Studies in evolutionary biology have led to the conclusion that human beings arose from ancestral primates. This association was angrily debated among scientists in Darwin's day. But today there is no significant scientific doubt about the close evolutionary relationships among all primates or humans. The theory of evolution explains how life on Earth has changed. In scientific terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. Scientific theories are explanations of natural phenomena built up logically from testable o

Many of the most important advances in paleontology over the past century relate to the evolutionary history of humans. Many connecting links between along various branches of the human family tree have been found as fossils. These fossils have been found in geological deposits of intermediate age. They inform the time and rate at which primate and human evolution occurred. Scientists have found thousands of fossil specimens representing members of the human family. A large number of these cannot be assigned to the modern human species, Homo sapiens. Most of these specimens have been well dated, often by radiometric techniques. They reveal a branched tree, parts of which trace a general evolutionary sequence leading from ape-like forms to modern humans. Paleontologists have discovered numerous species of extinct apes in rock strata that are older than four million years, but never a member of the human family at that great age. Australopithecus, whose earliest known fossils are about four million years old, species with some features closer to apes and some closer to modern humans. In brain size, Australopithecus

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