Forty thousand years ago, the last Neanderthal men and women died out, leaving modern humans alone on a planet they inherited from an entire family of different kinds of humans.
Long before that, humans had diverged evolutionarily from the Neanderthals, but not so much that individual humans and Neanderthals couldn't occasionally make babies together. As a result of these ancient relationships, most of us carry a few Neanderthal genes -- 2.5 to 4 percent is typical.
Oddly, though, if you look at the Y chromosomes of modern men, that's one place you don't see Neanderthal genes.
This is the conclusion of a paper that comes out today in The American Journal of Human Genetics. The team of four researchers, three from Stanford and one from the Max Planck Institute, report they are the first to examine the gene sequences on the Y chromosome of a Neanderthal man.
The particular Neanderthal man who provided the Y chromosome for the study lived 49,000 years ago and left his bones near El Sidrón, Spain.
The mystery is why modern humans would carry Neanderthal genes on other chromosomes but not on the Y. One clue comes from the kind of chromosome that Y is.
The Y chromosome is one of two human sex chromosomes. A person with two X chromosomes is normally female, while someone with an X and Y is normally male.
As I wrote in my press release:
The Neanderthal Y chromosome genes could have simply drifted out of the human gene pool by chance over the millennia. Another possibility is that Neanderthal Y chromosomes include genes that are incompatible with other human genes...
Stay tuned, the debate -- and the research -- is ongoing.
Previously: Humans owe important disease-fighting genes to trysts with cavemen, Humans share history - and a fair amount of genetic material - with Neanderthals
Photo of Neanderthal reconstruction by ICHTO