How many human species roamed Africa when the genus Homo emerged there about 2.4m years ago is a vexed question. Conventional wisdom suggests that there were as many as four-Homo erectws, Homo ergaster, Homo habilts and Homo rudol/ensis-and that one of these, Homo erectus, then both spread into Asia and diversified into other species including Homo sapiens. But skulls are rare and often fragmentary, and some researchers think the differences between them, which are used to separate the four alleged species, are actually no greater than differences in morphology between modern Homo sapiens skulls.
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