History of Earth
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- For the history of human beings on Earth, see “History of the world”.
The history of Earth covers approximately 4 billion years (4,567,000,000 years), from Earth’s 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 4.567 billion years ago, at the formation of Earth, and ending now. Each second of this period represents approximately 53,000 years (or 53 millennia). The Big Bang and origin of the universe, estimated at occurring 13.7 billion years ago,<ref name="NASA-WMAP">"New Image of Infant Universe Reveals Era of First Stars, Age of Cosmos, and More". NASA (February 11, 2003). Retrieved on 2006-03-26.</ref> is equivalent to taking place almost three days ago—two whole days before our clock began to tick.
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[edit] Origin
Earth formed as part of the birth of the solar system: what eventually became the solar system initially existed as a large, rotating cloud of dust, rocks, and gas. It was composed of hydrogen and helium produced in the Big Bang, as well as heavier elements ejected by supernova explosions. Then, about 4.6 billion years ago, it's assumed a nearby star became a supernova. The explosion sent a shock wave toward the solar nebula and caused it to implode. As the cloud continued to rotate, gravity and inertia flattened the cloud into a proto-planetary disc, perpendicular to its axis of rotation. Most of the mass concentrated in the middle and began to heat up. The impossibility of kinetic heat, produced by the infall of matter escaping caused the centre to heat up sufficiently to enable the centre of the concentration to produce its own internal heat source through nuclear fusion of hydrogen into helium, starting as a T Tauri star, our early sun. 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.<ref>Chaisson, Eric J. (2005). Solar System Modeling. Cosmic Evolution. Tufts University. Retrieved on 2006-03-27.</ref> These included one collection approximately 150 million kilometers from the center: Earth. As the Sun condensed and heated, fusion began, and the resulting T Tauri solar wind cleared out most of the material in the disc that had not already condensed into larger bodies.
[edit] Moon
The origin of the Moon is still uncertain, although 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 Lagrangian 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, an estimated 4.533 billion years ago (perhaps 12:05 a.m. on our clock),<ref>Münker, Carsten, Jörg A. Pfänder, Stefan Weyer, Anette Büchl, Thorsten Kleine, Klaus Mezger (July 4, 2003). "Evolution of Planetary Cores and the Earth-Moon System from Nb/Ta Systematics". Science 301 (5629): 84–87. DOI:10.1126/science.1084662.</ref> 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, this became a more spherical body: the Moon.<ref>Taylor, G. Jeffrey (April 26, 2004). Origin of the Earth and Moon. NASA. Retrieved on 2006-03-27.</ref> 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° with no recognizable seasons.) It may also have sped up Earth’s rotation and initiated the planet’s plate tectonics.
[edit] The Hadean eon
The early Earth, during the very early Hadean eon, 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, caused the planet at this stage to be fully molten. Heavier elements sank to the center while lighter ones rose to the surface, producing Earth's various layers (see "Structure of the Earth"). Earth's early atmosphere would have comprised surrounding material from the solar nebula, especially light gases such as hydrogen and helium, but the solar wind and Earth's own heat would have driven off this atmosphere.
This changed when Earth was about 40% its present radius, and gravitational attraction allowed the retention of an atmosphere which included water. Temperatures plummeted and the crust of the planet was accumulated on a solid surface, with areas melted by large impacts on the scale of decades to hundreds of years between impact. Large impacts would have caused localized melting and partial differentiation, with some lighter elements on the surface or released to the moist atmosphere. <ref name="Alfven1976">Alfvén, Hannes, Gustaf Arrhenius (1976). “ORIGIN OF THE EARTH'S OCEAN AND ATMOSPHERE”, Evolution of the Solar System. Washington, D.C.: National Aeronautics and Space Administration. Retrieved on 2006-08-22.</ref>
The surface cooled quickly, forming the solid crust within 150 million years (around 12:45 a.m. on our clock).<ref name="zircon">Wilde, Simon A., John W. Valley, William H. Peck, and Colin M. Graham (January 11, 2001). "Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago". Nature 409: 175-178. (PDF).</ref> From 4 to 3.8 billion years ago (around 3 to 4 a.m.), Earth underwent a period of heavy asteroidal bombardment.<ref name="space.com-bombardment">Britt, Robert Roy (2002-07-24). Evidence for Ancient Bombardment of Earth. Space.com. Retrieved on 2006-04-15.</ref> Steam escaped from the crust while more gases were released by volcanoes, completing the second atmosphere. Additional water was imported by bolide collisions, probably from asteroids ejected from the outer asteroid belt under the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain gave rise to the oceans within 750 million years (3.8 billion years ago, around 4:00 a.m. on our clock), but probably earlier. (Recent evidence suggests the oceans may have begun forming by 4.2 billion years ago-1:50 a.m. on our clock.)<ref name="Cavosie_etal_2005">Cavosie, A. J., J. W. Valley, S. A., Wilde, and E.I.M.F. (July 15, 2005). "Magmatic δ18O in 4400-3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean". Earth and Planetary Science Letters 235 (3-4): 663-681. DOI:10.1016/j.epsl.2005.04.028.</ref> 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 entry, ultraviolet radiation flooded the surface.
[edit] Beginnings of life
The details of the origin of life are unknown, though the broad principles have been established. It has been proposed that life, or at least organic components, may have arrived on Earth from space (see “Panspermia”), while others argue that terrestrial origins are more likely. The mechanisms by which life would initially arise are nevertheless held to be similar.<ref name="Scientific-American-panspermia">Warmflash, David, Benjamin Weiss (November 2005). "Did Life Come From Another World?". Scientific American: 64–71.</ref> If life arose on Earth, the timing of this event is highly speculative—perhaps it arose around 4 billion years ago (around 3:00 a.m. on our clock).<ref>Chaisson, Eric J. (2005). Chemical Evolution. Cosmic Evolution. Tufts University. Retrieved on 2006-03-27.</ref> Somehow, in the energetic chemistry of early Earth, a molecule (or even something else) gained the ability to make copies of itself–the replicator. The nature of this molecule is unknown, its function having long since been superseded by life’s current replicator, DNA. 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 raw materials (“food”) became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.<ref name="Dawkins-Ancestors-563">Dawkins, Richard (2004). “Canterbury”, The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company, 563–578. ISBN 0-618-00583-8.</ref>
Several different models have been proposed explaining how a replicator might have developed. Different replicators have been posited, including organic chemicals such as modern proteins of nucleic acids, phospholipids, crystals,<ref name="Dawkins-Watchmaker-150">Dawkins, Richard [1986] (1996). “Origins and miracles”, The Blind Watchmaker. New York: W. W. Norton & Company, 150–157. ISBN 0-393-31570-3.</ref> or even quantum systems.<ref name="Davies">Davies, Paul (October 6, 2005). "A quantum recipe for life". Nature 437 (7060): 819. (subscription required).</ref> There is currently no method of determining which of these models, if any, closely fits the origin of life on Earth. One of the older theories, and one which has been worked out in some detail, will serve as an example of how this might occur. The high energy from volcanoes, lightning, and ultraviolet radiation could help drive chemical reactions producing more complex molecules from simple compounds such as methane and ammonia.<ref name="Fortey-38">Fortey, Richard [1997] (September 1999). “Dust to Life”, Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Vintage Books, 38. ISBN 0-375-70261-X.</ref> 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.<ref name="Fortey-39">Fortey, Richard [1997] (September 1999). “Dust to Life”, Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Vintage Books, 39. ISBN 0-375-70261-X.</ref> 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 proper began. Other theories posit a different replicator. In any case, DNA took over the function of the replicator at some point; all known life (with the exception of some viruses and prions) use DNA as their replicator, in an almost identical manner (see genetic code).
[edit] The first cell
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 form a bilayer spontaneously when placed in water. Under certain conditions, many such spheres can be formed (see “The bubble theory”).<ref name="Fortey-40">Fortey, Richard [1997] (September 1999). “Dust to Life”, Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Vintage Books, 40. ISBN 0-375-70261-X.</ref> 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 (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 known as “black smokers”.<ref name="Fortey-42">Fortey, Richard [1997] (September 1999). “Dust to Life”, Life: A Natural History of the First Four Billion Years of Life on Earth. New York: Vintage Books, 42–44. ISBN 0-375-70261-X.</ref> or even hot, deep rocks.<ref name="Dawkins-Ancestors-580">Dawkins, Richard (2004). “Canterbury”, The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company, 580. ISBN 0-618-00583-8.</ref> However, it is believed that out of this multiplicity of cells, or protocells, only one survived. Current evidence suggests that the last universal common ancestor lived during the early Archean eon, perhaps roughly 3.5 billion years ago (5:30 a.m. on our imaginary clock) or earlier.<ref name="Penny-LUCA">Penny, David, Anthony Poole (December 1999). "The nature of the last universal common ancestor". Current Opinions in Genetics and Development 9 (6): 672–677. PMID 1060760. (PDF)</ref>,<ref name="Munster">Earliest Life. University of Münster (2003). Retrieved on 2006-03-28.</ref> This “LUCA” cell is the ancestor of all cells and hence all life on Earth. It was probably a prokaryote, possessing a cell membrane and probably ribosomes, but lacking a nucleus or membrane-bound organelles such as 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. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes in lateral gene transfer.<ref name="Penny-LUCA" />
[edit] Photosynthesis and oxygen
It is likely that the initial cells were all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and an energy source.<ref name="Dawkins-Ancestors-564">Dawkins, Richard (2004). “Canterbury”, The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company, 564–566. ISBN 0-618-00583-8.</ref> 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. Estimates vary, but by about 3 billion years ago<ref name="De-Marais-photosynthesis">De Marais, David J. (September 8, 2000). "Evolution: When Did Photosynthesis Emerge on Earth?". Science 289 (5485): 1703–1705. PMID 11001737. (full text)</ref> (around 8:00 a.m. on our clock), something similar to modern photosynthesis had probably developed. 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 o