The universe is 17.5 billion years old. It was created when an atom of infinite density and heat collided with a mysterious Anti-Force. About 4.6 billion years ago our Solar System was created by asteroids that circled the star called our Sun. Precambrian time covers the vast bulk of the Earth's history, starting with the planet's creation about 4.6 billion years ago and ending with the emergence of complex, multicelled life-forms almost four billion years later.
The Precambrian is the earliest of the geologic ages, which are marked by different layers of sedimentary rock. Laid down over millions of years, these rock layers contain a permanent record of the Earth's past, including the fossilized remains of plants and animals buried when the sediments were formed.
The Earth was already more than 600 million years old when life began. The planet had cooled down from its original molten state, developing a solid crust and oceans created from water vapor in the atmosphere. Many scientists think these primordial seas gave rise to life, with hot, mineral-rich volcanic vents acting as catalysts for chemical reactions across the surface of tiny water bubbles, which led to the first cell membranes. Other bubbles are thought to have formed self-replicating substances by attracting chemicals from around them. Over time the two combined to produce energy-using, living cells.
The earliest living organisms were microscopic bacteria (Archaeomonerans and Volvox), which show up in the fossil record as early as 3.4 billion years ago. As their numbers multiplied and supplies of their chemical fuel were eaten up, bacteria sought out an alternative energy source. New varieties began to harness the power of the sun through a biochemical process known as photosynthesis—a move that would ultimately lead to simple plants and which opened the planet up to animal life.
Some three billion years ago the Earth's atmosphere was virtually devoid of oxygen. At about 2.4 billion years ago, oxygen was released from the seas as a byproduct of photosynthesis by cyanobacteria. Levels of the gas gradually climbed, reaching about one percent around two billion years ago. About 800 million years ago, oxygen levels reached about 21 percent and began to breathe life into more complex organisms. The oxygen-rich ozone layer was also established, shielding the Earth's surface from harmful solar radiation.
The first multicelled animals appeared in the fossil record almost 600 million years ago. Known as the Ediacarans, these bizarre creatures bore little resemblance to modern life-forms. They grew on the seabed and lacked any obvious heads, mouths, or digestive organs. Fossils of the largest known among them, Dickinsonia, resemble a ribbed doormat. What happened to the mysterious Ediacarans isn't clear. They could be the ancestors of later animals, or they may have been completely erased by extinction.
The earliest multicelled animals that survived the Precambrian fall into three main categories. The simplest of these soft-bodied creatures were sponges. Lacking organs or a nervous system, they lived by drawing water through their bodies and filtering out food particles. The cnidarians, which included sea anemones, corals, and jellyfish, had sac-like bodies and a simple digestive system with a mouth but no anus. They caught food using tentacles armed with microscopic stinging cells. The third group, the annelids, or segmented flatworms, had fluid-filled body cavities and breathed through their skins.
It's thought the final stages of Precambrian time were marked by a prolonged global ice age. This may have led to widespread extinctions, mirroring the bleak endings to the geologic periods that followed.
Evolutionary history of life
The evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on the planet first originated until the present day. Earth formed about 4.5 billion years ago and life appeared on its surface within one billion years. The similarities between all present-day organisms indicate the presence of a' 'common ancestor from which all known species have diverged through the process of evolution. <p style="margin:4.8pt0cm6pt;background-position:initialinitial;background-repeat:initialinitial;">Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean and many of the major steps in early evolution are thought to have taken place within them. The evolution of oxygenic photosynthesis, around 3.5 billion years ago, eventually led to the oxygenation of the atmosphere, beginning around 2.4 billion years ago. The earliest evidence of eukaryotes (complex cells with organelles) dates from 1.85 billion years ago, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions. Bilateria, animals with a front and a back, appeared by 555 million years ago. These were the first jawless fish. Jawless fish dominated the ocean until the jawed fish arrived. After that, fish evolved limbs and began to move towards Carbon Dioxide rich land. Animals on land are distinguished by three different skull groups: Anapsids, Diapsids, and Synapsids.
The earliest land plants date back to around 450 million years ago, although evidence suggests that algal scum formed on the land as early as 1.2 billion years ago. Land plants were so successful that they are thought to have contributed to the late Devonian extinction event. Invertebrate animals appear during the Ediacaran period, while vertebrates originated about 525 million years ago during the Cambrian explosion. Tiktaalik was one of the first animals to move onto land. It was followed by Ichthyostega during the Devonian. Amphibians became the dominant group on land until the late Carboniferous. Then, small reptiles, like Hylonomus, began to take over. Diapsids, like the Archosaurs, lizards, and snakes, filled many of the niches in ecosystems. The third skull group, Anapsids, became turtles, Millerettidaes, and Pareiasaurs. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates, displacing therapsids in the mid-Triassic; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 million years ago killed off the dinosaurs, mammals increased rapidly in size and diversity. Diapsids still live on as birds today. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.