We were so close, in fact, that our species interbred with theirs. Tracing back our lineages, there must have been a last common ancestor of Homo sapiens and Neanderthals sometime in prehistory. (Related: "Geno 2.0 Can Reveal How Neanderthal You Are.")

But who was this mystery human?

Picking out direct ancestors in the fossil record is tricky. To figure out when the last common ancestor of Homo sapiens and Neanderthals lived, paleoanthropologists have been sifting through both genetic and anatomical evidence.

In the last five years, anthropologists have used DNA to reconstruct the evolutionary history of humans. Researchers have suggested a range of dates for when the last common ancestor of our lineage and Neanderthals could have lived. (Related: "Last