History of Earth
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 of this period, summarizing the leading scientific theories for each time period. Due to the difficulty of grasping very large amounts of time, an analogy to a clock will be used, with midnight beginning at the formation of Earth and the next midnight occuring now. Each second on this "clock" represents approximately 53,000 years, and the Big Bang and origin of the universe took place almost three days ago, two days before the first "midnight".
Origin
The formation of Earth occurred as part of the formation of the solar system. What eventually would become the solar system initally existed as a large rotating penis 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 (that's fifteen to thirty minutes before midnight on our imaginary clock) a nearby star probably exploded as a supernova. This sent a shock wave towards the solar nebula, causing it to contract. As the cloud continued rotating, 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, the rest of the disc started to break up into rings as gravity caused matter to condense around dust particles. Small fragments collided to become larger fragments, including one collection approximately 150 million kilometers from the center, named Earth by one of the life forms that later arose on its surface. As the Sun condensed and heated, fusion initiated and the solar wind cleared out most of the material in the disc that had not condensed into larger bodies.
Moon
The origin of the Moon is still uncertain, although considerable evidence exists for the giant impact hypothesis. Earth was probably not the only planet forming 150 million kilometers from the Sun. It is hypothesized that another collection occurred 150 million kilometers from 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 have initially stable, but destabilized as Earth increased its mass. It swung back and forth, relative to Earth, until finally, 4.553×109 years ago (perhaps 12:33 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 weeks; it would become a more uniform, spherical body probably within a year. The impact also probably knocked the 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 origins of Earth's plate tectonics.
Early days: Hadean eon
The early Earth was very different than the Earth of 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, combined with heat from radioactive breakdown, residual heat, and heat from the pressure of contraction likely caused the Earth to be initially fully molten. Heavier elements sunk to the center, while lighter ones rose to the surface, giving rise to Earth's various layers (see Structure of the Earth). The initial atmosphere would have been surrounding material from the solar nebula, mainly light gases like hydrogen and helium. What atmosphere was there was driven off by the solar wind and Earth's heat. The surface slowly cooled, forming the solid crust probably within 200 million years (around 1 a.m. on our clock). Steam escaped from the crust, and more gases were released by volcanos, giving rise to the atmosphere. Additional water was brought by meteorite and comet collisions. Earth cooled, clouds formed, and 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, Earth's second 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 high, and without an ozone layer, large amounts of ultraviolet radiation rained on the surface.
Beginnings of life
The high energy from volcanos, lightning, and ultraviolet radiation helped drive chemical reactions producing more complex molecules from simple molecules like methane and ammonia. Among these were many of the simple organic compounds that are the building blocks of life. As the amount of this "organic soup" increased, different molecules would react with one another, essentially randomly. Sometimes more complex molecules would result—perhaps clay provided a framework to collect and concentrate organic material. Some of the molecules could help speed up a chemical reaction. This continued for a very long time, with reactions occurring more or less randomly, until by chance there arose a truly bizarre molecule: the replicator. This molecule had the property of promoting the chemical reactions which produced a copy of itself, and evolution was born. The nature of the first replicator is unknown—it may have been similar to our current replicator DNA, or perhaps it was phospholipid or even crystalline. The timing of this is highly speculative as well—perhaps it occurred around 4×109 years ago (around 3 a.m. on our hypothetical clock). Although the nature of this molecule and the details of these events are unknown, the broad principles have been reasonably well established. The replicator continued making copies of itself, but occasionally (or often) a copy would have an error and not precisely match the original. If the change destroyed the copying ability of the molecule, there would be no more copies and the line would "die out". Some changes might make the molecule replicate faster or more efficiently, and those "strains" might become more numerous. As choice building blocks ("food") in the organic soup became depleted, strains which could use different materials or perhaps block other strains became more numerous.
The first cell
Of course, 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 uncertain 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 later that assisted in an orderly division would promote proliferation of those cell lines. RNA is a likely candidate for an early replicator, since it can both store genetic information as well as catalyze reactions; however, 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 population 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 cell is the likely ancestor of all cells and all life on Earth. It would be what we now call a prokaryote, possessing a cell membrane and ribosomes, but no 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 protein enzymes to catalyze reactions.
Photosynthesis and oxygen
The initial cells were likely all heterotrophs, using surrounding organic molecules (including those from other cells) as raw material and as an energy source. As the inital group of cells multiplied, they continued to consume the organic material. As the food supply diminished, a new strategy evolved in some organisms. These cells used sunlight as an energy source instead of relying on the diminishing amounts of free-existing organic molecules. Eventually, by about 3×109 years ago (around 8 a.m. on the imaginary clock), something similar to modern photosynthesis had evolved. This harnessessing of the sun's energy made it available not only to autotrophs but also to heterotrophs who 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). Oxygen was produced as a waste product. At first, it combined with iron and other minerals. Once the available minerals were bound, oxygen slowly began to accumulate in the atmosphere. Although each cell only produced a minute amount of oxygen, the combined metabolism of them over a long period of time eventually transformed Earth's atmosphere to its current state, Earth's third atmosphere. Some of the oxygen reacted to form ozone, which collected in a layer near the upper part of the atmosphere. This layer absorbed a significant amount of the ultraviolet radiation that previously passed through the atmosphere. This would allow cells to colonize the surface of the ocean and eventually the land, since without the ozone layer, the ultraviolet radiation reaching the surface would cause significant and probably lethal mutations in any cell present. Finally, in addition to making large amounts of energy available for life, and to blocking ultraviolet radiation, photosynthesis had a third major, world-changing effect. Oxygen was toxic; probably much life on Earth died out as the oxygen lives rose. Those that resisted it survived, and thrived. Some developed the ability to use oxygen to greatly increase the efficiency of their metabolism, deriving more energy from the same food.
Three domains and endosymbiosis
Modern taxonomy classifies all life into three domains. The nature and time of the origin of these domains are highly speculative. First, the Bacteria domain probably split off from the other forms of life, sometimes called Neomura (this is controversial). Soon after this, perhaps 2×109 years ago (around 2 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 recently coming to light. Somewhere 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 far more energy. Some of this surplus energy was provided to the host. The smaller cell replicated inside the larger one, and soon a stable symbiotic relationship between them developed. Over time, the host cell took over some of the genes of the smaller cells, and the two became dependent on each other—the larger cell could not survive without the large amounts of energy produced by the smaller ones, and they could not survive without the raw materials provided by the larger. Such synchrony developed between the larger cell and the population of smaller cells inside that they are considered to be a single organism, with the smaller cells being classified as organelles named mitochondria. A similar event took place with photosynthetic cyanobacteria entering larger heterotrophic cells and eventually becoming chloroplasts. There were probably several inclusion events, as the figure to the right suggests. In addition to the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, it as 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; however, none of these is generally accepted.
See also
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
- Fortey, Richard. Life: A Natural History of the First Four Billion Years of Life on Earth. Vintage Books: New York, 1997.
- http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_chem.html
- Dawkins, Richard. The Ancestor's Tale. Houghton Mifflin Company: Boston, 2004.
- http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/fr_1/fr_1_bio.html