Humans may be the way they are because 37,000 years ago our species mated with Neanderthals.
This is what Howard Hughes Medical Institute investigators at the University of Chicago propose based on a genetic analysis of a brain-size gene in modern humans.
Most humans these days have a certain version of the mircocephalin gene. If this gene spontaneously mutates the bearer has a much smaller head and brain. Homo sapiens has a good version of the gene.
They apparently acquired the D allelle of the microcephalin gene about 37,000 years ago. Since then D has become predominant, because humans animals with normal heads reproduce better than humans with small brains and heads.
Where the D allelle came from has been an open question. The Iniversity of Chicago scientists say it's possible -- even likely -- that modern humans picked it up by breeding with Neanderthals.
Neanderthals and humans diverged from each other about 1.1 million years ago, though both apparently remained sufficiently similar anatomically.
But these were different species. Did modern humans take advantage of Neanderthals routinely? If so, why? Because they were there?
The experience had to be eerie. Imagine if another member of the Homo family was extant and perhaps even attractive. Would there be laws against humans and Neanderthals fooling around?
They inevitably would interbreed. Then what?
We may owe our reign on at least one act of what we might now consider beastiality.
The idea makes the controversy over homosexual marriage seem like a colossal nothing.