At one point between three and two million years ago, it appears there may have been as many as six hominid types coexisting in Africa. Only one, however, was fated to last: Homo, which emerged from the mists beginning about two million years ago. No one knows quite what the relationship was between australopithecines and Homo, but what is known is that they coexisted for something over a million years before all the australopithecines, robust and gracile alike, vanished mysteriously, and possibly abruptly, over a million years ago. No one knows why they disappeared. "Perhaps," suggests Matt Ridley, "we ate them."

Conventionally, the Homo line begins with Homo habilis, a creature about whom we know almost nothing, and concludes with us, Homo sapiens (literally "man the thinker"). In between, and depending on which opinions you value, there have been half a dozen other Homo species: Homo ergaster, Homo neanderthalensis, Homo rudolfensis, Homo heidelbergensis, Homo erectus, and Homo antecessor.