Fossils and fragments of jaws suggest to scientists that the ancestors of monkeys, apes, and humans began their evolution approximately 50 million years ago. Additional evidence for this evolution has been derived from experiments in biochemistry and changes that occur in the DNA of cells. Mutation rates in the DNA have been calculated, and evolutionary changes can be determined from these calculations. By noting mutation rates, scientists can estimate the time since two species diverged from a single common ancestor, and they can develop a family tree.

Scientific evidence indicates that the first hominids (humanlike creatures) belonged to a group called Australopithecus. Members of the genus Australopithecus are believed to have displayed a critical step in human evolution: the ability to walk upright on two feet.

In 1924, the complete skull of a young child was found in a limestone quarry in South Africa. The fossil was unlike any ever seen before. The fossil was named Australopithecus africanus. Since then, several other A. africanus fossils have been found, showing heavy jaws, low foreheads, and small brain capacities.

In 1974, a team led by Donald Johanson and Tim White found a nearly complete skeleton of a female in the Afar region of Ethiopia. The fossil, which is earlier than A. africanus, is widely known as “Lucy,” but it is classified as Australopithecus afarensis.

In 1977, Mary Leakey and her group made another important discovery: a set of fossil hominid footprints. The footprints, dated at some 4 million years of age, show that the two Australopithecenes that made the footprints walked erect.

Members of the Australopithecus genus are considered the first hominids but not the first humans. Their brains were small in comparison with human brains, and they had long, monkeylike arms. Other members of the Australopithecus genus have been classified as A. robustus and A. boisei. These different species of Australopithecus lived in Africa and are believed to have been primarily plant eaters. Members of the Australopithecus group eventually died out about 1 million years ago.