In the 4-5 million years that the hominid lineage has been diverging from its common ancestor with the apes, dozens of hominid species have arisen, often with several species coexisting in time and space. As recently as 30,000 years ago, Homo sapiens coexisted with Homo neanderthalensis. Both species had large brains and advanced intellects. That these traits belonged to both species is most easily explained by