of movement in western North America. Note, that after the ridge is subducted, convergent turns to transform (with divergent inland).

8.8.2 Cenozoic Evolution

Anthropocene and Extinction

The changes that have occurred since the inception of Earth are vast and significant. From the oxygenation of the atmosphere, the progression of life forms, the assembly and deconstruction of several supercontinents, to the extinction of more life forms than exist today, having a general understanding of these changes can put present change into a more rounded perspective.

Chapter Contents

• 1.1.1.1 Spectroscopy
• 1.1.1.2 Redshift
• 1.1.1.3 Cosmic Microwave Background Radiation
• 1.1.2.1 Birth of a star
• 1.1.2.2 Fusion
• 1.1.2.3 Death of a Star
• 1.2.1.1 Pluto and planet definition
• 1.3.1 Origin of Earth’s Crust
• 1.3.2 Origin of the Moon
• 1.3.3 Origin of Earth’s Water
• 1.4.1 Late Heavy Bombardment
• 1.4.2 Origin of the Continents
• 1.4.3 First Life on Earth
• 1.5.1 Rodinia
• 1.5.2 Life Evolves
• 1.6.1 Paleozoic Tectonics and Paleogeography
• 1.6.2.1 Permian Mass Extinction
• 1.7.1 8.7.1 Mesozoic Tectonics and Paleogeography
• 1.7.2.1 K-T Extinction
• 1.8.1 8.8.1 Cenozoic Tectonics and Paleogeography
• 1.8.2.1 Anthropocene and Extinction
• 1.9 Summary
• 1.10 References

July 1, 2005

19 min read

Evolution of Earth

The evolution of this planet and its atmosphere gave rise to life, which shaped Earth's subsequent development. Our future lies in interpreting this geologic past and considering what changes--good and bad--may lie ahead

By Claude J. Allègre & Stephen H. Schneider

Like the lapis lazuli gem it resembles, the blue, cloud-enveloped planet the we recognize immediately from satellite pictures seems remarkably stable. Continents and oceans, encircled by an oxygen-rich atmosphere, support familiar life-forms. Yet this constancy is an illusion produced by the human experience of time. Earth and its atmosphere are continuously altered. Plate tectonics shift the continents, raise mountains and move the ocean floor while processes not fully understood alter the climate.

Such constant change has characterized Earth since its beginning some 4.5 billion years ago. From the outset, heat and gravity shaped the evolution of the planet. These forces were gradually joined by the global effects of the emergence of life. Exploring this past offers us the only possibility of understanding the origin of life and, perhaps, its future.

Scientists used to believe the rocky planets, including Earth, Mercury, Venus and Mars, were created by the rapid gravitational collapse of a dust cloud, a deation giving rise to a dense orb. In the 1960s the Apollo space program changed this view. Studies of moon craters revealed that these gouges were caused by the impact of objects that were in great abundance about 4.5 billion years ago. Thereafter, the number of impacts appeared to have quickly decreased. This observation rejuvenated the theory of accretion postulated by Otto Schmidt. The Russian geophysicist had suggested in 1944 that planets grew in size gradually, step by step.

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According to Schmidt, cosmic dust lumped together to form particulates, particulates became gravel, gravel became small balls, then big balls, then tiny planets, or planetesimals, and, nally, dust became the size of the moon. As the planetesimals became larger, their numbers decreased. Consequently, the number of collisions between planetesimals, or meteorites, decreased. Fewer items available for accretion meant that it took a long time to build up a large planet. A calculation made by George W. Wetherill of the Carnegie Institution of Washington suggests that about 100 million years could pass between the formation of an object measuring 10 kilometers in diameter and an object the size of Earth.

The process of accretion had significant thermal consequences for Earth, consequences that forcefully directed its evolution. Large bodies slamming into the planet produced immense heat in its interior, melting the cosmic dust found there. The resulting furnace--situated some 200 to 400 kilometers underground and called a magma ocean--was active for millions of years, giving rise to volcanic eruptions. When Earth was young, heat at the surface caused by volcanism and lava ows from the interior was intensified by the constant bombardment of huge objects, some of them perhaps the size of the moon or even Mars. No life was possible during this period.

Beyond clarifying that Earth had formed through accretion, the Apollo program compelled scientists to try to reconstruct the subsequent temporal and physical development of the early Earth. This undertaking had been considered impossible by founders of geology, including Charles Lyell, to whom the following phrase is attributed: No vestige of a beginning, no prospect for an end. This statement conveys the idea that the young Earth could not be re-created, because its remnants were destroyed by its very activity. But the development of isotope geology in the 1960s had rendered this view obsolete. Their imaginations red by Apollo and the moon ndings, geochemists began to apply this technique to understand the evolution of Earth.

Dating rocks using so-called radioactive clocks allows geologists to work on old terrains that do not contain fossils. The hands of a radioactive clock are isotopes--atoms of the same element that have different atomic weights--and geologic time is measured by the rate of decay of one isotope into another [see "The Earliest History of the Earth," by Derek York; Scientific American , January 1993]. Among the many clocks, those based on the decay of uranium 238 into lead 206 and of uranium 235 into lead 207 are special. Geochronologists can determine the age of samples by analyzing only the daughter product--in this case, lead--of the radioactive parent, uranium.

Panning for zircons ISOTOPE GEOLOGY has permitted geologists to determine that the accretion of Earth culminated in the differentiation of the planet: the creation of the core--the source of Earth's magnetic field--and the beginning of the atmosphere. In 1953 the classic work of Claire C. Patterson of the California Institute of Technology used the uranium-lead clock to establish an age of 4.55 billion years for Earth and many of the meteorites that formed it. In the early 1990s, however, work by one of us (Allègre) on lead isotopes led to a somewhat new interpretation.

As Patterson argued, some meteorites were indeed formed about 4.56 billion years ago, and their debris constituted Earth. But Earth continued to grow through the bombardment of planetesimals until some 120 million to 150 million years later. At that time--4.44 billion to 4.41 billion years ago--Earth began to retain its atmosphere and create its core. This possibility had already been suggested by Bruce R. Doe and Robert E. Zartman of the U.S. Geological Survey in Denver two decades ago and is in agreement with Wetherills estimates.

The emergence of the continents came somewhat later. According to the theory of plate tectonics, these landmasses are the only part of Earth's crust that is not recycled and, consequently, destroyed during the geothermal cycle driven by the convection in the mantle. Continents thus provide a form of memory because the record of early life can be read in their rocks. Geologic activity, however, including plate tectonics, erosion and metamorphism, has destroyed almost all the ancient rocks. Very few fragments have survived this geologic machine.

Nevertheless, in recent decades, several important nds have been made, again using isotope geochemistry. One group, led by Stephen Moorbath of the University of Oxford, discovered terrain in West Greenland that is between 3.7 billion and 3.8 billion years old. In addition, Samuel A. Bowring of the Massachusetts Institute of Technology explored a small area in North America--the Acasta gneiss--that is thought to be 3.96 billion years old.

Ultimately, a quest for the mineral zircon led other researchers to even more ancient terrain. Typically found in continental rocks, zircon is not dissolved during the process of erosion but is deposited in particle form in sediment. A few pieces of zircon can therefore survive for billions of years and can serve as a witness to Earths more ancient crust. The search for old zircons started in Paris with the work of Annie Vitrac and Jol R. Lancelot, later at the University of Marseille and now at the University of Nmes, respectively, as well as with the efforts of Moorbath and Allgre. It was a group at the Australian National University in Canberra, directed by William Compston, that was nally successful. The team discovered zircons in western Australia that were between 4.1 billion and 4.3 billion years old.

Zircons have been crucial not only for understanding the age of the continents but for determining when life rst appeared. The earliest fossils of undisputed age were found in Australia and South Africa. These relics of blue-green algae are about 3.5 billion years old. Manfred Schidlowski of the Max Planck Institute for Chemistry in Mainz studied the Isua formation in West Greenland and argued that organic matter existed as long ago as 3.8 billion years. Because most of the record of early life has been destroyed by geologic activity, we cannot say exactly when it rst appeared--perhaps it arose very quickly, maybe even 4.2 billion years ago.

Stories from gases ONE OF THE MOST important aspects of the planet's evolution is the formation of the atmosphere, because it is this assemblage of gases that allowed life to crawl out of the oceans and to be sustained. Researchers have hypothesized since the 1950s that the terrestrial atmosphere was created by gases emerging from the interior of the planet. When a volcano spews gases, it is an example of the continuous outgassing, as it is called, of Earth. But scientists have questioned whether this process occurred suddenly--about 4.4 billion years ago when the core differentiated--or whether it took place gradually over time.

To answer this question, Allègre and his colleagues studied the isotopes of rare gases. These gases--including helium, argon and xenon--have the peculiarity of being chemically inert, that is, they do not react in nature with other elements. Two of them are particularly important for atmospheric studies: argon and xenon. Argon has three isotopes, of which argon 40 is created by the decay of potassium 40. Xenon has nine, of which xenon 129 has two different origins. Xenon 129 arose as the result of nucleosynthesis before Earth and solar system were formed. It was also created from the decay of radioactive iodine 129, which does not exist on Earth anymore. This form of iodine was present very early on but has died out since, and xenon 129 has grown at its expense.

Like most couples, both argon 40 and potassium 40 and xenon 129 and iodine 129 have stories to tell. They are excellent chronometers. Although the atmosphere was formed by the outgassing of the mantle, it does not contain any potassium 40 or iodine 129. All argon 40 and xenon 129, formed in Earth and released, are found in the atmosphere today. Xenon was expelled from the mantle and retained in the atmosphere; therefore, the atmosphere-mantle ratio of this element allows us to evaluate the age of differentiation. Argon and xenon trapped in the mantle evolved by the radioactive decay of potassium 40 and iodine 129. Thus, if the total outgassing of the mantle occurred at the beginning of Earths formation, the atmosphere would not contain any argon 40 but would contain xenon 129.

The major challenge facing an investigator who wants to measure such ratios of decay is to obtain high concentrations of rare gases in mantle rocks because they are extremely limited. Fortunately, a natural phenomenon occurs at mid-ocean ridges during which volcanic lava transfers some silicates from the mantle to the surface. The small amounts of gases trapped in mantle minerals rise with the melt to the surface and are concentrated in small vesicles in the outer glassy margin of lava ows. This process serves to concentrate the amounts of mantle gases by a factor of 10 4 or 10 5 . Collecting these rocks by dredging the seaoor and then crushing them under vacuum in a sensitive mass spectrometer allows geochemists to determine the ratios of the isotopes in the mantle. The results are quite surprising. Calculations of the ratios indicate that between 80 and 85 percent of the atmosphere was outgassed during Earths rst one million years; the rest was released slowly but constantly during the next 4.4 billion years.

The composition of this primitive atmosphere was most certainly dominated by carbon dioxide, with nitrogen as the second most abundant gas. Trace amounts of methane, ammonia, sulfur dioxide and hydrochloric acid were also present, but there was no oxygen. Except for the presence of abundant water, the atmosphere was similar to that of Venus or Mars. The details of the evolution of the original atmosphere are debated, particularly because we do not know how strong the sun was at that time. Some facts, however, are not disputed. It is evident that carbon dioxide played a crucial role. In addition, many scientists believe the evolving atmosphere contained sufficient quantities of gases such as ammonia and methane to give rise to organic matter.

Still, the problem of the sun remains unresolved. One hypothesis holds that during the Archean eon, which lasted from about 4.5 billion to 2.5 billion years ago, the suns power was only 75 percent of what it is today. This possibility raises a dilemma: How could life have survived in the relatively cold climate that should accompany a weaker sun? A solution to the faint early sun paradox, as it is called, was offered by Carl Sagan and George Mullen of Cornell University in 1970. The two scientists suggested that methane and ammonia, which are very effective at trapping infrared radiation, were quite abundant. These gases could have created a super-greenhouse effect. The idea was criticized on the basis that such gases were highly reactive and have short lifetimes in the atmosphere.

What controlled co? IN THE LATE 1970s Veerabhadran Ramanathan, now at the Scripps Institution of Oceanography, and Robert D. Cess and Tobias Owen of Stony Brook University proposed another solution. They postulated that there was no need for methane in the early atmosphere because carbon dioxide was abundant enough to bring about the super-greenhouse effect. Again this argument raised a different question: How much carbon dioxide was there in the early atmosphere? Terrestrial carbon dioxide is now buried in carbonate rocks, such as limestone, although it is not clear when it became trapped there. Today calcium carbonate is created primarily during biological activity; in the Archean eon, carbon may have been primarily removed during inorganic reactions.

The rapid outgassing of the planet liberated voluminous quantities of water from the mantle, creating the oceans and the hydrologic cycle. The acids that were probably present in the atmosphere eroded rocks, forming carbonate-rich rocks. The relative importance of such a mechanism is, however, debated. Heinrich D. Holland of Harvard University believes the amount of carbon dioxide in the atmosphere rapidly decreased during the Archean and stayed at a low level.

Understanding the carbon dioxide content of the early atmosphere is pivotal to understanding climatic control. Two conicting camps have put forth ideas on how this process works. The rst group holds that global temperatures and carbon dioxide were controlled by inorganic geochemical feedbacks; the second asserts that they were controlled by biological removal.

James C. G. Walker, James F. Kasting and Paul B. Hays, then at the University of Michigan at Ann Arbor, proposed the inorganic model in 1981. They postulated that levels of the gas were high at the outset of the Archean and did not fall precipitously. The trio suggested that as the climate warmed, more water evaporated, and the hydrologic cycle became more vigorous, increasing precipitation and runoff. The carbon dioxide in the atmosphere mixed with rainwater to create carbonic acid runoff, exposing minerals at the surface to weathering. Silicate minerals combined with carbon that had been in the atmosphere, sequestering it in sedimentary rocks. Less carbon dioxide in the atmosphere meant, in turn, less of a greenhouse effect. The inorganic negative feedback process offset the increase in solar energy.

This solution contrasts with a second paradigm: biological removal. One theory advanced by James E. Lovelock, an originator of the Gaia hypothesis, assumed that photosynthesizing microorganisms, such as phytoplankton, would be very productive in a high carbon dioxide environment. These creatures slowly removed carbon dioxide from the air and oceans, converting it into calcium carbonate sediments. Critics retorted that phytoplankton had not even evolved for most of the time that Earth has had life. (The Gaia hypothesis holds that life on Earth has the capacity to regulate temperature and the composition of Earth's surface and to keep it comfortable for living organisms.)

In the early 1990s Tyler Volk of New York University and David W. Schwartzman of Howard University proposed another Gaian solution. They noted that bacteria increase carbon dioxide content in soils by breaking down organic matter and by generating humic acids. Both activities accelerate weathering, removing carbon dioxide from the atmosphere. On this point, however, the controversy becomes acute. Some geochemists, including Kasting, now at Pennsylvania State University, and Holland, postulate that while life may account for some carbon dioxide removal after the Archean, inorganic geochemical processes can explain most of the sequestering. These researchers view life as a rather weak climatic stabilizing mechanism for the bulk of geologic time.

Oxygen from algae THE ISSUE OF CARBON remains critical to how life inuenced the atmosphere. Carbon burial is a key to the vital process of building up atmospheric oxygen concentrations--a prerequisite for the development of certain life-forms. In addition, global warming is taking place now as a result of humans releasing this carbon. For one billion or two billion years, algae in the oceans produced oxygen. But because this gas is highly reactive and because there were many reduced minerals in the ancient oceans--iron, for example, is easily oxidized--much of the oxygen produced by living creatures simply got used up before it could reach the atmosphere, where it would have encountered gases that would react with it.

Even if evolutionary processes had given rise to more complicated life-forms during this anaerobic era, they would have had no oxygen. Furthermore, un ltered ultraviolet sunlight would have likely killed them if they left the ocean. Researchers such as Walker and Preston Cloud, then at the University of California at Santa Barbara, have suggested that only about two billion years ago, after most of the reduced minerals in the sea were oxidized, did atmospheric oxygen accumulate. Between one billion and two billion years ago oxygen reached current levels, creating a niche for evolving life.

By examining the stability of certain minerals, such as iron oxide or uranium oxide, Holland has shown that the oxygen content of the Archean atmosphere was low before two billion years ago. It is largely agreed that the present-day oxygen content of 20 percent is the result of photosynthetic activity. Still, the question is whether the oxygen content in the atmosphere increased gradually over time or suddenly. Recent studies indicate that the increase of oxygen started abruptly between 2.1 billion and 2.03 billion years ago and that the present situation was reached 1.5 billion years ago.

The presence of oxygen in the atmosphere had another major bene t for an organism trying to live at or above the surface: it ltered ultraviolet radiation. Ultraviolet radiation breaks down many molecules--from DNA and oxygen to the chlorouorocarbons that are implicated in stratospheric ozone depletion. Such energy splits oxygen into the highly unstable atomic form O, which can combine back into O 2 and into the very special molecule O 3 , or ozone. Ozone, in turn, absorbs ultraviolet radiation. It was not until oxygen was abundant enough in the atmosphere to allow the formation of ozone that life even had a chance to get a root-hold or a foothold on land. It is not a coincidence that the rapid evolution of life from prokaryotes (single-celled organisms with no nucleus) to eukaryotes (single-celled organisms with a nucleus) to metazoa (multicelled organisms) took place in the billion-year-long era of oxygen and ozone.

Although the atmosphere was reaching a fairly stable level of oxygen during this period, the climate was hardly uniform. There were long stages of relative warmth or coolness during the transition to modern geologic time. The composition of fossil plankton shells that lived near the ocean oor provides a measure of bottom water temperatures. The record suggests that over the past 100 million years bottom waters cooled by nearly 15 degrees Celsius. Sea levels dropped by hundreds of meters, and continents drifted apart. Inland seas mostly disappeared, and the climate cooled an average of 10 to 15 degrees C. Roughly 20 million years ago permanent ice appears to have built up on Antarctica.

About two million to three million years ago the paleoclimatic record starts to show signi cant expansions and contractions of warm and cold periods in 40,000-year or so cycles. This periodicity is interesting because it corresponds to the time it takes Earth to complete an oscillation of the tilt of its axis of rotation. It has long been speculated, and recently calculated, that known changes in orbital geometry could alter the amount of sunlight coming in between winter and summer by about 10 percent or so and could be responsible for initiating or ending ice ages.

The warm hand of man MOST INTERESTING and perplexing is the discovery that between 600,000 and 800,000 years ago the dominant cycle switched from 40,000-year periods to 100,000-year intervals with very large uctuations. The last major phase of glaciation ended about 10,000 years ago. At its height 20,000 years ago, ice sheets about two kilometers thick covered much of northern Europe and North America. Glaciers expanded in high plateaus and mountains throughout the world. Enough ice was locked up on land to cause sea levels to drop more than 100 meters below where they are today. Massive ice sheets scoured the land and revamped the ecological face of Earth, which was ve degrees C cooler on average than it is currently.

The precise causes of the longer intervals between warm and cold periods are not yet sorted out. Volcanic eruptions may have played a signi cant role, as shown by the effect of El Chichón in Mexico and Mount Pinatubo in the Philippines. Tectonic events, such as the development of the Himalayas, may have inuenced world climate. Even the impact of comets can inuence short-term climatic trends with catastrophic consequences for life [see "What Caused the Mass Extinction? An Extraterrestrial Impact," by Walter Alvarez and Frank Asaro; and "What Caused the Mass Extinction? A Volcanic Eruption," by Vincent E. Courtillot; Scientific American , October 1990]. It is remarkable that despite violent, episodic perturbations, the climate has been buffered enough to sustain life for 3.5 billion years.

One of the most pivotal climatic discoveries of the past 30 years has come from ice cores in Greenland and Antarctica. When snow falls on these frozen continents, the air between the snow grains is trapped as bubbles. The snow is gradually compressed into ice, along with its captured gases. Some of these records can go back more than 500,000 years; scientists can analyze the chemical content of ice and bubbles from sections of ice that lie as deep as 3,600 meters (2.2 miles) below the surface.

The ice-core borers have determined that the air breathed by ancient Egyptians and Anasazi Indians was very similar to that which we inhale today--except for a host of air pollutants introduced over the past 100 or 200 years. Principal among these added gases, or pollutants, are extra carbon dioxide and methane. Since about 1860--the expansion of the Industrial Revolution--carbon dioxide levels in the atmosphere have increased more than 30 percent as a result of industrialization and deforestation; methane levels have more than doubled because of agriculture, land use and energy production. The ability of increased amounts of these gases to trap heat is what drives concerns about climate change in the 21st century [see "The Changing Climate," by Stephen H. Schneider; Scientific American , September 1989].

The ice cores have shown that sustained natural rates of worldwide temperature change are typically about one degree C per millennium. These shifts are still signi cant enough to have radically altered where species live and to have potentially contributed to the extinction of such charismatic megafauna as mammoths and saber-toothed tigers. But a most extraordinary story from the ice cores is not the relative stability of the climate during the past 10,000 years. It appears that during the height of the last ice age 20,000 years ago there was 50 percent less carbon dioxide and less than half as much methane in the air than there has been during our epoch, the Holocene. This nding suggests a positive feedback between carbon dioxide, methane and climatic change.

The reasoning that supports the idea of this destabilizing feedback system goes as follows. When the world was colder, there was less concentration of greenhouse gases, and so less heat was trapped. As Earth warmed up, carbon dioxide and methane levels increased, accelerating the warming. If life had a hand in this story, it would have been to drive, rather than to oppose, climatic change. It appears increasingly likely that when humans became part of this cycle, they, too, helped to accelerate warming. Such warming has been especially pronounced since the mid-1800s because of greenhouse gas emissions from industrialization, land-use change and other phenomena. Once again, though, uncertainties remain.

Nevertheless, most scientists would agree that life could well be the principal factor in the positive feedback between climatic change and greenhouse gases. There was a rapid rise in average global surface temperature at the end of the 20th century [ see illustration on opposite page ]. Indeed, the period from the 1980s onward has been the warmest of the past 2,000 years. Nineteen of the 20 warmest years on record have occurred since 1980, and the 12 warmest have all occurred since 1990. The all-time record high year was 1998, and 2002 and 2003 were in second and third places, respectively. There is good reason to believe that the decade of the 1990s would have been even hotter had not Mount Pinatubo erupted: this volcano put enough dust into the high atmosphere to block some incident sunlight, causing global cooling of a few tenths of a degree for several years.

Could the warming of the past 140 years have occurred naturally? With ever increasing certainty, the answer is no.

The box at the right shows a remarkable study that attempted to push back the Northern Hemisphere's temperature record a full 1,000 years. Climatologist Michael Mann of the University of Virginia and his colleagues performed a complex statistical analysis involving some 112 different factors related to temperature, including tree rings, the extent of mountain glaciers, changes in coral reefs, sunspot activity and volcanism.

The resulting temperature record is a reconstruction of what might have been obtained had thermometer-based measurements been available. (Actual temperature measurements are used for the years after 1860.) As shown by the confidence range, there is considerable uncertainty in each year of this 1,000-year temperature reconstruction. But the overall trend is clear: a gradual temperature decrease over the first 900 years, followed by a sharp temperature upturn in the 20th century. This graph suggests that the decade of the 1990s was not only the warmest of the century but of the entire past millennium.

By studying the transition from the high carbon dioxide, low-oxygen atmosphere of the Archean to the era of great evolutionary progress about half a billion years ago, it becomes clear that life may have been a factor in the stabilization of climate. In another example--during the ice ages and interglacial cycles--life seems to have the opposite function: accelerating the change rather than diminishing it. This observation has led one of us (Schneider) to contend that climate and life coevolved rather than life serving solely as a negative feedback on climate.

If we humans consider ourselves part of life--that is, part of the natural system--then it could be argued that our collective impact on Earth means we may have a signi cant co-evolutionary role in the future of the planet. The current trends of population growth, the demands for increased standards of living and the use of technology and organizations to attain these growth-oriented goals all contribute to pollution. When the price of polluting is low and the atmosphere is used as a free sewer, carbon dioxide, methane, chlorouorocarbons, nitrous oxides, sulfur oxides and other toxics can build up.

Drastic changes ahead IN THEIR REPORT Climate Change 2001 , climate experts on the Intergovernmental Panel on Climate Change estimated that the world will warm between 1.4 and 5.8 degrees C by 2100. The mild end of that range--a warming rate of 1.4 degrees C per 100 years--is still 14 times faster than the one degree C per 1,000 years that historically has been the average rate of natural change on a global scale. Should the higher end of the range occur, then we could see rates of climatic change nearly 60 times faster than natural average conditions, which could lead to changes that many would consider dangerous. Change at this rate would almost certainly force many species to attempt to move their ranges, just as they did from the ice age/interglacial transition between 10,000 and 15,000 years ago. Not only would species have to respond to climatic change at rates 14 to 60 times faster, but few would have undisturbed, open migration routes as they did at the end of the ice age and the onset of the interglacial era. The negative effects of this significant warming--on health, agriculture, coastal geography and heritage sites, to name a few--could also be severe.

To make the critical projections of future climatic change needed to understand the fate of ecosystems on Earth, we must dig through land, sea and ice to learn as much from geologic, paleoclimatic and paleoecological records as we can. These records provide the backdrop against which to calibrate the crude instruments we must use to peer into a shadowy environmental future, a future increasingly inuenced by us.

THE AUTHORS CLAUDE J. ALLGRE and STEPHEN H. SCHNEIDER study various aspects of Earths geologic history and its climate. Allgre is professor at the University of Paris and directs the department of geochemistry at the Paris Geophysical Institute. He is a foreign member of the National Academy of Sciences. Schneider is professor in the department of biological sciences at Stanford University and co-director of the Center for Environmental Science and Policy. He was honored with a MacArthur Prize Fellowship in 1992 and was elected to membership in the National Academy of Sciences in 2002.

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The origin of life on Earth, explained

The origin of life on Earth stands as one of the great mysteries of science. Various answers have been proposed, all of which remain unverified. To find out if we are alone in the galaxy, we will need to better understand what geochemical conditions nurtured the first life forms. What water, chemistry and temperature cycles fostered the chemical reactions that allowed life to emerge on our planet? Because life arose in the largely unknown surface conditions of Earth’s early history, answering these and other questions remains a challenge.

Several seminal experiments in this topic have been conducted at the University of Chicago, including the Miller-Urey experiment that suggested how the building blocks of life could form in a primordial soup.

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• When did life on Earth begin?

Where did life on Earth begin?

What are the ingredients of life on earth, what are the major scientific theories for how life emerged, what is chirality and why is it biologically important, what research are uchicago scientists currently conducting on the origins of life, when did life on earth begin .

Earth is about 4.5 billion years old. Scientists think that by 4.3 billion years ago, Earth may have developed conditions suitable to support life. The oldest known fossils, however, are only 3.7 billion years old. During that 600 million-year window, life may have emerged repeatedly, only to be snuffed out by catastrophic collisions with asteroids and comets.

The details of those early events are not well preserved in Earth’s oldest rocks. Some hints come from the oldest zircons, highly durable minerals that formed in magma. Scientists have found traces of a form of carbon—an important element in living organisms— in one such 4.1 billion-year-old zircon . However, it does not provide enough evidence to prove life’s existence at that early date.

Two possibilities are in volcanically active hydrothermal environments on land and at sea.

Some microorganisms thrive in the scalding, highly acidic hot springs environments like those found today in Iceland, Norway and Yellowstone National Park. The same goes for deep-sea hydrothermal vents. These chimney-like vents form where seawater comes into contact with magma on the ocean floor, resulting in streams of superheated plumes. The microorganisms that live near such plumes have led some scientists to suggest them as the birthplaces of Earth’s first life forms.

Organic molecules may also have formed in certain types of clay minerals that could have offered favorable conditions for protection and preservation. This could have happened on Earth during its early history, or on comets and asteroids that later brought them to Earth in collisions. This would suggest that the same process could have seeded life on planets elsewhere in the universe.

The recipe consists of a steady energy source, organic compounds and water.

Sunlight provides the energy source at the surface, which drives photosynthesis. On the ocean floor, geothermal energy supplies the chemical nutrients that organisms need to live.

Also crucial are the elements important to life . For us, these are carbon, hydrogen, oxygen, nitrogen, and phosphorus. But there are several scientific mysteries about how these elements wound up together on Earth. For example, scientists would not expect a planet that formed so close to the sun to naturally incorporate carbon and nitrogen. These elements become solid only under very cold temperatures, such as exist in the outer solar system, not nearer to the sun where Earth is. Also, carbon, like gold, is rare at the Earth’s surface. That’s because carbon chemically bonds more often with iron than rock. Gold also bonds more often with metal, so most of it ends up in the Earth’s core. So, how did the small amounts found at the surface get there? Could a similar process also have unfolded on other planets?

The last ingredient is water. Water now covers about 70% of Earth’s surface, but how much sat on the surface 4 billion years ago? Like carbon and nitrogen, water is much more likely to become a part of solid objects that formed at a greater distance from the sun. To explain its presence on Earth, one theory proposes that a class of meteorites called carbonaceous chondrites formed far enough from the sun to have served as a water-delivery system.

There are several theories for how life came to be on Earth. These include:

Life emerged from a primordial soup

As a University of Chicago graduate student in 1952, Stanley Miller performed a famous experiment with Harold Urey, a Nobel laureate in chemistry. Their results explored the idea that life formed in a primordial soup.

Miller and Urey injected ammonia, methane and water vapor into an enclosed glass container to simulate what were then believed to be the conditions of Earth’s early atmosphere. Then they passed electrical sparks through the container to simulate lightning. Amino acids, the building blocks of proteins, soon formed. Miller and Urey realized that this process could have paved the way for the molecules needed to produce life.

Scientists now believe that Earth’s early atmosphere had a different chemical makeup from Miller and Urey’s recipe. Even so, the experiment gave rise to a new scientific field called prebiotic or abiotic chemistry, the chemistry that preceded the origin of life. This is the opposite of biogenesis, the idea that only a living organism can beget another living organism.

Seeded by comets or meteors

Some scientists think that some of the molecules important to life may be produced outside the Earth. Instead, they suggest that these ingredients came from meteorites or comets.

“A colleague once told me, ‘It’s a lot easier to build a house out of Legos when they’re falling from the sky,’” said Fred Ciesla, a geophysical sciences professor at UChicago. Ciesla and that colleague, Scott Sandford of the NASA Ames Research Center, published research showing that complex organic compounds were readily produced under conditions that likely prevailed in the early solar system when many meteorites formed.

Meteorites then might have served as the cosmic Mayflowers that transported molecular seeds to Earth. In 1969, the Murchison meteorite that fell in Australia contained dozens of different amino acids—the building blocks of life.

Comets may also have offered a ride to Earth-bound hitchhiking molecules, according to experimental results published in 2001 by a team of researchers from Argonne National Laboratory, the University of California Berkeley, and Lawrence Berkeley National Laboratory. By showing that amino acids could survive a fiery comet collision with Earth, the team bolstered the idea that life’s raw materials came from space.

In 2019, a team of researchers in France and Italy reported finding extraterrestrial organic material preserved in the 3.3 billion-year-old sediments of Barberton, South Africa. The team suggested micrometeorites as the material’s likely source. Further such evidence came in 2022 from samples of asteroid Ryugu returned to Earth by Japan’s Hayabusa2 mission. The count of amino acids found in the Ryugu samples now exceeds 20 different types .

In 1953, UChicago researchers published a landmark paper in the Journal of Biological Chemistry that marked the discovery of the pro-chirality concept , which pervades modern chemistry and biology. The paper described an experiment showing that the chirality of molecules—or “handedness,” much the way the right and left hands differ from one another—drives all life processes. Without chirality, large biological molecules such as proteins would be unable to form structures that could be reproduced.

Today, research on the origin of life at UChicago is expanding. As scientists have been able to find more and more exoplanets—that is, planets around stars elsewhere in the galaxy—the question of what the essential ingredients for life are and how to look for signs of them has heated up.

Nobel laureate Jack Szostak joined the UChicago faculty as University Professor in Chemistry in 2022 and will lead the University’s new interdisciplinary Origins of Life Initiative to coordinate research efforts into the origin of life on Earth. Scientists from several departments of the Physical Sciences Division are joining the initiative, including specialists in chemistry, astronomy, geology and geophysics.

“Right now we are getting truly unprecedented amounts of data coming in: Missions like Hayabusa and OSIRIS-REx are bringing us pieces of asteroids, which helps us understand the conditions that form planets, and NASA’s new JWST telescope is taking astounding data on the solar system and the planets around us ,” said Prof. Ciesla. “I think we’re going to make huge progress on this question.”

Last updated Sept. 19, 2022.

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Clara Blättler

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Jack Szostak

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Planet Earth, explained

Our home planet provides us with life and protects us from space.

Earth, our home planet, is a world unlike any other. The third planet from the sun, Earth is the only place in the known universe confirmed to host life.

With a radius of 3,959 miles, Earth is the fifth largest planet in our solar system, and it's the only one known for sure to have liquid water on its surface. Earth is also unique in terms of monikers. Every other solar system planet was named for a Greek or Roman deity, but for at least a thousand years, some cultures have described our world using the Germanic word “earth,” which means simply “the ground.”

Our dance around the sun

Earth orbits the sun once every 365.25 days. Since our calendar years have only 365 days, we add an extra leap day every four years to account for the difference.

Though we can't feel it, Earth zooms through its orbit at an average velocity of 18.5 miles a second. During this circuit, our planet is an average of 93 million miles away from the sun, a distance that takes light about eight minutes to traverse. Astronomers define this distance as one astronomical unit (AU), a measure that serves as a handy cosmic yardstick.

Earth rotates on its axis every 23.9 hours, defining day and night for surface dwellers. This axis of rotation is tilted 23.4 degrees away from the plane of Earth's orbit around the sun, giving us seasons. Whichever hemisphere is tilted closer to the sun experiences summer, while the hemisphere tilted away gets winter. In the spring and fall, each hemisphere receives similar amounts of light. On two specific dates each year—called the equinoxes—both hemispheres get illuminated equally.

Many layers, many features

About 4.5 billion years ago, gravity coaxed Earth to form from the gaseous, dusty disk that surrounded our young sun. Over time, Earth's interior—which is made mostly of silicate rocks and metals—differentiated into four layers.

For Hungry Minds

At the planet's heart lies the inner core, a solid sphere of iron and nickel that's 759 miles wide and as hot as 9,800 degrees Fahrenheit. The inner core is surrounded by the outer core, a 1,400-mile-thick band of iron and nickel fluids. Beyond the outer core lies the mantle, a 1,800-mile-thick layer of viscous molten rock on which Earth's outermost layer, the crust, rests. On land, the continental crust is an average of 19 miles thick, but the oceanic crust that forms the seafloor is thinner—about three miles thick—and denser.

Like Venus and Mars, Earth has mountains, valleys, and volcanoes. But unlike its rocky siblings, almost 70 percent of Earth's surface is covered in oceans of liquid water that average 2.5 miles deep. These bodies of water contain 97 percent of Earth's volcanoes and the mid-ocean ridge , a massive mountain range more than 40,000 miles long.

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Earth's crust and upper mantle are divided into massive plates that grind against each other in slow motion. As these plates collide, tear apart, or slide past each other, they give rise to our very active geology. Earthquakes rumble as these plates snag and slip past each other. Many volcanoes form as seafloor crust smashes into and slides beneath continental crust. When plates of continental crust collide, mountain ranges such as the Himalaya are pushed toward the skies.

Protective fields and gases

Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen, and one percent other gases such as carbon dioxide, water vapor, and argon. Much like a greenhouse, this blanket of gases absorbs and retains heat. On average, Earth's surface temperature is about 57 degrees Fahrenheit; without our atmosphere, it'd be zero degrees . In the last two centuries, humans have added enough greenhouse gases to the atmosphere to raise Earth's average temperature by 1.8 degrees Fahrenheit . This extra heat has altered Earth's weather patterns in many ways .

The atmosphere not only nourishes life on Earth, but it also protects it: It's thick enough that many meteorites burn up before impact from friction, and its gases—such as ozone—block DNA-damaging ultraviolet light from reaching the surface. But for all that our atmosphere does, it's surprisingly thin. Ninety percent of Earth's atmosphere lies within just 10 miles of the planet's surface .

The silhouette of a woman is seen on a Norwegian island beneath the Northern Lights ( aurora borealis ).

We also enjoy protection from Earth's magnetic field, generated by our planet's rotation and its iron-nickel core. This teardrop-shaped field shields Earth from high-energy particles launched at us from the sun and elsewhere in the cosmos. But due to the field's structure, some particles get funneled to Earth's Poles and collide with our atmosphere, yielding aurorae, the natural fireworks show known by some as the northern lights.

Spaceship Earth

Earth is the planet we have the best opportunity to understand in detail—helping us see how other rocky planets behave, even those orbiting distant stars. As a result, scientists are increasingly monitoring Earth from space. NASA alone has dozens of missions dedicated to solving our planet's mysteries.

At the same time, telescopes are gazing outward to find other Earths. Thanks to instruments such as NASA's Kepler Space Telescope, astronomers have found more than 3,800 planets orbiting other stars, some of which are about the size of Earth , and a handful of which orbit in the zones around their stars that are just the right temperature to be potentially habitable. Other missions, such as the Transiting Exoplanet Survey Satellite, are poised to find even more.

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Earth Timeline: A Guide to Earth’s Geological History and Events [Infographic]

Earth’s Timeline and History

4,567,000,000 years ago, Earth was covered in molten lava. Earth was completely unrecognizable. In its earliest stage of formation, it was uninhabitable as it clumped from a cloud of dust .

About 1,000,000,000 years ago, Earth had its first signs of life. Single-celled organisms consumed the sun’s energy. As a waste product, these cyanobacteria eventually filled the oceans and atmosphere with oxygen.

Next, an oxygenated atmosphere paved the way for more complex life forms to exist. At about 100,000,000 years ago, dinosaurs roamed the Earth until their abrupt extinction.

Finally, 10,000,000 years ago was the age of mammals. And these mammals are our direct ancestors. It was only 100,000 years ago when homo sapiens truly evolved.

In this Earth timeline, we lay out all the pieces on the floor. What sequence of events has unfolded for Earth to support life and for it to evolve?

The Sculpting of Earth (4.6 – 4.0 billion years ago )

The Big Bang created all matter in the universe. This includes the sun, planets, and our solar system. At the center, the sun swept in smaller elements like hydrogen and helium. Farther away, heavier elements formed planets. Based on the core accretion model, gravity was the driver coalescing Earth from a cloud of dust.

In this early stage of Earth’s creation, the heaviest material like iron sank to the core . Lighter material remained on top to form a crust. Because the solid inner core heats the outer liquid layer, it produces convection currents. This geodynamo is Earth’s magnetic field . Without it, Earth would be blasted by harmful rays from the sun.

In the late Hadean Eon, the Earth was still in its late heavy bombardment stage . Earth was hit by asteroids, comets, and foreign objects left, right, and center. We know it wasn’t only Earth because we can see these impacts on other places in our solar system. Some scientists believe that water originated from the bombardment of comets at this time. In addition, there’s reason to believe that these collisions could have sparked the chemical building blocks for life – DNA .

One of the most important events for Earth was the formation of our moon. The giant impact hypothesis describes it as an object the size of Mars heading towards Earth at tremendous speed. After delivering a glancing blow to Earth, gravity pulls this object into orbit. Ever since this grande event in the Hadean Eon , it has remained in orbit ever since.

Earth Cooling and Primitive Life (4.0 – 2.5 billion years ago )

The collision of the moon into Earth significantly impacted climate, oceans, and life on Earth. Because the moon’s orbit drags Earth, it slowed Earth’s rotation significantly from 6-hour days to 24 hours. Having the moon in orbit also stabilized the Earth from wobbling. But most importantly, the collision of the moon tilted Earth on its axis . And because the Earth is tilted on its axis, Earth now had seasons.

Eventually, the climate on Earth became more stable in the Archean Eon . Instead of a molten state, the Earth started to cool down. Water vapor condensed to form oceans. And the Earth cooled down enough to create continents. Though opinions vary, “Vaalbara” became Earth’s first supercontinent .

As the Earth cooled down, a new form of life began to prosper. When oxygen was absent in the atmosphere , cyanobacteria could convert sunlight to energy. In shallow water, they metabolized their own food. As a waste product, cyanobacteria released oxygen into the oceans.

When oxygen mixed with iron in the oceans, rusted iron collected on the seafloor. The banded iron formation continued until there was no more iron in the oceans to rust. Oxygen had nowhere to go but into the atmosphere. That’s why this event is the Great Oxygenation Event .

An Oxygenated Atmosphere (2500 – 541 million years ago)

Earth now had an oxygenated atmosphere for new life to flourish on Earth. But it wasn’t cyanobacteria flourishing. Because oxygen was toxic for cyanobacteria, they poisoned all anaerobic life on Earth including themselves. Imagine a dominant species polluting the planet until extinction. The oxygen byproduct from cyanobacteria created an oxygen crisis on Earth.

At this time, methane was more abundant in the atmosphere. One thing that methane did very well was trap heat in the atmosphere. It’s one of the most efficient greenhouse gases there are. So when oxygen combined with methane, it produced carbon dioxide. All of a sudden, the greenhouse effect wasn’t as strong. As a result, the whole planet froze. It was “Snowball Earth” as the Earth went into an ice age for the next 300,000,000 years.

Another important consequence of an oxygen-filled atmosphere was the emergence of aerobic eukaryotes. Before the oxygenation, life was anaerobic. Eventually, aerobic respiration organisms emerged because of the enriched atmosphere. This increased the complexity of life. For example, multi-cellular organisms became apparent in this eon. But the abundance of CO 2 held eukaryotes from diversifying.

As oxygen filled the atmosphere, Earth’s ozone layer thickened. Before the presence of an ozone layer, life was restricted to shallow water. Because water shielded harmful radiation, that’s where life existed. Eventually, a thicker ozone layer (O 3 ) enabled life to diversify on land in the Proterozoic Eon .

The Cambrian Explosion and Fossil Records (541 – 245 million years ago)

The Cambrian explosion was the largest diversification of life in Earth’s history. Everything before this era was Precambrian. We couldn’t identify life because we didn’t have fossilized shells or animals . This is when hard-shelled invertebrates originated in the oceans. The Cambrian explosion started with the Age of Invertebrates. And life got more diverse from there.

Next came the Age of Fish when thousands of fish species arose. Then, the first vertebrate land animal made its leap ashore. Amphibians took a breath of fresh air and colonized the empty continent of Gondwana. This was the start of the Age of Amphibians. We share similar characteristics to our vertebrate ancestors . For example, humans have spines, jaws, and mouths originating from fish.

In the Paleozoic Era , lush rainforests flourished on land. But due to an abrupt shift in global warming, a major marine and terrestrial extinction event began. This event was the Carboniferous Rainforest Collapse. Plants and swamps were buried, compacted, and cooked into underground coal deposits . It left behind vast deserts for reptiles to eventually dominate the continental interior.

The Paleozoic Era ended with the largest extinction in Earth’s history. The Permian-Triassic Extinction vanquished 96% of all marine species. About 70% of terrestrial vertebrate species were wiped out. Opinions vary about the Permian-Triassic Extinction cause. But the consensus is that it was from a major asteroid impact event .

The Age of Reptiles and Dinosaurs (245 – 66 million years ago)

When Earth’s climate became hotter and drier, rainforests collapsed triggering the Age of Reptiles. Reptiles are different from amphibians because they lay their hard-shelled eggs on land. They essentially adapted to the land by cutting all ties with the ocean. Because reptiles evolved to dryland conditions , they gained a unique ecological advantage.

As conditions changed, dinosaurs (also known as terrible lizards) began to evolve. These reptile-like mammals had scaly skin and hatched eggs like reptiles. Some dinosaurs adapted as herbivores and some as carnivores. For the next 160 million years, dinosaurs were the dominant land vertebrates on Earth.

The Age of Conifers in the Mesozoic Era provided the spread of seeded plants. Conifers store vast amounts of carbon. As a result, oxygen content in the atmosphere jumped to 35% compared to 21% today. In addition, they provided habitat, shelter, and a source of food for specific animal species to survive.

Also notable is that Pangea existed as one supercontinent in this era. Dinosaurs lived on one supercontinent. Plate tectonics was the mechanism that eventually tore continents apart. Don’t forget that dinosaurs existed for 160 million years. So continental drift gradually rifted dinosaurs apart. We know this because we can find the same fossils on separate continents.

The Age of Mammals and Homo Sapiens (66 million years ago – now)

Ultimately, the start of the Cenozoic Era was the demise of dinosaurs. After a 6-mile wide asteroid hit Earth, a dust cloud blocked the sun. This caused temperatures to plummet which was the heart of the damage from the Cretaceous–Paleogene extinction event. Because of the worldwide climate disruption, it was responsible for the extinction of the dinosaurs.

Mammals existed long before the Cenozoic Era. But they kept a low profile because dinosaurs dominated the land. The extinction of the dinosaurs marked the Age of Mammals. When dinosaurs roamed the Earth, mammals remained small and furry. And because dinosaurs went extinct, mammals emerged as the largest land animals at this time.

Apes remained in trees for their primary food source. Eventually, grass began to spread in places like the African Savannah and there were fewer trees. This forced apes to walk to new food sources. With their heads above the grass to see predators, apes evolved by walking on two legs. It also helped to have their hands available when they were traveling.

As the timeline to modern human evolution begins, hominids were the early proto-humans. They were known for sharpening objects with silicon rocks. They began to master the use of their hands and fingers. In the Stone Age, early humans had fire under control. This enabled them to cook their food giving them more calories. Modern humans learned to make more complex sounds and share information in groups. So humans have only existed for about 0.004% of the age of the Earth.

Geologic time is vast

It’s almost unimaginable the story of Earth. It spans over 4.5 billion years.

Scientists are still fitting all the pieces to the puzzle.

Since its creation, oceans, continents, and life as we know it has remarkably transformed. Life has evolved and adapted.

This article assembles some of the key events. From evolution to extinction, these are events that have unfolded as part of the Earth timeline. Though opinions vary, we’ve leaped through 4.5 billion years of Earth’s history and provided a general guideline.

A Guide to Earth’s Geological History and Events

Earth is a constantly changing planet. Each era has its own unique geological history and events.

Earth’s geological history will be of interest to students, professionals, and enthusiasts of earth science.

Now, it’s time to let you take the stage. What are some important events we missed? Please let us know with a comment below.

Coal Formation: How Coal Forms

Convergent Plate Boundaries: The Collision of Plate Tectonics

What Are the 3 Types of Rocks?

Banded Iron Formation (BIF): How These Rocks Got Their Stripes

How Volcanoes Form Igneous Rocks

Metamorphic Rocks: Heat, Pressure and Metamorphism

What is Isostatic Rebound?

What is Earth’s Geological Time Scale?

Mountain Building: How Mountains are Made

Plate Tectonic Types: Divergent, Convergent and Transform Plates

The Cenozoic Era: From Dinosaur Extinction to Human Evolution

What Happened in the Archean Eon? [4.0 to 2.5 billion years ago]

African Plate and the East African Rift Zone

What is a Cinder Cone Volcano (Scoria Cone)?

39 comments.

Hi W. These are 2 separate topics of themselves. I’ve decided to break the “Big Bang” and “Human Evolution” into separate posts. My post on human evolution is not as comprehensive for covering appendages and organs (eyes, ears, etc.). But I think my article on the Big Bang is pretty detailed. Here they are:

• The History of the Universe: The Big Bang and Beyond – https://earthhow.com/history-of-the-universe/
• Human Evolution: A Timeline of Early Hominids – https://earthhow.com/human-evolution-timeline/
• Theory of Evolution: Charles Darwin and Natural Selection – https://earthhow.com/theory-of-evolution/

Very disappointing. Leaves much to be desired and explains nothing about how any species evolved simultaneous breeding PAIRS, not to mention how development could focus on appendages and organs (eyes, ears, etc.) for which no “simple” organism could know existed before such development. And, of course, it fails to explain from where the elements to cause the “big bang” came. It’s a nice hypothesis, but it has less proof and more holes than an intelligent-design theory.

Do not need to have any typo’s in your writing.

esta bonita la historia

Life is born newly every second. Same time life is dying every second

As per our Hindu Vedas we have 33,000,000 million species on our earth. So we human beings are also changing like our mobile phones. Finally all is like a dream.

This is cute, watching people fight over the idea that they know any “facts ” about the universe. It’s quite literally beyond the comprehension of a human brain. We aren’t able to understand the size, how it started or even the most basic principles, yet you are arguing as if you understand any part of it.

It’s like watching fruit flys fight over 401ks. Guys, seriously. You simply don’t know the first thing about anything yet.

What an incredibly accurate and detailed piece! You broke it down layer by layer and it really helped making sense of things I previously hadn’t known.

Great article. Good work.

The Big Bang did not create our solar system. It happened approx 13-14 billion years ago; our sun and solar system formed 9 billion years later. Our sun is only about 4.6 billion years old. It is described as a “third-generation star”. The Big Bang created all matter in the universe but that matter was not in the form it is now – for example none of the heavy metals like iron, gold, carbon, etc existed then (what we and our planet are made out of) those elements were all formed later by reactions within the first generations of stars.

I found this article very interesting.

I think there is a typo in the “age of dinosaurs” section. Should be “reptile-like animals”, not “reptile-like mammals”. Dinosaurs were reptiles, not mammals.

Dinosaur Pictures is an interactive map that lets you see how the Earth looked like up to 750 million years ago.

Do have like a map of the earth through out the history.

That’s a wonderful history of earth

Please check your six eras legends are mixed.

This is very helpful

We can write a thousand page book in fine print and miss one or even ten specific things that one may search for, its only natural.

And the topic is nature by the way

Wow this really does help!!

Can you please give us your references?

it was amazing and very helpful.

What seems to be missing is the actual geological changes in the earth’s surface that gave rise to the different land masses and bodies of water.

Worst case: “Since about 1,000,000,000 years ago, Earth had its first signs of life.” NOT! Life emerged at approximately 4.4 BYA. No excuse, guys! Very inaccurate science. Dates are WAY off on numerous topics. DK

Very helpful charts look amazing thanks for writing this

This was really helpful 👍🏾

The Permian-Triassic mass extinction was NOT due to a meteorite impact. Consensus is that it was due to the eruption of the Siberian Steps. There is overwhelming evidence that the Siberian Traps erupted continuously for approximately 1 million years, emitting trillions of tons of CO2 into the atmosphere, disrupting the carbon cycle and causing a greenhouse gas climate catastrophe which warmed the oceans, which in turn caused methane hydrates at the bottom of the ocean to melt and release methane into the atmosphere.

What seems to be missing is the main discussion of geology of the earth, which is about the rocks and physical processes that formed them. What is not missing is a long discussion of the supposed evolution of biological creatures.

A volcanic event was to have occurred in what is now Siberia around 250 million years ago that wiped out the plants and herbivores that per-existed dinosaurs. Fossil evidence is said to exist of these herbivores. The event is to have spewed out enough lava to cover the entire USA 1000 ft deep, along with noxious particulates, sulfurous gases and co2 that changed the climate. The acid gases poisoned the seas resulting in massive extinctions.

Nice summary, thank you. I am, however, a bit puzzled that you refer to dinosaurs as (reptile-like) mammals. Are you sure they were mammals?

I have throughly enjoyed reading this work. It has simplified processes of which I was unsure. Thank you for all the work that you have put in to enable me to be able understand the geological history of the planet upon which I live.

In the second paragraph of the Cenozoic Era, it says”When dinosaurs roamed the Earth, mammals remained small and fury.” It should say small and furry, not fury.

By the way, despite the minor errors I’ve pointed out, this is an excellent summary.

While the meteor that hit what is now present day Yucatan wiped out ALMOST all dinosaurs, especially the large ones, the avian dinosaurs survived and gave rise to modern birds. I mention this because you say in the first paragraph of the section on the Cenozoic Era that the dinosaurs went extinct. My point is, that they didn’t ALL go extinct, as previously thought.

In the second paragraph in the Mesozoic era section you say “As conditions changed, dinosaurs (also known as terrible lizards) began to evolve. These reptile-like mammals had scaly skin and hatched eggs like reptiles.” While there may have been a common ancestor to dinosaurs and mammals, I’m pretty sure there were no mammals at this point.

In a PBS video it said that Pangea had formed during the Paleozoic, not the Mesozoic.

Hi, nice write up, however, I noticed the color coding of the bar graph and the corresponding era isn’t matching at the very top of the page.

It doesn’t say anything about the major types of plants in the Mesozoic era like conifers and gingkos

Great synopsis and very helpful to my home-schooled children. I would suggest a modification to the presentation of the giant impact hypothesis. The hypothesis promotes that Mars-sized body, known as Theia, collided with Earth, throwing vaporized chunks of the earth’s crust into space (as opposed to delivering a glancing blow, implying that Theia survived and is now the moon). Gravity bound the ejected particles together, creating the moon. The idea is supported by the similarities in composition of earth and moon matter. Thank you.

Very Helpful for my class project! Thank you so much!

Thanks, fixed it

Hello Your first paragraph has a typo………..”In its earliest stage of formation, it was inhabitable as it clumped from a cloud of dust” ”inhabitable” obviously should be ”uninhabitable” Regards Mike

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The History of Earth in Exactly 2000 Words

The history of Earth is obviously a ginormous topic. Really, almost everything humans know is part of the history of Earth. Here, we will cover the geologic and biologic history of Earth, dating all the way back to its creation. Given that earth formed 4.6 billion years old and I only have 2000 words to cover the entire history of earth, this will necessarily be a sweeping overview. We’ve got to cover 2 million years per word!

Pre-Earth – Solar System Formation

The universe is 14 billion years old. Earth, in contrast, is merely a third of that age. The earth’s formation occurred around the team time as our solar system’s formation. 

Our solar system was probably caused by the supernova of another star. A supernova is an incredibly powerful explosion of a star. That explosion creates a shock wave that travels outwards. The  solar nebula hypothesis  proposes that the shockwave from a supernova swept up any space dust that was out in the ether towards the front of a sort of wave. Think of it like sweeping up dust from your floor. As all of this dust, which was spread out over unimaginable distances, becomes (relatively) concentrated in one area at the front of the shock wave, the gravitational pull from each piece of dust began to pull the dust together. This process is called accretion. Accretion continued and the dust turned into rocks, which turned into asteroids, and got bigger until our planet and sun formed. 

Geologic Time

Geologists break time into geologic time. Eons are the largest units on the geologic time scale, lasting 500 million to 2 billion years. Smaller units of time are eras, periods, and epochs. For example, we are in the Holocene epoch, which is part of the Quaternary period, which is part of the Cenozoic era, which is part of the Phanerozoic eon. Here, we will explore planet Earth’s four eons in broad strokes.

Hadean Eon – Hell on Earth 

4.6 – 4 billion years ago

The first eon in the history of Earth was the Hadean Eon. The earth’s surface was a molten ball of magma during this eon. The Hadean eon was incredibly violent, with lots of major impacts from accretion still happening. 

Theia and Our Moon

One particularly noteworthy impact was when the planetary body Theia dealt a glancing blow to early Earth. Both celestial bodies were molten.  When Theia hit Earth , Theia tore off a chunk of Earth’s mass. Theia continued on her orbit, but the torn-off chunk of Earth did not. As it cooled, this chunk remained in the Earth’s orbit and turned into our moon. 

As with everything in geologic time, scientists are currently  rethinking  the creation of the moon, with some arguing that it wasn’t Theia’s impact. 

The Late Heavy Bombardment

The  late heavy bombardment  marks the end of Hadean eon and the beginning of the next eon, which is the Archean. The late heavy bombardment was caused when gas giants Uranus and Neptune fled further away from the sun. This change in solar system gravity caused tons of asteroids, comets, and meteorites from the asteroid belt to be pulled closer towards the sun, and therefore Earth. Tons and tons of these asteroids impacted Earth, Venus, Mars, and Mercury. These collisions made Earth gain mass and heat.  

We have no rocks from the Hadean Eon because the Earth was all lava. Scientists believe life couldn’t have formed before the incredibly riotous time of the late heavy bombardment.

Archean Eon – The Origins of Life

4 – 2.5 billion years ago

The Archean Eon wasn’t nearly as violent as the Hadean, but Earth still wasn’t a place you would want to visit. After the late heavy bombardment, the crust of the earth was able to solidify into a rock, rather than magma. Earth had no atmosphere and the land was completely inhospitable. 

The First Fossils, the First Life

The first fossils of life on earth appear about  4-3.7 billion years ago  during the first part of the Archean era. Of course, early life was incredibly basic compared to what we’re used to today. Viruses also appeared around this time.  Some scientists  think the relatively rapid appearance of living organisms on earth, being only a few hundred million years after the planet itself appeared, is evidence supporting the existence of life all over the universe. The  oldest fossils  we know about are in Australia and date to 3.5 billion years ago. 

Photosynthesis Begins

About 3.5 billion years ago, life figured out how to capture energy from the sun via photosynthesis. These ocean-dwelling microbes released oxygen as a byproduct. However, this oxygen didn’t help create an atmosphere just yet. Instead, the high concentration of iron in the oceans reacted with the oxygen, preventing it from escaping out of the ocean. 

Proterozoic Eon – Something and Then Nothing

2.5 billion – 550 million years ago

The Proterozoic eon is, by far, the longest in Earth’s history, at about 2 billion years. Earth’s atmosphere and lithosphere (rocks) changed significantly during the first part of this period. The second billion years of the Proterozoic, in contrast, were not very exciting.

The Great Oxygenation 

Oxygen began to accumulate in Earth’s atmosphere  around 2.3 billion years ago . The accumulation of oxygen distinguishes the Archaean Eon from the Proterozoic Eon. 

The first snowball Earth occurred during this time. This glaciation was one of the major drivers that pumped oxygen into the atmosphere. When climate change caused the end of this ice age around 2.3 billion years ago, the melting ice introduced vast amounts of oxygen into the atmosphere.

Ironically, the addition of oxygen to the oceans and atmosphere was catastrophic for life on Earth, at least in the short run. Life in the oceans had evolved to live in an anaerobic, or oxygen-free, environment. The introduction of oxygen led to the extinction of many of these anaerobic bacteria. 

The Boring Billion

As the name implies, the period from about 1.8 to 0.8 billion years ago was remarkable in only one way; that Earth was stable and not much happened. Tectonics didn’t shift much, life didn’t really evolve, there weren’t many available nutrients, and the climate didn’t change. 

Some scientists think that eukaryotes (life with a nucleus) evolved all-important characteristics during the boring billion, such as sexual reproduction, which made the Cambrian explosion possible. Additionally, algae and the beginnings of plants evolved during this time. But considering this was over 1000 million years, those evolutionary accomplishments seem fairly minimal. I guess all of us, the Earth included, need periods of rest. 

Plate Tectonics

Geologists aren’t exactly sure when the Earth’s crust divided into different plates, causing plate tectonics. This may have happened anywhere from  1-3 billion years ago . Scientists who argue for the more recent evolution of plate tectonics believe that this process  couldn’t have happened  when the Earth was still a ball of magma or otherwise very hot. Ever since the late heavy bombardment, the earth has been cooling. As the crust of the earth turned from magma to rock, tectonic plates were able to form. 

Plate tectonics has been  essential to the development of life on Earth  over the past billion years. These tectonics create mountain ranges and oceanic troughs. Along with glaciers that crushed rocks, the eroding of mountains made via plate tectonics provided the nutrients necessary for life to finally end the boring billion. 

Phanerozoic Eon – Our Modern Earth

550 million years ago to present

All the nutrients pouring into the ocean from eroding mountains provided the building blocks for life to diversify. The beginning of the Phanerozoic Eon is marked by the  Cambrian Explosion . Tons of exciting life forms first appeared during this time. Importantly for us, the first animals evolved during this biological explosion. While we don’t have fossils of the very first animals, creatures like  hallucigenia  and jellyfish give us clues into the first animals that swam (they were all sea-dwelling) the earth. Here are just a  few examples  of new life that evolved during the explosion; sea creatures evolved eyes and legs, mollusks appeared, predators evolved jaws and teeth, and worms evolved plume-like gills. 

Complex Plants!

About 470 million years ago plants finally made the move from the ocean to land. About 100 million years later, these plants began to resemble the plants we have today with roots, leaves, and stems. Fun fact: grass hadn’t evolved by the time dinosaurs walked the Earth. 

Supercontinents – A Crucial Step in the History of the Earth

The formation and breakup of the  supercontinents Gondwana and Pangea  had massive impacts on the history of the earth. The size of landmass, the location of that landmass relative to the poles of the earth, and the volcanic activity of that landmass all contributed to the different climates of the Phanerozoic eon. 

Dinosaurs – The Mesozoic Era

Of course, dinosaurs are an integral part of the history of Earth and our current eon. Dinos evolved about 250 million years ago and dominated the earth until 66 million years ago. You may recognize the words Triassic, Jurassic, and Cretaceous. These are all periods of geologic time, which are smaller than eras, which in turn are smaller than eons. Dinosaurs roamed during these three geologic periods, which together make the Mesozoic era. 

Mass Extinctions

Since the Cambrian explosion, there have been six mass extinctions ( including the human-caused one happening today ). These extinctions were major events in the evolution of life because they culled down the number of species on the planet. Over the next tens of millions of years, new forms of life evolved from those that made it through the extinction, eventually leading to the world of life we know today.

Humans Evolve – The Antropocene

Some people speak of evolution as if the appearance of humans was the end goal of evolution. Evolution has no goals and will continue long after humans go extinct. We are merely a byproduct of natural processes. As demonstrated above, if the age of the earth was condensed into 12 hours, humans would appear 11 hours 59 minutes 58 seconds into that 12 hours. Our presence is  very  new to this planet. 

However, a history of Earth necessarily ends with the evolution of homo sapiens because that’s where we are today. Some scientists are fighting for the Anthropocene to become our present-day geologic epoch. This geologic period would start after World War II, when humans began to change the climate and create vast amounts of waste, especially plastic. Evidence for this new geologic period is the presence of plastic all over the world, which will eventually become incorporated into rocks and earth’s lithosphere, the rapid warming of the planet, and the mass extinction event happening currently. 

Geologists as a whole haven’t accepted the designation of the Anthropocene as its own geologic period, but that may change in the coming years. 

The History of Earth Continues

As far as we know, Earth is the only planet with life.  Space exploration , especially with the launch of the  James Webb Space Telescope , is revealing  earth-like planets  at an astonishing rate. Perhaps as we learn more about other Earth-like planets out in the universe, we will learn more about the history of Earth, our pale blue dot. 

Check us out on  EarthSnap , a free app brought to you by Eric Ralls and Earth.com.

Essay on Earth

500 words essay on earth.

The earth is the planet that we live on and it is the fifth-largest planet. It is positioned in third place from the Sun. This essay on earth will help you learn all about it in detail. Our earth is the only planet that can sustain humans and other living species. The vital substances such as air, water, and land make it possible.

All About Essay on Earth

The rocks make up the earth that has been around for billions of years. Similarly, water also makes up the earth. In fact, water covers 70% of the surface. It includes the oceans that you see, the rivers, the sea and more.

Thus, the remaining 30% is covered with land. The earth moves around the sun in an orbit and takes around 364 days plus 6 hours to complete one round around it. Thus, we refer to it as a year.

Just like revolution, the earth also rotates on its axis within 24 hours that we refer to as a solar day. When rotation is happening, some of the places on the planet face the sun while the others hide from it.

As a result, we get day and night. There are three layers on the earth which we know as the core, mantle and crust. The core is the centre of the earth that is usually very hot. Further, we have the crust that is the outer layer. Finally, between the core and crust, we have the mantle i.e. the middle part.

The layer that we live on is the outer one with the rocks. Earth is home to not just humans but millions of other plants and species. The water and air on the earth make it possible for life to sustain. As the earth is the only livable planet, we must protect it at all costs.

Get the huge list of more than 500 Essay Topics and Ideas

There is No Planet B

The human impact on the planet earth is very dangerous. Through this essay on earth, we wish to make people aware of protecting the earth. There is no balance with nature as human activities are hampering the earth.

Needless to say, we are responsible for the climate crisis that is happening right now. Climate change is getting worse and we need to start getting serious about it. It has a direct impact on our food, air, education, water, and more.

The rising temperature and natural disasters are clear warning signs. Therefore, we need to come together to save the earth and leave a better planet for our future generations.

Being ignorant is not an option anymore. We must spread awareness about the crisis and take preventive measures to protect the earth. We must all plant more trees and avoid using non-biodegradable products.

Further, it is vital to choose sustainable options and use reusable alternatives. We must save the earth to save our future. There is no Planet B and we must start acting like it accordingly.

Conclusion of Essay on Earth

All in all, we must work together to plant more trees and avoid using plastic. It is also important to limit the use of non-renewable resources to give our future generations a better planet.

FAQ on Essay on Earth

Question 1: What is the earth for kids?

Answer 1: Earth is the third farthest planet from the sun. It is bright and bluish in appearance when we see it from outer space. Water covers 70% of the earth while land covers 30%. Moreover, the earth is the only planet that can sustain life.

Question 2: How can we protect the earth?

Answer 2: We can protect the earth by limiting the use of non-renewable resources. Further, we must not waste water and avoid using plastic.

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Earth Day 2024

By: History.com Editors

Updated: April 22, 2024 | Original: October 27, 2009

Earth Day was founded in 1970 as a day of education about environmental issues, and Earth Day 2024 is on Monday, April 22. The holiday is now a global celebration that’s sometimes extended into Earth Week, a full seven days of events focused on green living and confronting the climate crisis. The brainchild of Senator Gaylord Nelson and inspired by the protests of the 1960s, Earth Day began as a “national teach-in on the environment” and was held on April 22 to maximize the number of students that could be reached on university campuses. By raising public awareness of pollution, Nelson hoped to bring environmental causes into the national spotlight.

Earth Day History

By the early 1960s, Americans were becoming aware of the effects of pollution on the environment. Rachel Carson’s 1962 bestseller Silent Spring raised the specter of the dangerous effects of pesticides on the American countryside. Later in the decade, a 1969 fire on Cleveland’s Cuyahoga River shed light on the problem of chemical waste disposal. Until that time, protecting the planet’s natural resources was not part of the national political agenda, and the number of activists devoted to large-scale issues such as industrial pollution was minimal. Factories pumped pollutants into the air, lakes and rivers with few legal consequences. Big, gas-guzzling cars were considered a sign of prosperity. Only a small portion of the American population was familiar with–let alone practiced–recycling.

Did you know? A highlight of the United Nations' Earth Day celebration in New York City is the ringing of the Peace Bell, a gift from Japan, at the exact moment of the vernal equinox.

Who Started Earth Day?

Elected to the U.S. Senate in 1962, Senator Gaylord Nelson, a Democrat from Wisconsin, was determined to convince the federal government that the planet was at risk. In 1969, Nelson, considered one of the leaders of the modern environmental movement, developed the idea for Earth Day after being inspired by the anti- Vietnam War “teach-ins” that were taking place on college campuses around the United States. According to Nelson, he envisioned a large-scale, grassroots environmental demonstration “to shake up the political establishment and force this issue onto the national agenda.”

Nelson announced the Earth Day concept at a conference in Seattle in the fall of 1969 and invited the entire nation to get involved. He later recalled:

“The wire services carried the story from coast to coast. The response was electric. It took off like gangbusters. Telegrams, letters and telephone inquiries poured in from all across the country. The American people finally had a forum to express its concern about what was happening to the land, rivers, lakes and air—and they did so with spectacular exuberance.”

Denis Hayes, a young activist who had served as student president at Stanford University, was selected as Earth Day’s national coordinator, and he worked with an army of student volunteers and several staff members from Nelson’s Senate office to organize the project. According to Nelson, “Earth Day worked because of the spontaneous response at the grassroots level. We had neither the time nor resources to organize 20 million demonstrators and the thousands of schools and local communities that participated. That was the remarkable thing about Earth Day. It organized itself.”

On the first Earth Day on April 22, 1970, rallies were held in Philadelphia, Chicago , Los Angeles and most other American cities, according to the Environmental Protection Agency. In New York City , Mayor John Lindsay closed off a portion of Fifth Avenue to traffic for several hours and spoke at a rally in Union Square with actors Paul Newman and Ali McGraw. In Washington, D.C. , thousands of people listened to speeches and performances by singer Pete Seeger and others, and Congress went into recess so its members could speak to their constituents at Earth Day events.

The first Earth Day was effective at raising awareness about environmental issues and transforming public attitudes. According to the Environmental Protection Agency, “Public opinion polls indicate that a permanent change in national priorities followed Earth Day 1970. When polled in May 1971, 25 percent of the U.S. public declared protecting the environment to be an important goal, a 2,500 percent increase over 1969.” Earth Day kicked off the “Environmental decade with a bang,” as Senator Nelson later put it. During the 1970s, a number of important pieces of environmental legislation were passed, among them the Clean Air Act, the Water Quality Improvement Act, the Endangered Species Act, the Toxic Substances Control Act and the Surface Mining Control and Reclamation Act. Another key development was the establishment in December 1970 of the Environmental Protection Agency, which was tasked with protecting human health and safeguarding the natural environment—air, water and land.

What Do You Do For Earth Day?

Since 1970, Earth Day celebrations have grown. In 1990, Earth Day went global, with 200 million people in over 140 nations participating, according to the Earth Day Network (EDN), a nonprofit organization that coordinates Earth Day activities. In 2000, Earth Day focused on clean energy and involved hundreds of millions of people in 184 countries and 5,000 environmental groups, according to EDN. Activities ranged from a traveling, talking drum chain in Gabon, Africa, to a gathering of hundreds of thousands of people at the National Mall in Washington, D.C. 

Today, the Earth Day Network collaborates with more than 17,000 partners and organizations in 174 countries. According to EDN, more than 1 billion people are involved in Earth Day activities, making it “the largest secular civic event in the world.”

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An essay toward a natural history of the earth: and terrestrial bodies, especially minerals: as also of the sea, rivers, and springs. With an account of the universal deluge: and of the effects that it had upon the earth

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What can early Earth teach us about the search for life?

by Evan Gough, Universe Today

Earth is the only life-supporting planet we know of, so it's tempting to use it as a standard in the search for life elsewhere. But the modern Earth can't serve as a basis for evaluating exoplanets and their potential to support life. Earth's atmosphere has changed radically over its 4.5 billion years.

A better way is to determine what biomarkers were present in Earth's atmosphere at different stages in its evolution and judge other planets on that basis.

That's what a group of researchers from the UK and the U.S. did. Their research is titled " The early Earth as an analogue for exoplanetary biogeochemistry ," and it appears on the pre-print server arXiv . The lead author is Eva E. Stüeken, a Ph.D. student at the School of Earth & Environmental Sciences, University of St Andrews, UK.

When Earth formed about 4.5 billion years ago, its atmosphere was nothing like it is today. At that time, the atmosphere and oceans were anoxic. About 2.4 billion years ago, free oxygen began to accumulate in the atmosphere during the Great Oxygenation Event, one of the defining periods in Earth's history. But the oxygen came from life itself, meaning life was present when the Earth's atmosphere was much different.

This isn't the only example of how Earth's atmosphere has changed over geological time . But it's an instructive one and shows why searching for life means more than just searching for an atmosphere like modern Earth's. If that's the way we conducted the search, we'd miss worlds where photosynthesis hadn't yet appeared.

In their research, the authors point out how Earth hosted a rich and evolving population of microbes under different atmospheric conditions for billions of years.

"For most of this time, Earth has been inhabited by a purely microbial biosphere albeit with seemingly increasing complexity over time," the authors write. "A rich record of this geobiological evolution over most of Earth's history thus provides insights into the remote detectability of microbial life under a variety of planetary conditions."

It's not just life that's changed over time. Plate tectonics have changed and may have been 'stagnant lid' tectonics for a long time. In stagnant lid tectonics, plates don't move horizontally. That can have consequences for atmospheric chemistry.

The main point is that Earth's atmosphere does not reflect the solar nebula the planet formed in. Multiple intertwined processes have changed the atmosphere over time. The search for life involves not only a better understanding of these processes, but how to identify what stage exoplanets might be in.

It's axiomatic that biological processes can have a dramatic effect on planetary atmospheres. "On the modern Earth, the atmospheric composition is very strongly controlled by life," the researchers write. "However, any potential atmospheric biosignature must be disentangled from a backdrop of abiotic (geological and astrophysical) processes that also contribute to planetary atmospheres and would be dominating on lifeless worlds and on planets with a very small biosphere."

The authors outline what they say are the most important lessons that the early Earth can teach us about the search for life.

The first is that the Earth has actually had three different atmospheres throughout its long history. The first one came from the solar nebula and was lost soon after the planet formed. That's the primary atmosphere. The second one formed from outgassing from the planet's interior.

The third one, Earth's modern atmosphere, is complex. It's a balancing act involving life, plate tectonics, volcanism, and even atmospheric escape. A better understanding of how Earth's atmosphere has changed over time gives researchers a better understanding of what they see in exoplanet atmospheres.

The second is that the further we look back in time, the more the rock record of Earth's early life is altered or destroyed. Our best evidence suggests life was present by 3.5 billion years ago, maybe even by 3.7 billion years ago. If that's the case, the first life may have existed on a world covered in oceans, with no continental land masses and only volcanic islands.

If there had been abundant volcanic and geological activity between 3.5 and 3.7 billion years ago, there would've been large fluxes of CO 2 and H 2 . Since these are substrates for methanogenesis, then methane may have been abundant in the atmosphere and detectable.

The third lesson the authors outline is that a planet can host oxygen-producing life for a long time before oxygen can be detected in an atmosphere. Scientists think that oxygenic photosynthesis appeared on Earth in the mid-Archean eon. The Archean spanned from 4 billion to 2.5 billion years ago, so mid-Archean is sometime around 3.25 billion years ago. But oxygen couldn't accumulate in the atmosphere until the Great Oxygenation Event about 2.4 billion years ago.

Oxygen is a powerful biomarker, and if it is found in an exoplanet's atmosphere, it would be cause for excitement. But life on Earth was around for a long time before atmospheric oxygen would've been detectable.

The fourth lesson involves the appearance of horizontal plate tectonics and its effect on chemistry. "From the GOE onwards, the Earth looked tectonically similar to today," the authors write. The oceans were likely stratified into an anoxic layer and an oxygenated surface layer. However, hydrothermal activity constantly introduced ferrous iron into the oceans. That increased the sulfate levels in the seawater which reduced the methane in the atmosphere. Without that methane, Earth's biosphere would've been much less detectable.

"Planet Earth has evolved over the past 4.5 billion years from an entirely anoxic planet with possibly a different tectonic regime to the oxygenated world with horizontal plate tectonics that we know today," the authors explain. All that complex evolution allowed life to appear and to thrive, but it also makes detecting earlier biospheres on exoplanets more complicated.

We're at a huge disadvantage in the search for life on exoplanets. We can literally dig into Earth's ancient rock to try to untangle the long history of life on Earth and how the atmosphere evolved over billions of years. When it comes to exoplanets, all we have is telescopes. Increasingly powerful telescopes, but telescopes nonetheless. While we are beginning to explore our own solar system, especially Mars and the tantalizing ocean moons orbiting the gas giants, other solar systems are beyond our physical reach.

"We must instead remotely recognize the presence of alien biospheres and characterize their biogeochemical cycles in planetary spectra obtained with large ground- and space-based telescopes," the authors write. "These telescopes can probe atmospheric composition by detecting absorption features associated with specific gases." Probing atmospheric gases is our most powerful approach right now, as the JWST shows.

But as scientists get better tools, they'll start to go beyond atmospheric chemistry. "We might also be able to recognize global-scale surface features, including light interaction with photosynthetic pigments and 'glint' arising from specular reflection of light by a liquid ocean."

Understanding what we're seeing in exoplanet atmospheres parallels our understanding of Earth's long history. Earth could be the key to our broadening and accelerating search for life.

"Unraveling the details of Earth's complex biogeochemical history and its relationship with remotely observable spectral signals is an important consideration for instrument design and our own search for life in the universe," the authors write.

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Title: information content of jwst spectra of wasp-39b.

Abstract: WASP-39b was observed using several different JWST instrument modes and the spectra were published in a series of papers by the ERS team. The current study examines the information content of these spectra measured using the different instrument modes, focusing on the complexity of the temperature-pressure profiles and number of chemical species warranted by the data. We examine if H2O, CO, CO2, K, H2S, CH4, and SO2 are detected in each of the instrument modes. Two Bayesian inference methods are used to perform atmospheric retrievals: standard nested sampling and supervised machine learning of the random forest (trained on a model grid). For nested sampling, Bayesian model comparison is used as a guide to identify the set of models with the required complexity to explain the data. Generally, non-isothermal transit chords are needed to fit the transmission spectra of WASP-39b, although the complexity of the Tp-profile required is mode-dependent. The minimal set of chemical species needed to fit a spectrum is mode-dependent as well, and also depends on whether grey or non-grey clouds are assumed. When a non-grey cloud model is used to fit the G395H spectrum, it generates a spectral continuum that compensates for the H2O opacity. The same compensation is absent when fitting the non-grey cloud model to the PRISM spectrum (which has broader wavelength coverage), suggesting that it is spurious. The interplay between the cloud spectral continuum and the H2O opacity determines if SO2 is needed to fit either spectrum. The inferred elemental abundances of carbon and oxygen and the carbon-to-oxygen (C/O) ratios are all mode- and model-dependent, and should be interpreted with caution. Bayesian model comparison does not always offer a clear path forward for favouring specific retrieval models (e.g. grey versus non-grey clouds) and thus for enabling unambiguous interpretations of exoplanet spectra.

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