History of Earth

The planet Earth, photographed in the year 1972.

The history of Earth covers approximately 4.55×10⁹ years, from its formation out of the solar nebula to the present. This article presents a broad overview, summarizing the leading scientific theories. Due to the difficulty of comprehending very large amounts of time, the analogy of a single 24-hour period will be used, beginning exactly 4.55×10⁹ years ago, at the formation of Earth, and ending now. Each second of this period represents approximately 53,000 years. The Big Bang and origin of the universe took place almost three days ago—two whole days before our clock began to tick.

Origin

Artist's impression of a protoplanetary disc forming around a binary star system.

Main articles: Solar system § Origin and age of the solar system, and Planet § Planetary formation

The formation of Earth occurred as part of the birth of the solar system: what eventually became the solar system initally existed as a large, rotating cloud of dust and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements produced in numerous supernovae from stars long gone. Then, about 4.6×10⁹ years ago (fifteen to thirty minutes before our imaginary clock started), a nearby star probably became a supernova. The explosion sent a shock wave toward the solar nebula and caused it to contract. As the cloud continued to rotate, gravity and inertia flattened the cloud into a disc, perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. Meanwhile, as gravity caused matter to condense around dust particles, the rest of the disc started to break up into rings. Small fragments collided and became larger fragments. These included one collection approximately 150 million kilometers from the center—Earth. As the Sun condensed and heated, fusion started and the resulting solar wind cleared out most of the material in the disc that had not already condensed into larger bodies.

Moon

Animation of Theia forming in Earth's L₅ point and then drifting into impact. The animation progresses in one-year steps making Earth appear not to move. The view is of the south pole.

Main articles: Moon § Origin and history, and Giant impact hypothesis

The origin of the Moon is still uncertain, though much evidence exists for the giant impact hypothesis. Earth may not have been the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from both the Sun and the Earth, at the fourth or fifth Lagrange point. This planet, named Theia, is thought to have been smaller than the current Earth, probably about the size and mass of Mars. Its orbit may at first have been stable, but destabilized as Earth increased its mass by the accretion of more and more material. Theia swung back and forth relative to Earth until, finally, 4.533×10⁹ years ago (perhaps 12:05 a.m. on our clock), it collided at a low, oblique angle. The low speed and angle were not enough to destroy Earth, but a large portion of its crust was ejected. Heavier elements from Theia sank to Earth's core, while the remaining material and ejecta condensed into a single body within a couple of weeks. Under the influence of its own gravity, and probably within a year, this became a more spherical body: the Moon. The impact is also thought to have changed Earth's axis to produce the large 23.5° axial tilt that is responsible for Earth's seasons (a simple, ideal model of the planets' origins would have axial tilts of 0° and no recognizable seasons). It may have also sped up Earth's rotation and been instrumental in the inception of the planet's plate tectonics.

Early days: Hadean eon

Volcanic eruptions would have been common in Earth's early days.

The early Earth was very different from the world known today. There were no oceans and no oxygen in the atmosphere. It was bombarded by planetoids and other material left over from the formation of the solar system. This bombardment, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction, probably caused the planet at this stage to be fully molten. Heavier elements sank to the center; lighter ones rose to the surface, producing Earth's various layers (see Structure of the Earth). Earth's very first atmosphere would have comprised surrounding material from the solar nebula—mainly light gases like hydrogen and helium. The solar wind and Earth's own heat would have driven off this atmosphere. The surface cooled slowly, forming the solid crust, probably within 200 million years (around 1:00 a.m. on our clock). Steam escaped from the crust and more gases were released by volcanoes, giving rise to a second atmosphere. Additional water was imported by meteorite and comet collisions. The planet cooled. Clouds formed. Rain gave rise to the oceans by about 700 million years (around 3:45 a.m. on our clock), but probably earlier. The new atmosphere probably contained ammonia, methane, water vapor, carbon dioxide, and nitrogen, as well as smaller amounts of other gases. Any free oxygen would have been bound by hydrogen or minerals on the surface. Volcanic activity was intense and, without an ozone layer to hinder its passage, unimpeded ultraviolet radiation flooded the surface.

Beginnings of life

The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems highly elaborate.

Main article: Origin of life

The high energy from volcanoes, lightning, and ultraviolet radiation helped drive chemical reactions producing more complex molecules from simple compounds like methane and ammonia. Among these were many of the relatively simple organic compounds that are the building blocks of life. As the amount of this "organic soup" increased, different molecules reacted with one another. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material. The presence of certain molecules could speed up a chemical reaction. All this continued for a very long time, with reactions occurring more or less at random, until by chance there arose a new molecule: the replicator. This had the bizarre property of promoting the chemical reactions which produced a copy of itself, and evolution began. The nature of the first replicator is unknown. Perhaps it was similar to our current replicator, DNA, or perhaps it was a phospholipid or even a crystal. The timing of this is highly speculative as well—perhaps around 4×10⁹ years ago (around 3:00 a.m. on our clock). Though the nature of this molecule and the details of these events remain unknown, the broad principles have been reasonably well established. In making copies of itself, the replicator did not always perform accurately: some copies contained an "error." If the change destroyed the copying ability of the molecule, there could be no more copies, and the line would "die out." On the other hand, a few rare changes might make the molecule replicate faster or better: those "strains" would become more numerous and "successful." As choice building blocks ("food") in the organic soup became depleted, strains which could exploit different materials—or perhaps check the progress of other strains—became more numerous.

The first cell

A small section of a cell membrane. This modern cell membrane is far more sophisticated than the original simple phospholipid bilayer (the small blue spheres with two tails). Proteins and carbohydrates serve various functions in regulating the passage of material through the membrane and in reacting to the environment.

Modern life has its replicating material packaged neatly inside a cellular membrane. It is easier to understand the origin of the cell membrane than the origin of the replicator, since the phospholipid molecules that make up a cell membrane will often spontaneously form a bilayer when placed in water. Under certain conditions, many such spheres can be formed (see the Bubble Theory). It is not known whether this process preceded or succeeded the origin of the replicator (or perhaps it was the replicator). The prevailing theory is that the replicator, perhaps RNA by this point (see the RNA world hypothesis), along with its replicating apparatus and maybe other biomolecules, had already evolved. Initial protocells may have simply burst when they grew too large; the scattered contents may then have recolonized other "bubbles." Proteins that stabilized the membrane, or that later assisted in an orderly division, would have promoted the proliferation of those cell lines. RNA is a likely candidate for an early replicator since it can both store genetic information and catalyze reactions. At some point DNA took over the genetic storage role from RNA, and proteins known as enzymes took over the catalysis role, leaving RNA to transfer information and modulate the process. There is increasing belief that these early cells may have evolved in association with underwater volcanic vents ("black smokers"). However, it is believed that out of this multiplicity of cells, or protocells, only one survived. Current evidence suggests that perhaps roughly 3.5×10⁹ years ago (5:30 a.m. on our imaginary clock) the last universal common ancestor lived. This "LUCA" cell is the likely ancestor of all cells and hence all life on Earth. It would have been what we now call a prokaryote, possessing a cell membrane and ribosomes, but lacking a nucleus or membrane-bound organelles like mitochondria or chloroplasts. Like all modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to catalyze reactions.

Photosynthesis and oxygen

The harnessing of the sun's energy led to several major changes in life on Earth.

It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material, and as an energy source. As the food supply diminished, a new strategy evolved in some cells. Instead of relying on the diminishing amounts of free-existing organic molecules, these cells adopted sunlight as an energy source. Eventually, by about 3×10⁹ years ago (around 8:00 a.m. on our clock), something similar to modern photosynthesis had arisen. This made the sun's energy available not only to autotrophs but also to the heterotrophs that consumed them. Photosynthesis used the plentiful carbon dioxide and water as raw materials and, with the energy of sunlight, produced energy-rich organic molecules (carbohydrates). Moreover, oxygen is a waste product of photosynthesis. At first it became bound up with iron and other minerals but, once all available minerals were bound, oxygen began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast period of time transformed Earth's atmosphere to its current state. This, then, is Earth's third atmosphere. Some of the oxygen reacted to form ozone, which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and ultimately the land: without the ozone layer, ultraviolet radiation bombarding the surface would have caused unsustainable levels of mutation in exposed cells. Besides making large amounts of energy available to life-forms and blocking ultraviolet radiation, the effects of photosynthesis had a third, major, and world-changing impact. Oxygen was toxic; probably much life on Earth died out as its levels rose. Resistant forms survived and thrived, and some developed the ability to use oxygen to enhance their metabolism and derive more energy from the same food.

Endosymbiosis and the three domains of life

Some of the pathways by which the various endosymbionts might have arisen.

Modern taxonomy classifies life into three domains. The nature and time of the origin of these domains are speculative. First, the Bacteria domain probably split off from the other forms of life, sometimes called Neomura, but this supposition is is controversial. Soon after this, perhaps 2×10⁹ years ago (around 2:00 p.m. on our clock), the Neomura split into the Archaea and the Eukarya. Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now coming to light. Around this time period, a bacterial cell related to today's Rickettsia entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest the smaller one but failed (maybe due to the evolution of prey defenses). Perhaps the smaller cell attempted to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it was able to metabolize the larger cell's waste products and derive more energy. Some of this surplus energy was returned to the host. The smaller cell replicated inside the larger one, and soon a stable symbiotic relationship developed. Over time the host cell acquired some of the genes of the smaller cells, and the two kinds became dependent on each other—the larger cell could not survive without the energy produced by the smaller ones, and these in turn could not survive without the raw materials provided by the larger cell. Synchrony developed between the larger cell and the population of smaller cells inside it to the extent that they are considered to have become a single organism, the smaller cells being classified as organelles called mitochondria. A similar event took place with photosynthetic cyanobacteria entering larger heterotrophic cells and becoming chloroplasts. Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes some time before 10⁹ years ago (around 6:00 p.m. on our clock). There were probably several such inclusion events, as the figure at right suggests. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it has been suggested that cells gave rise to peroxisomes, spirochetes gave rise to cilia and flagella, and that perhaps a DNA virus gave rise to the cell nucleus, though none of these theories are generally accepted.

Multicellularity

Volvox aureus is believed to be similar to the first multicellular plants.

Archeans, bacteria, and eukaryotes continued to diversify, and to become more sophisticated and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archea and bacteria. The plant, animal, and fungi lines had all split, though they still existed as solitary cells. Some of these lived in colonies, and gradually some division of labor began to take place; for instance, cells on the periphery might assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 10⁹ years ago (around 7:00 p.m. on our clock), the first multicellular plants emerged, probably initially similar to green algae like Volvox. Also around this time, the supercontinent Rodinia formed. Earlier continental movements are not well established. A hundred million years or so later, true multicellularity had also evolved in animals. At first it probably somewhat resembled that of today's sponges, where all cells were totipotent and a disrupted organism could reassemble itself. As the division of labor became more complete in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die. Around 750 million years ago (8:00 p.m. on our clock) Rodinia began to break up.

Colonization of land

File:Mars Twin Peaks (1024px).jpg

For most of Earth's history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth's neighboring planets.

As we have already seen, the accumulation of oxygen in Earth's atmosphere resulted in the formation of ozone, forming a layer that absorbed much of the sun's ultraviolet radiation. As a result, unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. The chronology of this process is unknown, but it probably happened prior to the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 million years ago and then broke apart 60 million years later (from about 8:50 p.m. to 9:10 p.m. on our imaginary clock). Fish, the earliest vertebrates, evolved in the oceans around 510 million years ago (9:20 p.m). A major extinction event occurred 488 million years ago (9:25 p.m.).

At some time during the Ordovician period (488.3 to 443.7 million years ago, 9:40 p.m. on our clock), plants (probably resembling algae) started growing at the edges of the water, and then out of it. Initially remaining close to the water's edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is controversial: it is generally believed that arthropods appeared on the surface around 450 million years ago (9:40 p.m.), perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. Alternatively, some believe that arthropods preceded the first plants, appearing on land as early as 530 million years ago (9:12 p.m.), perhaps enjoying a safe place to lay their eggs as a survival advantage. Around 380 to 375 million years ago (10:00 p.m.) the first tetrapods evolved from the fish. It is thought that fins evolved to become limbs which allowed the first tetrapods to lift their heads out of the water to breathe air. This would let them survive in oxygen-poor water or pursue small prey in shallow water. They may have later ventured on land for brief periods. Eventually the amphibians evolved. They hatched in the water but spent their adult lives on land. Around 360 million years ago (10:06 p.m.), another period of extinction occurred. Around this time, plants evolved seeds, which dramatically accelerated their spread on land.

Pangaea, the most recent supercontinent, existed from 300 to 180 million years ago. The outlines of the modern continents and other land masses are indicated on this map.

Twenty million years later (340 million years ago, 10:12 p.m. on our clock), the evolution of the amniotic egg allowed eggs to be laid on land, probably a survival advantage for the tetrapod embryos. This resulted in the divergence of reptiles from amphibians. Another thirty million years (310 million years ago, 10:22 p.m.) saw the divergence of the mammals from the reptiles. Of course, other groups of organisms continued to evolve and lines diverged—in fish, insects, bacteria, and so on—but not as much is known of the details. 300 million years ago (10:25 p.m.) the most recent supercontinent formed, called Pangaea. The most severe extinction event to date took place 252 million years ago (10:40 p.m. on our clock); soon afterwards, dinosaurs split off from their reptilian ancestors and came to be dominant among the vertebrates. Existing mammals were probably similar to modern shrews. By 180 million years ago (11:03 p.m.), Pangea broke up into Laurasia and Gondwana. Fifty million years later (130 million years ago, 11:19 p.m.), flowers evolved and, in another ten million years (120 million years ago, 11:22 p.m.), the line of birds separated from other reptiles. Competition with birds drove many pterosaurs to extinction, and the dinosaurs were probably already in decline for various reasons when, 65 million years ago (11:39 p.m.), a meteorite struck Earth just off the Yucatán peninsula, ejecting vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. Most large animals, including the non-avian dinosaurs, became extinct. Thereafter, mammals diversified, grew larger, and became the dominant vertebrates. Around 2 million years later (63 million years ago, 11:40 p.m.), the line of primates diverged from the rest of the mammals. By 38 million years ago (11:48 p.m.), the terrestrial ancestors of cetaceans had returned to the oceans to become the dolphins and whales.

Humanity

File:Austrolopithecus africanus.jpg

Australopithecus africanus, an early hominid.

Main article: Human evolution

A small African ape, the last common ancestor of modern humans and the chimpanzees, lived around six million years ago (11:58 p.m. on our clock). The line then forked: One branch gave rise to humans and to no other surviving animals. The other branch later forked again, into the common chimpanzee and bonobo. For various reasons that are still debated, one of the apes gained the ability to walk upright, probably soon after the initial split. Brain size increased rapidly, and by 2.4 million years ago (11:59:14 p.m., or 46 seconds before midnight) the very first animals classified in the genus Homo had appeared. The ability to control fire began in Homo erectus, and perhaps the rudiments of modern language did also. Though the precise chronology is uncertain, this probably happened between 800 million and 300 million years (between fifteen and six seconds) ago. As brain size increased, babies had to be born sooner, requiring a longer period of dependence. This, along with increased plasticity and ability to learn, led to more and more complex social skills. In parallel, humans became better at devising and using tools. All this fed back into language, and thus further cooperation and brain-development. Modern humans—Homo sapiens—are believed to have originated 100,000 to 200,000 years (two to four seconds) ago in Africa, spreading around the world. Spirituality began in ancestors of Homo sapiens (evidenced by care of the lame and burial of the dead), but evidence of more sophisticated beliefs, such as the early cave paintings (perhaps with magical or religious significance) did not appear until some 32,000 years (0.6 seconds) ago. By 11,000 years (0.2 seconds) ago, Homo sapiens had reached the tip of South America. Verbal skills and tool use continued to improve.

Civilization

Main article: History of the world

File:Mohenjodaro computergeneratedimage1.jpg

A computer-generated reconstruction of the city of Mohenjo-daro of the Indus Valley civilization, one of the ancient civilizations.

Throughout more than ninety percent of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers. As intelligence increased and language became more complex, the ability to remember and transmit information resulted in a new sort of replicator: the meme. Ideas could be rapidly exchanged and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Somewhere between 8500 and 7000 BCE (0.20 to 0.17 seconds ago), people in the Fertile Crescent in Mesopotamia began the systematic husbandry of plants and animals: agriculture (Tudge, 1998). This spread to neighboring regions, and also developed independently elsewhere. No longer nomadic, humans began to make permanent settlements. The relative security provided by farming allowed the population to expand. Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing division of labor. This led to Earth's first civilization at Sumer in the Middle East, between 4000 and 3000 BCE (around 0.10 seconds ago). Others quickly arose in ancient Egypt and the Indus River valley.

Starting around 3000 BCE (0.09 seconds ago on our clock), Hinduism, the oldest religion still practiced today, began to form. Others soon followed. The invention of writing enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival—curiosity and education drove the pursuit of knowledge and wisdom. Various disciplines, including science, arose. New civilizations sprang up, traded with one another, and engaged in war for territory and resources: empires began to form. By around 500 BCE (0.048 seconds ago), there were empires in the Middle East, India, China and Greece, approximately on equal footing; at times one empire expanded, only to decline or be driven back later.

Vitruvian Man by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.

In the 1300s (about 0.012 seconds ago), the Renaissance began in Italy with advances in religion, art and science. Starting around 1500 (0.0096 seconds ago), European civilization began to undergo changes that would eventually see its world domination, for reasons that are still debated. Explorers reached the American continent where they set up colonies and displaced the native inhabitants. Raw materials from the colonies accelerated Europe's progress. The scientific and industrial revolutions saw major advances in Europe: that continent exerted political and cultural dominance over humans around the planet. In 1776 (0.004 seconds ago), one of Europe's colonies rebelled and became a new country, calling itself the United States of America. From 1914 to 1918 (about 0.0017 seconds ago) and 1939 to 1945 (about 0.0012 seconds ago), nations around the world were embroiled in world wars. Created after World War I, the League of Nations was a first step toward a world government; after World War II it was replaced by the United Nations. The breakup of the Soviet Union in 1991 left the United States as the sole superpower; however, the following year, several European nations joined together in the European Union. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This globalization has often produced discord, although increased collaboration has resulted as well.

Recent events

After four and a half billion years, one of Earth's life forms broke free of the biosphere. For the first time in history, Earth was viewed from the vantage of space.

Change has continued at a breakneck pace in the last millisecond of our notional 24-hour period, from the mid 1950s to today. There is increasing awareness of the effect humans have on their environment, as well as of the necessary steps to limit or reverse damage; and there is increasing concern about mass extinction and global warming. Pessimists argue that it is too late to avert ecological catastrophe; optimists argue that the ever more-rapid progress of science and technology will provide solutions. Of all the recent scientific discoveries, genetic engineering may be the most significant. Humans can now directly modify the genetic material of other species, a process which until this point had been the exclusive province of nature. More than this: science has unravelled the very genetic code of Homo sapiens himself. Humans have also begun to take their first tentative steps off their home planet. In 1957, the U.S.S.R. launched the first artificial object to achieve orbit and, soon afterwards, Yuri Gagarin became the first human in space. Since 2000 there has been a continuous human presence in space aboard the International Space Station. Future developments can at best only be sketchily imagined today—the possible advances in mathematics, physics, chemistry, biology, electronics, and all the other disciplines which may permit the colonization of distant worlds.

See also

“A mote of dust suspended in a sunbeam”. Carl Sagan describes Earth, seen in this (cropped) photograph entitled “Pale Blue Dot” taken by Voyager 1 at a distance of 3.7 billion miles. Earth is a tiny dot about halfway down the streak of sunlight. (see full image)

• Timeline of the Big Bang
• Geologic timescale
• Timeline of evolution

References

• http://www.nasa.gov/worldbook/earth_worldbook.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_1.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_2.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_plan_4.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_chem.html
• http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_bio.html
• Dawkins, Richard. The Ancestor's Tale. Houghton Mifflin Company, Boston, 2004. ISBN 0618005838
• Dawkins, Richard. The Ancestor's Tale. Weidenfeld and Nicolson, London, 2004. ISBN 0297825038
• Fortey, Richard. Life: A Natural History of the First Four Billion Years of Life on Earth. Alfred A. Knopf, New York, 1998. ISBN 0375401199
• McNeill, William H. A World History (4th ed.). Oxford University Press, Oxford, 1999. ISBN 0195025547
• McNeill, William H. A World History (4th ed.). Oxford University Press, New York, 1999. ISBN 0195116151
• Tudge, Colin. Neanderthals, Bandits and Farmers—How Agriculture Really Began, Weidenfeld & Nicolson, London, 1998. ISBN 0297842587
• http://www.bbc.co.uk/religion/religions/hinduism/history/