I have written several articles the environment. A list of links have been provided at bottom of this article for your convenience. This article will, however address different aspects on the environment and the planet in general.
It is theorized that the true age of the earth is about 4.6 billion years old and was formed at about the same time as the rest of our solar system. Irregardless of how old our planet is, nobody can deny that our creation was nothing short of a miracle. I am going to tackle a herculean task and cover the creation of our planet and cover our natural history to the present day. I am doing this to show how rare the creation of a planet like earth is with our advanced life. I want people to cherish this gift. Because that is what it is, a gift. We have been treating it like our personal play thing, with no repercussions resulting from our actions.
The Story Begins Long Before Earth Existed
Earth was not around at the beginning of the universe. In fact, very little of what we see in the cosmos today was around when the universe formed some 13.8 billion years ago. However, to get to Earth, it’s important to start at the beginning, when the universe was young.
It all started out with only two elements: hydrogen and helium, and a small trace of lithium. The first stars formed out of the hydrogen that existed. Once that process started, generations of stars were born in clouds of gas. As they aged, those stars created heavier elements in their cores, elements such as oxygen, silicon, iron, and others. When the first generations of stars died, they scattered those elements to space, which seeded the next generation of stars. Around some of those stars, the heavier elements formed planets.
The Birth of the Solar System Gets a Kick-start
Some five billion years ago, in a perfectly ordinary place in the galaxy, something happened. It might have been a supernova explosion pushing a lot of its heavy-element wreckage into a nearby cloud of hydrogen gas and interstellar dust. Or, it could have been the action of a passing star stirring up the cloud into a swirling mixture. Whatever the kick-start was, it pushed the cloud into action which eventually resulted in the birth of the solar system. The mixture grew hot and compressed under its own gravity. At its center, a protostellar object formed. It was young, hot, and glowing, but not yet a full star. Around it swirled a disk of the same material, which grew hotter and hotter as gravity and motion compressed the dust and rocks of the cloud together.
The hot young protostar eventually “turned on” and began to fuse hydrogen to helium in its core. The Sun was born. The swirling hot disk was the cradle where Earth and its sister planets formed. It wasn’t the first time such a planetary system was formed. In fact, astronomers can see just this sort of thing happening elsewhere in the universe.
While the Sun grew in size and energy, beginning to ignite its nuclear fires, the hot disk slowly cooled. This took millions of years. During that time, the components of the disk began to freeze out into small dust-sized grains. Iron metal and compounds of silicon, magnesium, aluminum, and oxygen came out first in that fiery setting. Bits of these are preserved in chondrite meteorites, which are ancient materials from the solar nebula. Slowly these grains settled together and collected into clumps, then chunks, then boulders, and finally bodies called planetesimals large enough to exert their own gravity.
The standard model for the formation of the Solar System (including the Earth) is the solar nebula hypothesis. In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of hydrogen and helium created shortly after the Big Bang 13.8 Ga (billion years ago) and heavier elements ejected by supernovae. About 4.5 Ga, the nebula began a contraction that may have been triggered by the shock wave from a nearby supernova. A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its angular momentum, gravity, and inertia flattened it into a protoplanetary disk perpendicular to its axis of rotation. Small perturbations due to collisions and the angular momentum of other large debris created the means by which kilometer-sized protoplanets began to form, orbiting the nebular center.
The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until nuclear fusion of hydrogen into helium began. After more contraction, a T Tauri star ignited and evolved into the Sun. Meanwhile, in the outer part of the nebula gravity caused matter to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway accretion, successively larger fragments of dust and debris clumped together to form planets. Earth formed in this manner about 4.54 billion years ago (with an uncertainty of 1%) and was largely completed within 10–20 million years. The solar wind of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets.
Earth Is Born in Fiery Collisions
As time went by, planetesimals collided with other bodies and grew larger. As they did, the energy of each collision was tremendous. By the time they reached a hundred kilometers or so in size, planetesimal collisions were energetic enough to melt and vaporize much of the material involved. The rocks, iron, and other metals in these colliding worlds sorted themselves into layers. The dense iron settled in the center and the lighter rock separated into a mantle around the iron, in a miniature of Earth and the other inner planets today. Planetary scientists call this settling process differentiation. It didn’t just happen with planets, but also occurred within the larger moons andthe largest asteroids. The iron meteorites that plunge to Earth from time to time come from collisions between these asteroids in the distant past.
At some point during this time, the Sun ignited. Although the Sun was only about two-thirds as bright as it is today, the process of ignition (the so-called T-Tauri phase) was energetic enough to blow away most of the gaseous part of the protoplanetary disk. The chunks, boulders, and planetesimals left behind continued to collect into a handful of large, stable bodies in well-spaced orbits. Earth was the third one of these, counting outward from the Sun. The process of accumulation and collision was violent and spectacular because the smaller pieces left huge craters on the larger ones. Studies of the other planets show these impacts and the evidence is strong that they contributed to catastrophic conditions on the infant Earth.
At one point early in this process a very large planetesimal struck Earth an off-center blow and sprayed much of the young Earth’s rocky mantle into space. The planet got most of it back after a period of time, but some of it collected into a second planetesimal circling Earth. Those leftovers are thought to have been part of the Moon’s formation story.
Volcanoes, Mountains, Tectonic Plates, and an Evolving Earth
The oldest surviving rocks on Earth were laid down some five hundred million years after the planet first formed. It and other planets suffered through what’s called the “late heavy bombardment” of the last stray planetesimals around four billion years ago). The ancient rocks have been dated by the uranium-lead method and appear to be about 4.03 billion years old. Their mineral content and embedded gases show that there were volcanoes, continents, mountain ranges, oceans, and crustal plates on Earth in those days.
Hadean and Archean Eons
The first eon in Earth’s history, the Hadean, begins with the Earth’s formation and is followed by the Archean eon at 3.8 Ga. The oldest rocks found on Earth date to about 4.0 Ga, and the oldest detrital zircon crystals in rocks to about 4.4 Ga, soon after the formation of the Earth’s crust and the Earth itself. The giant impact hypothesis for the Moon’s formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the mantle and crust into space and created the Moon.
From crater counts on other celestial bodies, it is inferred that a period of intense meteorite impacts, called the Late Heavy Bombardment, began about 4.1 Ga, and concluded around 3.8 Ga, at the end of the Hadean. In addition, volcanism was severe due to the large heat flow and geothermal gradient. Nevertheless, detrital zircon crystals dated to 4.4 Ga show evidence of having undergone contact with liquid water, suggesting that the Earth already had oceans or seas at that time.
By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not have survived at Earth’s surface, because the Archean atmosphere lacked oxygen hence had no ozone layer to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate fossils dated to around 3.5 Ga. Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4 Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth’s surface.
Some slightly younger rocks (about 3.8 billion years old) show tantalizing evidence of life on the young planet. While the eons that followed were full of strange stories and far-reaching changes, by the time the first life did appear, Earth’s structure was well-formed and only its primordial atmosphere was being changed by the onset of life. The stage was set for the formation and spread of tiny microbes across the planet. Their evolution ultimately resulted in the modern life-bearing world still filled with mountains, oceans, and volcanoes that we know today. It’s a world that is constantly changing, with regions where continents are pulling apart and other places where new land is being formed. These actions affect not just the planet, but life on it.
Formation of the Moon
Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the Earth) and a small metallic core. Second, there is virtually no water or other volatiles on the Moon. Third, the Earth and Moon have the same oxygen isotopic signature (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: The giant impact hypothesis proposes that the Moon originated after a body the size of Mars (sometimes named Theia) struck the proto-Earth a glancing blow.
The collision released about 100 million times more energy than the more recent Chicxulub impact that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of the Earth’s outer layers and melt both bodies. A portion of the mantle material was ejected into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material, explaining its abnormal composition. The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.
The evidence for the story of Earth’s formation and evolution is the result of patient evidence-collecting from meteorites and studies of the geology of the other planets. It also comes from analyses of very large bodies of geochemical data, astronomical studies of planet-forming regions around other stars, and decades of serious discussion among astronomers, geologists, planetary scientists, chemists, and biologists. The story of Earth is one of the most fascinating and complex scientific stories around, with plenty of evidence and understanding to back it up.
Mantle convection, the process that drives plate tectonics, is a result of heat flow from the Earth’s interior to the Earth’s surface. It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by subduction into the mantle at subduction zones. During the early Archean (about 3.0 Ga) the mantle was much hotter than today, probably around 1,600 °C (2,910 °F), so convection in the mantle was faster. Although a process similar to present-day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.
The initial crust, formed when the Earth’s surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was basaltic in composition, like today’s oceanic crust, because little crustal differentiation had yet taken place. The first larger pieces of continental crust, which is a product of differentiation of lighter elements during partial melting in the lower crust, appeared at the end of the Hadean, about 4.0 Ga. What is left of these first small continents are called cratons. These pieces of late Hadean and early Archean crust form the cores around which today’s continents grew.
The oldest rocks on Earth are found in the North American craton of Canada. They are tonalites from about 4.0 Ga. They show traces of metamorphism by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then. Cratons consist primarily of two alternating types of terranes. The first are so-called greenstone belts, consisting of low-grade metamorphosed sedimentary rocks. These “greenstones” are similar to the sediments today found in oceanic trenches, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of felsic magmatic rocks. These rocks are mostly tonalite, trondhjemite or granodiorite, types of rock similar in composition to granite (hence such terranes are called TTG-terranes). TTG-complexes are seen as the relicts of the first continental crust, formed by partial melting in basalt.
Oceans and atmosphere
Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth’s heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances. After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released by volcanoes, completing a second atmosphere rich in greenhouse gases but poor in oxygen. Finally, the third atmosphere, rich in oxygen, emerged when bacteria began to produce oxygen about 2.8 Ga.
In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of volatiles from the Earth’s interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as impact degassing in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as the Earth formed. The new atmosphere probably contained water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.
Planetesimals at a distance of 1 astronomical unit (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long. The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5 AU. Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than Neptune, computer simulations show that they were originally far more common in the inner parts of the Solar System.
As the Earth cooled, clouds formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4 Ga. By the start of the Archean eon, they already covered much of the Earth. This early formation has been difficult to explain because of a problem known as the faint young Sun paradox. Stars are known to get brighter as they age, and at the time of its formation the Sun would have been emitting only 70% of its current power. Thus, the Sun has become 30% brighter in the last 4.5 billion years. Many models indicate that the Earth would have been covered in ice. A likely solution is that there was enough carbon dioxide and methane to produce a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. Another greenhouse gas, ammonia, would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.
Evolution of Life:
The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago, during the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean Eon. There are microbial mat fossils such as stromatolites found in 3.48 billion-year-old sandstone discovered in Western Australia. Other early physical evidence of a biogenic substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland as well as “remains of biotic life” found in 4.1 billion-year-old rocks in Western Australia. According to one of the researchers, “If life arose relatively quickly on Earth … then it could be common in the universe.”
Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion, have gone extinct. Estimates on the number of Earth’s current species range from 10 million to 14 million, of which about 1.2 million are documented, but over 86 percent have not been described. However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of one percent described. Now that I have given a brief introduction to the creation of life, I will cover it more in depth.
One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism.
The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life. An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning. Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules. Computer simulations show that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth.
Additional complexity could have been reached from at least three possible starting points: self-replication, an organism’s ability to produce offspring that are similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances.
Replication first: RNA world
Even the simplest members of the three modern domains of life use DNA to record their “recipes” and a complex array of RNA and protein molecules to “read” these instructions and use them for growth, maintenance, and self-replication.
The discovery that a kind of RNA molecule called a ribozyme can catalyze both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA. They could have formed an RNA world in which there were individuals but no species, as mutations and horizontal gene transfers would have meant that the offspring in each generation were quite likely to have different genomes from those that their parents started with. RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have. Ribozymes remain as the main components of ribosomes, the “protein factories” of modern cells.
Although short, self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest ribozymes may have been formed of simpler nucleic acids such as PNA, TNA or GNA, which would have been replaced later by RNA. Other pre-RNA replicators have been posited, including crystals and even quantum systems.
In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about 100 °C (212 °F) and at ocean-bottom pressures near hydrothermal vents. In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes.
Metabolism first: iron–sulfur world
Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of proteins, are easily synthesized in plausible prebiotic conditions, as are small peptides (polymers of amino acids) that make good catalysts. A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence of carbon monoxide and hydrogen sulfide with iron sulfide and nickel sulfide as catalysts. Most of the steps in their assembly required temperatures of about 100 °C (212 °F) and moderate pressures, although one stage required 250 °C (482 °F) and a pressure equivalent to that found under 7 kilometers (4.3 mi) of rock. Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents.
A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have “compositional genomes” (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection.
Membranes first: Lipid world
It has been suggested that double-walled “bubbles” of lipids like those that form the external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form liposomes, double-walled “bubbles”, and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to natural selection for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.
The clay theory
Some clays, notably montmorillonite, have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their crystalline pattern, are subject to an analog of natural selection (as the clay “species” that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters.
Research in 2003 reported that montmorillonite could also accelerate the conversion of fatty acids into “bubbles”, and that the bubbles could encapsulate RNA attached to the clay. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest cells may have been aided by similar processes.
A similar hypothesis presents self-replicating iron-rich clays as the progenitors of nucleotides, lipids and amino acids.
Last universal ancestor
It is believed that of this multiplicity of protocells, only one line survived. Current phylogenetic evidence suggests that the last universal ancestor (LUA) lived during the early Archean eon, perhaps 3.5 Ga or earlier. This LUA cell is the ancestor of all life on Earth today. 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 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 by lateral gene transfer.
The earliest cells absorbed energy and food from the surrounding environment. They used fermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an anaerobic (oxygen-free) environment. The evolution of photosynthesis made it possible for cells to derive energy from the Sun.
Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product. Some organisms, including purple bacteria and green sulfur bacteria, use an anoxygenic form of photosynthesis that uses alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such extremophile organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents.
The simpler anoxygenic form arose about 3.8 Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4 Ga, but some researchers put it back as far as 3.2 Ga. The latter “probably increased global productivity by at least two or three orders of magnitude” Among the oldest remnants of oxygen-producing lifeforms are fossil stromatolites.
At first, the released oxygen was bound up with limestone, iron, and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the Siderian period (between 2500 Ma and 2300 Ma). When most of the exposed readily reacting minerals were oxidized, oxygen finally 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 time transformed Earth’s atmosphere to its current state. This was Earth’s third atmosphere.
Some oxygen was stimulated by solar 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 eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.
Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the oxygen catastrophe. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food.
Emergence of eukaryotes
Modern taxonomy classifies life into three domains. The time of their origin is uncertain. The Bacteria domain probably first split off from the other forms of life (sometimes called Neomura), but this supposition is controversial. Soon after this, by 2 Ga, 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 becoming known. The earliest fossils possessing features typical of fungi date to the Paleoproterozoic era, some 2.4 ago; these multicellular benthic organisms had filamentous structures capable of anastomosis.
Around this time, the first proto-mitochondrion was formed. A bacterial cell related to today’s Rickettsia, which had evolved to metabolize oxygen, entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to the evolution of prey defenses). The smaller cell may have tried to parasitize the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it metabolized the larger cell’s waste products and derived more energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable symbiosis developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some genes from 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. The whole cell is now considered a single organism, and the smaller cells are classified as organelles called mitochondria.
A similar event occurred with photosynthetic cyanobacteria entering large 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 more than 1 billion years ago. There were probably several such inclusion events. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes, spirochetes led to cilia and flagella, and that perhaps a DNA virus led to the cell nucleus, though none of them are widely accepted.
Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Each domain repeatedly split into multiple lineages, although little is known about the history of the archaea and bacteria. Around 1.1 Ga, the supercontinent Rodinia was assembling. The plant, animal, and fungi lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually a division of labor began to take place; for instance, cells on the periphery might have started to 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 1 billion years ago, the first multicellular plants emerged, probably green algae. Possibly by around 900 Ma true multicellularity had also evolved in animals.
At first, it probably resembled today’s sponges, which have totipotent cells that allow a disrupted organism to reassemble itself. As the division of labor was completed in all lines of multicellular organisms, cells became more specialized and more dependent on each other; isolated cells would die.
The rate of the evolution of life as recorded by fossils accelerated in the Cambrian period (542–488 Ma). The sudden emergence of many new species, phyla, and forms in this period is called the Cambrian Explosion. The biological fomenting in the Cambrian Explosion was unprecedented before and since that time. Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. The development of hard body parts such as shells, skeletons or exoskeletons in animals like molluscs, echinoderms, crinoids and arthropods (a well-known group of arthropods from the lower Paleozoic are the trilobites) made the preservation and fossilization of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian than about that of older periods. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples are Anomalocaris and Haikouichthys. More recently, however, these seem to have found a place in modern classification.
During the Cambrian, the first vertebrate animals, among them the first fishes, had appeared.A creature that could have been the ancestor of the fishes, or was probably closely related to it, was Pikaia. It had a primitive notochord, a structure that could have developed into a vertebral column later. The first fishes with jaws (Gnathostomata) appeared during the next geological period, the Ordovician. The colonisation of new niches resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm Dunkleosteus, which could grow 7 meters (23 ft) long.
The diversity of life forms did not increase greatly because of a series of mass extinctions that define widespread biostratigraphic units called biomeres. After each extinction pulse, the continental shelf regions were repopulated by similar life forms that may have been evolving slowly elsewhere. By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.
Colonization of land
Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun’s ultraviolet radiation, meaning 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. Prokaryote lineages had probably colonized the land as early as 2.6 Ga even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later. Fish, the earliest vertebrates, evolved in the oceans around 530 Ma. A major extinction event occurred near the end of the Cambrian period, which ended 488 Ma.
Several hundred million years ago, plants (probably resembling algae) and fungi started growing at the edges of the water, and then out of it. The oldest fossils of land fungi and plants date to 480–460 Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000 Ma and the plants 700 Ma. 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 not precisely known: the oldest clear evidence is of arthropods on land around 450 Ma, perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.
Evolution of tetrapods
At the end of the Ordovician period, 443 Ma, additional extinction events occurred, perhaps due to a concurrent ice age. Around 380 to 375 Ma, the first tetrapods evolved from fish. Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water. They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the amphibians. About 365 Ma, another period of extinction occurred, perhaps as a result of global cooling. Plants evolved seeds, which dramatically accelerated their spread on land, around this time (by approximately 360 Ma).
About 20 million years later (340 Ma), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of amniotes from amphibians. Another 30 million years (310 Ma) saw the divergence of the synapsids (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details.
After yet another, the most severe extinction of the period, around 230 Ma, dinosaurs split off from their reptilian ancestors. The Triassic–Jurassic extinction event at 200 Ma spared many of the dinosaurs, and they soon became dominant among the vertebrates. Though some mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews. The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx, traditionally considered one of the first birds, lived around 150 Ma. The earliest evidence for the angiosperms evolving flowers is during the Cretaceous period, some 20 million years later (132 Ma).
The first of five great mass extinctions was the Ordovician-Silurian extinction. Its possible cause was the intense glaciation of Gondwana, which eventually led to a snowball earth. 60% of marine invertebrates became extinct and 25% of all families.
The second mass extinction was the Late Devonian extinction, probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like CO2) or the eutrophication of water. 70% of all species became extinct.
The third mass extinction was the Permian-Triassic, or the Great Dying, event was possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, methane hydrate gasification, sea level fluctuations, and a major anoxic event. Either the proposed Wilkes Land crater in Antarctica or Bedout structure off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether either these or other proposed Permian-Triassic boundary craters are either real impact craters or even contemporaneous with the Permian-Triassic extinction event. This was by far the deadliest extinction ever, with about 57% of all families and 83% of all genera killed.
The fifth and most recent mass extinction was the K-T extinction. In 66 Ma, a 10-kilometer (6.2 mi) asteroid struck Earth just off the Yucatán Peninsula—somewhere in the southwestern tip of then Laurasia—where the Chicxulub crater is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of all life, including the non-avian dinosaurs, became extinct, marking the end of the Cretaceous period and Mesozoic era
Diversification of mammals
The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that filled the world by the late Triassic. The first mammals were very small, and were probably nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-Paleogene extinction event. By the early Paleocene the earth recovered from the extinction, and mammalian diversity increased. Creatures like Ambulocetus took to the oceans to eventually evolve into whales, whereas some creatures, like primates, took to the trees. This all changed during the mid to late Eocene when the circum-Antarctic current formed between Antarctica and Australia which disrupted weather patterns on a global scale. Grassless savanna began to predominate much of the landscape, and mammals such as Andrewsarchus rose up to become the largest known terrestrial predatory mammal ever, and early whales like Basilosaurus took control of the seas.
The evolution of grass brought a remarkable change to the Earth’s landscape, and the new open spaces created pushed mammals to get bigger and bigger. Grass started to expand in the Miocene, and the Miocene is where many modern- day mammals first appeared. Giant ungulates like Paraceratherium and Deinotherium evolved to rule the grasslands. The evolution of grass also brought primates down from the trees, and started human evolution. The first big cats evolved during this time as well. The Tethys Sea was closed off by the collision of Africa and Europe.
The formation of Panama was perhaps the most important geological event to occur in the last 60 million years. Atlantic and Pacific currents were closed off from each other, which caused the formation of the Gulf Stream, which made Europe warmer. The land bridge allowed the isolated creatures of South America to migrate over to North America, and vice versa. Various species migrated south, leading to the presence in South America of llamas, the spectacled bear, kinkajous and jaguars.
Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climactic changes due to the ice ages. The ice ages led to the evolution of modern man in Saharan Africa and expansion. The mega-fauna that dominated fed on grasslands that, by now, had taken over much of the subtropical world. The large amounts of water held in the ice allowed for various bodies of water to shrink and sometimes disappear such as the North Sea and the Bering Strait. It is believed by many that a huge migration took place along Beringia which is why, today, there are camels (which evolved and became extinct in North America), horses (which evolved and became extinct in North America), and Native Americans. The ending of the last ice age coincided with the expansion of man, along with a massive die out of ice age mega-fauna. This extinction is nicknamed “the Sixth Extinction“.
It is more difficult to establish the origin of language; it is unclear whether Homo erectus could speak or if that capability had not begun until Homo sapiens. As brain size increased, babies were born earlier, before their heads grew too large to pass through the pelvis. As a result, they exhibited more plasticity, and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development. Modern humans (Homo sapiens) are believed to have originated around 200,000 years ago or earlier in Africa; the oldest fossils date back to around 160,000 years ago.
The first humans to show signs of spirituality are the Neanderthals (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools. However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance) did not appear until 32,000 years ago. Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief. By 11,000 years ago, Homo sapiens had reached the southern tip of South America, the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD). Tool use and communication continued to improve, and interpersonal relationships became more intricate.
Throughout more than 90% of its history, Homo sapiens lived in small bands as nomadic hunter-gatherers. As language became more complex, the ability to remember and communicate information resulted, according to a theory proposed by Richard Dawkins, in a new replicator: the meme. Ideas could be exchanged quickly and passed down the generations. Cultural evolution quickly outpaced biological evolution, and history proper began. Between 8500 and 7000 BC, humans in the Fertile Crescent in the Middle East began the systematic husbandry of plants and animals: agriculture. This spread to neighboring regions, and developed independently elsewhere, until most Homo sapiens lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as Australia. However, among those civilizations that did adopt agriculture, the relative stability and increased productivity 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 BC. Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China. 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, enabling the first specialized occupations (e.g. craftsmen, merchants, priests, etc.). Curiosity and education drove the pursuit of knowledge and wisdom, and various disciplines, including science (in a primitive form), arose. This in turn led to the emergence of increasingly larger and more complex civilizations, such as the first empires, which at times traded with one another, or fought for territory and resources.
By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline. In 221 BC, China became a single polity that would grow to spread its culture throughout East Asia, and it has remained the most populous nation in the world. The fundamentals of Western civilization were largely shaped in Ancient Greece, with the world’s first democratic government and major advances in philosophy, science, and mathematics, and in Ancient Rome in law, government, and engineering. The Roman Empire was Christianized by Emperor Constantine in the early 4th century and declined by the end of the 5th. Beginning with the 7th century, Christianization of Europe began. In 610, Islam was founded and quickly became the dominant religion in Western Asia. The House of Wisdom was established in Abbasid-era Baghdad, Iraq. It is considered to have been a major intellectual center during the Islamic Golden Age, where Muslim scholars in Baghdad and Cairo flourished from the ninth to the thirteenth centuries until the Mongol sack of Baghdad in 1258 AD. In 1054 AD the Great Schism between the Roman Catholic Church and the Eastern Orthodox Church led to the prominent cultural differences between Western and Eastern Europe. I won’t cover any more of our history, because I will quote a famous radio icon Paul Harvey, “And now you know the rest of the story!”
I did it I covered the natural history of the world. Thanks to the wonderful contributors and editors of Wikipedia, a lot of the work was done for me. Since all this material was already available already, why did I feel that I needed to cover it again? Good question. I briefly stated the reason for this article at the beginning. I know it seemed like forever. Firstly by righting these articles I am trying to distill a massive amount of information and condense it down to a more succinct and manageable form. I have done this for sharks, and slavery to name a few. By covering such a broad subject matter and condensing it down like I do, it helps my reader become fairly knowledgeable on particular subjects. Also, I make no qualms about copying and pasting portions of articles, because I am not not trying to make any money from these articles, nor am I trying to get published or get any degrees. I have enough already, and I have a good job already. This blog allows me to get my word out and help give people a truly accurate picture of the events that are currently occurring in this country. I do give credit in my resource sections. If there is no resource sections in my article, I simply pulled the information from my head.
So back to this article. I wrote this article to show what is involved in the creation of a planet with intelligent life. It is simply amazing, literally millions of events had to come into place for us to be here living our lives. We tend to forget how lucky we truly are. We need to treat this planet with more care. We are the dominant species of this planet, it is up to us to leave it better off than before we were born. We need to be husbands of this our mother earth. We owe it to all the lesser species , many who have been here hundreds of millions of years before we emerged from the primordial ooze. Case in point the shark.
What truly scares me is the prospect that we actually may leave this planet and infect another unsuspecting world. That day may be nearing, as we make firm plans on making a manned flight to Mars. In a recent article I discussed UFOs and the odds of intelligent life from across the universe coming to visit us. If they do I hope they are more humane to us than we have been to our fellow human beings.
thoughtco.com,”The Birth of Earth, The Story of Our Planet’s Formation,”By Andrew Alden; en.wikipedia.org, “History of Earth,” By Wikipedia Editors;
In geochronology, time is generally measured in mya (million years ago), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four great eons, starting 4,540 mya with the formation of the planet. Each eon saw the most significant changes in Earth’s composition, climate and life. Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs.
|Hadean||4,540–4,000||The Earth is formed out of debris around the solar protoplanetary disk. There is no life. Temperatures are extremely hot, with frequent volcanic activity and hellish-looking environments (hence the eon’s name, which comes from Hades). The atmosphere is nebular. Possible early oceans or bodies of liquid water. The Moon is formed around this time probably due to a protoplanet’s collision into Earth.|
|Archean||4,000–2,500||Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process known as abiogenesis. The continents of Ur, Vaalbara and Kenorland may have existed around this time. The atmosphere is composed of volcanic and greenhouse gases.|
|Proterozoic||2,500–541||The name of this eon means “early life”. Eukaryotes, a more complex form of life, emerge, including some forms of multicellular organisms. Bacteria begin producing oxygen, shaping the third and current of Earth’s atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time. The early and late phases of this eon may have undergone “Snowball Earth” periods, in which all of the planet suffered below-zero temperatures. The early continents of Columbia, Rodinia and Pannotia, in that order, may have existed in this eon.|
|Phanerozoic||541–present||Complex life, including vertebrates, begin to dominate the Earth’s ocean in a process known as the Cambrian explosion. Pangaea forms and later dissolves into Laurasia and Gondwana, which in turn dissolve into the current continents. Gradually, life expands to land and familiar forms of plants, animals and fungi begin appearing, including annelids, insects and reptiles, hence the eon’s name, which means “visible life”. Several mass extinctions occur, among which birds, the descendants of non-avian dinosaurs, and more recently mammals emerge. Modern animals—including humans—evolve at the most recent phases of this eon.|
Geologic time scale
Main article: Geologic time scale
The history of the Earth can be organized chronologically according to the geologic time scale, which is split into intervals based on stratigraphic analysis. The following four timelines show the geologic time scale. The first shows the entire time from the formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent period is expanded in the fourth timeline.