The power of a metaphor is directly proportional to its ability to be turned into a science fiction film.

Io9 offers this up in explaining the divergence and evolution and re-union of the various subspecies of humanity circa 100,000 years ago:

As early as 700,000 years ago, a group of humans left Africa and spread out across Europe and Central Asia. These people became Neanderthals and Denisovans, and the people they left behind in Africa became modern Homo sapiens. These groups were not different species, but rather different human groups who were separated from each other for hundreds of thousands of years. When modern Homo sapiens came trooping out of Africa about 100 to 80 thousand years ago, they would have looked very different from their Neanderthal and Denisovan cousins. Neanderthals were short and barrel chested, pale-skinned with large jaws and sloping foreheads. Denisovans, who lived exclusively in Asia, may have been darker skinned but we don't have enough fossil evidence to say for sure what their bodies would have looked like.

What is certain is that when the dark-skinned, tall Homo sapiens met the Neanderthals, it was probably like two alien groups meeting on Star Trek. By that I mean one group had head lumps in slightly different places than the other — but both were both humanoids, capable of bearing children together.

OK, now that makes sense.