Whoever propelled these far-off bits of DNA into the modern African population haven't left behind any distinct fossils – at least none yet found – but there is some fossil evidence that very ancient groups of proto-humans survived in Africa until surprisingly recently, Marean said.

The most surprising is a skull found in Nigeria that looks like a proto-human called Homo heidelbergensis. That group flourished 500,000 years ago and may have been ancestors of Neanderthals. But carbon dating shows this individual lived just 16,000 to 11,000 years ago.

Not that ancient Africans necessarily intermixed with members of this group, said Marean. "But this is evidence that really ancient lineages were hanging around."