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Mary Leakey (1913-) palaeoanthropologist, seen here at the end of the trail of hominid footprints fossilized in volcanic ash. The trail was discovered by Mary Leakey's expedition at Laetoli, Tanzania in 1978. It dates from 3.6 million years and shows that hominids had acquired the upright, bipedal, free-striding gait of modern man by this date. The footprints probably belong to Australopithecus afarensis whose dates are from 3.7 to 3.0 million years ago. They show a well developed arch to the foot and no divergence of the big toe. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN READER)