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Paranthropus boisei skull. Cast of the fossil skull of a young male of Paranthropus boisei discovered by Mary Leakey in 1959 at Olduvai Gorge, Tanzania. This specimen, called OH5, is the holotype specimen and dates back 1.8 million years. P. boisei forms part of the human evolutionary tree, though a divergent branch to that which led to modern humans. (KEYSTONE/SCIENCE PHOTO LIBRARY/NATURAL HISTORY MUSEUM, LONDON/SCIENCE PHOTO LIBRARY)