In the drawing at right, the skeleton of Lucy, exemplar of an adult member of the species Australopithecus afarensis (with shaded bones representing those that were recovered), dates from the same geological period when the Laetoli footprints were made. For comparison, the skeleton of a modern human stands beside her.

skulls), and it appears to have spent part of its life climbing in trees, as indicated by its short legs and features of its upper limbs. But Australopithecus also walked upright, as humans do. Footprints left by one of the earliest Australopithecus species have been discovered preserved with remarkable clarity in hardened volcanic ash.

About 2.3 million years ago, the earliest species of Homo, the genus to which all modern humans belong, evolved in Africa. This species is known as Homo habilis (“handy” or “skillful man”). Its average brain size, as determined from skulls that postdate 2 million years ago, was probably about 50 percent larger than that of earlier Australopithecus. The earliest stone tools appear about 2.6 million years ago.

About 1.8 million years ago, a more evolved species, Homo erectus (“upright man”) appeared. This species spread from Africa to Eurasia. The subsequent fossil record includes the skeletal remains of additional species within the genus Homo. The more recent species generally had larger brains than the earlier ones.