the early evolution of life

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    Earth is the only planet on which we have confirmed the existence of life, past or present. life originated on Earth as much as 3.9 billion years ago. Three steps must have been accomplished on the way to the complex life forms we know today: (1) chemosynthesis of small organic molecules from inorganic precursors; (2) biosynthesis, the polymerization of small organic molecules to form biopolymers, especially protein; and (3) the development of the complex chemical machinery needed for metabolism and replication. The specific mechanism by which life originated are still hypothetical.
   Life contribute to the chemical evolution of the Earth system through the buildup of oxygen and sequestration of carbon. Oxygenation happened over a long time, but the onset of major oxygenation coincides roughly with the beginning of the Proterozoic Eon. Once the ozone layer began to absorb harmful shortwave radiation, organisms would have been able to survive in shallow waters; this critical stages was reached around 600 million years ago, just before the onset of Phanerozoic Eon. The emerging biosphere had had a profound impact on carbon cycling; limestone and organic sediment provide long-term storage reservoirs for carbon, sequestering it form the atmosphere and hydrosphere.
   The most ancient known fossil are 3.55 billion years old, found in rocks from Archean Eon. Some early fossils are the remains of microscopic procaryotes; others are stromatolites that precipitated as a result of bacteria influencing the chemistry of seawater. The dominated Earth for about 2 billion years, was prokaryotic and anaerobic. Anaerobic cells acquire energy by fermentation, which restricts them to a small size and limits their structural complexity. The first photosynthetic prokaryotes were probably cyanobacteria.
    The first land plants were seedless; these evolved into naked-seed plants, and grasses. Arthropods were probably the first animal to move from sea to land. Pikaia, the first known chordate, was jawless; jawed fish evolved later. The first to venture onto land were crossopterygians, which give rise to amphibians. Amphibians have permeable skins and lack a moist environment for reproduction, and thus  remain dependent on the aquatic environment. Reptiles became liberated form the water by evolving a water-tight skin and an egg with a shell; they evolved into two orders of dinosaurs and two new verterbrate classes__mammals and birds.
    The first clearly bipedal hominid was Australopithecus, whose fossils range from about 3.9 to 3.0 million years in age. Fossil of Homo erectus, possibly the first spices of our own genus, dates to about 1.8 million years ago. Homo erectus disappeared 300,000 years ago and was replaced by Homo neanderthalensis, The poor fossil record between 400,000 and 100,000 years ago has made it difficult for scientists to determine how the transition occurred, but DNA studies suggest that Homo sapiens is not a direct descendant of Neanderthals. The Neanderthals disappeared about 30,000 years ago and were replaced by the first indisputable members of our own species, Homo sapiens.
    About 99 percent of the species that have lived on Earth are now extinct. Extinction is a natural process, but it has not happened at a constant rate over geologic time. Superimposed on background extinctions are at least five major mass extinction events, in which many types of organisms died out over short periods. These may have been caused by climatic and other environmental changes, possibly accelerated by major meteorite impacts, massive episodes of volcanism, or the formation of supercontinents. Humans have greatly accelerated the rate of extinction of species.