Five fossilized human skulls show how the shape of the early human face evolved: (left to right) Australopithecus africanus, 2.5 million years old; Homo rudolfensis, 1.9 million years old; Homo erectus, 1 million years old; Homo heidelbergensis, 350,000 years old; Homo sapiens, 4,800 years old. Scientists believe that climate change had a major impact on the development of early humans. (Smithsonian Institution)
