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Early hominid footprints. Footprints of human ancestors (probably Australopithecus afarensis) with elephant footprints (top centre) at Laetoli, Tanzania. The rock bed on which they are preserved is 3.7 million years old. These footprints were discovered by Mary Leakey in 1977. They show that human ancestors had acquired an upright, bipedal gait by this stage in their evolution.(KEYSTONE/SCIENCE PHOTO LIBRARY SPL/JOHN READER/SCIENCE PHOTO LIBRARY)