More than 3.5 million years ago, two hominids walked upright across a field of newly fallen volcanic ash in eastern Africa. The footprints were covered by a subsequent ashfall until 1978, when they were unearthed by paleontologists. The Laetoli footprints, named after the site where they were found, are very early evidence of upright walking, a key acquisition in the lineage leading to humans.

But today there is no scientific doubt about the close evolutionary relationships between humans and all other primates. Using the same scientific methods and tools that have been employed to study the evolution of other species, researchers have compiled a large and increasing number of fossil discoveries and compelling new molecular evidence that clearly indicate that the same forces responsible for the evolution of all other life forms on Earth account for the biological evolution of human characteristics.

Based on the strength of evidence from DNA comparisons, the common ancestor of humans and chimpanzees lived approximately 6 to 7 million years ago in Africa. The evolutionary tree leading from this ancestral species to modern humans contains a number of side branches, representing populations and species that eventually went extinct. At various times in the past, the planet appears to have been populated by several human-like species.

About 4.1 million years ago, a species appeared in Africa that paleontologists place in the genus Australopithecus, which means “southern ape.” (A member of the genus was first discovered in southern Africa, although other fossils, including an almost complete skeleton of a 3-year-old female, have been found in eastern Africa.) The brain of an adult of this genus was about the same size as that of modern apes (as documented by the size of fossil

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