More than 75 years of digging in the ancient, arid sediments of East Africa has told scientists a great deal about the long evolutionary trail that led to modern human beings. They know about Lucy, the upright-walking proto-human australopithecine that strode the continent some 3.2 million years ago; about Homo habilis, the first known human species, which was making and using stone tools in the same region by 1.2 million years later; about Homo erectus, which emerged from Africa soon thereafter and spread across the world.
But while the broad outlines of this prehistoric genealogy have been well established, most...
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