When we talk about humans in a biological sense, we’re referring to one surviving species and fourteen or more extinct ones. And while the vast majority of human species no longer walk the Earth, we carry their remnants in our genes.
Among our more recently extinct human cousins are the Neanderthals, who—like the Denisovans and, most likely, a number of still-unidentified human species—bred with a significant number of anatomically modern humans (AMHs) before dying out 40,000 years ago. Since AMHs first evolved about 200,000 years ago, this means that for most of our species’ history—80% of it, give or take—our direct ancestors shared the world with Neanderthals. And some of them shared a little more.
Neanderthals originated in Eurasia and, near as we can tell, never made to Africa. Meanwhile, we AMHs originated in Africa and have been gradually migrating hither and yon ever since. Anyone who has ever been in a long-distance relationship knows what that means: it took a while for Neanderthals and anatomically modern humans to discover each other. And because of this geographical divide, it’s only AMHs who have ancestors outside of Africa who are carrying around vestigial Neanderthal gene markers. This gives us a general idea of where AMHs and Neanderthals met.
And, thanks to the DNA of a 45,000-year-old Siberian man, we’re also getting a better understanding of when. A British study published earlier this month suggests that this man had no close Neanderthal ancestors, but had a Neanderthal ancestor already dating back some 15,000 years earlier—and that ancestor’s genetic traces tell us things we didn’t know about AMH migration. Jennifer Viegas writes for Discovery News:
"Project leader Chris Stringer of the Natural History Museum in London explained to Discovery News that the Siberian man belonged to a population that was closely related to the ancestors of today's Europeans and Asians. He carried only slightly more Neanderthal DNA than they do ...
"He concluded, 'While it is still possible that modern humans did traverse southern Asia before 60,000 years ago, those groups could not have made a significant contribution to the surviving modern populations outside of Africa, which contain evidence of interbreeding with Neanderthals.’"
But Neanderthals are only part of the story. The vast majority of human species have never been genetically sequenced; whatever contributions they may have made to our DNA, our deep ancestral history, and our ancestors’ survival in Africa are beyond our capacity to trace, and may remain so forever. For now, it’s enough to know that we all have intelligent, brave human ancestors in our prehistory who do not resemble anatomically modern humans, ancestors about whom we know nearly nothing. All of them continue, in some way, to live on through us.