Prehistoric life is a term used to refer to diverse organisms that inhabited Earth from the origin of life about 3.5 billion years ago to the time when humans began to keep written records, about 3500 BC. During the course of evolution, new forms of life developed and many other forms, such as the dinosaurs, became extinct. Prehistoric life evolved over this vast timespan from simple bacteria-like cells in the oceans to algae and protozoans and complex multicellular forms such as worms, molluscs, crustaceans, fish, insects, land plants, amphibians, reptiles, birds, and mammals. On a geological timescale humans evolved very recently, about 4 million years ago at the earliest, although the exact dating is a matter of some debate. A few forms of prehistoric life like some plants and notable animals such as the coelacanth are ancient life forms and still exist today, making them living fossils. Others, like sharks, have changed little over eons. For most forms of prehistorical life that became extinct, the only record of them ever existing that remain today are fossils or rock imprints.