Encyclopedia
The
history of Earth covers approximately 4.567 billion 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. The
Big Bang and origin of the
universe, estimated at occurring 13.7 billion years ago, is equivalent to taking place almost three days ago—two whole days before our clock began to tick.
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 and gas. It was composed of
hydrogen and
helium produced in the Big Bang, as well as heavier
elements
produced by
stars long gone. Then, about 4.6 billion years ago , 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 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.
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.
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 , 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. It may also have sped up Earth’s rotation and initiated the planet’s
plate tectonics.
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 . 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.
The surface cooled quickly, forming the solid crust within 150 million years . From 4 to 3.8 billion years ago , Earth underwent a period of heavy asteroidal bombardment. 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 , 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 entry,
ultraviolet radiation flooded the surface.
Beginnings of life
The details of the origin of life are unknown, though the broad principles have been established. A minority of scientists believes that life, or at least organic components, may have arrived on Earth from space ; the mechanisms by which life would initially arise are nevertheless believed to be similar to those of life with terrestrial origins. Most scientists believe that life arose on Earth, but the timing of this event is highly speculative—perhaps it arose around 4 billion years ago . Somehow, in the energetic chemistry of early Earth, a molecule 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 became depleted, strains which could exploit different materials, or perhaps halt the progress of other strains and steal their resources, became more numerous.
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, or even quantum systems. 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. 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 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 use DNA as their replicator, in an almost identical manner .
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 . It is not known whether this process preceded or succeeded the origin of the replicator . The prevailing theory is that the replicator, perhaps
RNA by this point , 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”. or even hot, deep rocks. 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 or earlier.
, 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.hotosynthesis and oxygen
It is likely that the initial cells were all heterotrophs, using surrounding organic molecules as raw material and 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. Estimates vary, but by about 3 billion years ago , 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 organic molecules .
Moreover, oxygen was produced as a waste product of photosynthesis. At first it became bound up with limestone, iron, and other minerals. There is substantial proof of this in iron-oxide rich layers in geological strata that correspond with this time period. The oceans would have turned to a green color while oxygen was reacting with minerals. When the reactions stopped, oxygen could finally enter 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 was stimulated by incoming ultraviolet radiation 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 .
See also
The geological time scale is used by geologist [i]s and other scientists to describe the timing and ...
References
External links
- — a detailed look at events from the origin of the universe to the present
- Valley, John W. “” Scientific American is a popular-science [i] magazine [i], published since August 28 [i]...
. 2005 Oct:58–65. – discusses the timing of the formation of the oceans and other major events in Earth’s early history. - Davies, Paul. “”. The Guardian is a British [i] newspaper [i] owned by the Guardian Media Group [i]. ...
. 2005 Dec 20. – discusses speculation into the role of quantum systems in the origin of life - . Animated story of life since about 13,700,000,000 shows everything from the big bang to the formation of the earth and the development of bacteria and other organisms to the ascent of man.
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