(picture from New Scientist)


Males have only one X chromosome, which is necessary for reproduction. As is often the case with hybrids, the male offspring from these unions would probably have been infertile.

But the females, which have two X chromosomes, would have been fertile. If some of those hybrid females then bred with proto-chimp males, some of their male offspring would have received a working X from the chimp side of the family. They would have been fertile -- and with them the hybrid line would have been off and reproducing on its own. (Washington Post)

Ben's notes:
At first this just clarified to me the slutty nature of proto-human women and human females' propensity for ending up with the stupid dominant ape, but after pondering it awhile I tend to think that it shows more the unquenchable sex drive of male proto-chimps, which in turn is evident in its male human descendants.