Prehistoric life
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Prehistoric life are the diverse organisms that have inhabited Earth from the origin of life about 3.8 billion years ago (b.y.a.) to the Historic period (about 3500 BC) when humans began to keep written records.
During the course of evolution, new forms of life developed and many other forms, such as the dinosaurs, became extinct. (See Timeline of evolution).
Prehistoric life evolved over this vast timespan from simple bacteria-like cells in the oceans to algae and protozoa, and ultimately to complex multicellular forms such as fungi, land plants, worms, molluscs, crustaceans, insects, and vertebrates.
In geologic terms, humans evolved very recently, only about 2.5 million years ago (mya). (See Geologic time scale, Human evolution).
Very few species of prehistoric life (such as the coelacanth) still exist today unchanged, tens of millions of years later, thereby making them living fossils. Yet other creatures, like sharks, have changed but a little over millions of years.
However, most life forms -- over 99 percent -- have become extinct, and so the only record of them ever existing that remains today are rock imprints, casts or other fossils.
