The short version of the story is that a group of human ancestors left Africa and went to Europe, and later to the rest of the Old World. These were the Neanderthals. A half million years later, modern humans left Africa and settled all over the world. They interbred with Neanderthals for a few thousand years, and then the Neanderthals went extinct, except as a minor part of our Homo sapiens DNA. The only humans around today with no Neanderthal DNA are descended from the people who never left Sub-Saharan Africa.
Geneticists have found that snippets of Neanderthal DNA can be found in any part of our genome, except for the Y chromosome. What happened to the Neanderthal Y? Its demise could have been coincidental, or luck. Probably not, but that always needs to kept as a possibility. Or it could have been that it was always modern human men breeding with Neanderthal women, but since we haven't found any Neanderthal mitochondrial DNA in modern humans, which is only passed along by females, that doesn't seem likely. There are other scientific possibilities, which are explained at the Conversation. -via Damn Interesting
(Image credit: Clemens Vasters)