Evidence has been piling up for a while that early humans in Europe had children with the Neanderthals who had been living there for probably 500 thousand years before humans arrived. Very few Neanderthal genes are left in humans today, so what difference does it make? A lot, both genetically and philosophically.
First of all, we still aren't sure what genetic inheritance the Neanderthals left behind among non-African populations, and we're even less certain about mixtures that probably took place in the Middle East and Asia between Homo sapiens and hominins like the Denisovans. It's possible that Europeans inherited red hair from Neanderthals. And just this week, a new study in Nature linked Neanderthal genes to fatty acids that show up in the brains and other tissues of Europeans — but not in Asian populations.
Did my Neanderthal ancestors give me a predisposition to accumulate certain kinds of fat? Possibly. Though researchers aren't sure what role these extra fatty acids play in our bodies, one thing is certain. A large group of Homo sapiens have inherited genetic material from a group that was once considered another species.
So the first way that our Neanderthal heritage matters is that it may actually have a bearing on some aspects of our metabolism, our brain function, and more. Second, it reminds us that what at first might seem to be another species was probably just another kind of human. As humans, we tend to think of ourselves as grand exceptions in the animal world — even though we look like our ape cousins, we have table manners and Android devices and can follow the plot on Game of Thrones. Key to this sense of exceptionalism is the idea that no other creatures alive were ever like us.
Well, too bad. Because there were other humans on the Earth like us hundreds of thousands of years ago, and even though they were barrel-chested and had heavy brows, they had sex with our mothers and fathers. If you're a white person, you've got Neanderthal in your blood. If you want to get cute about it, you can claim that Europeans are the result of paleolithic miscegenation, making whitey a much less "pure" version of Homo sapiens than Africans.
But even if you don't want to make that joke, there's still a point to be made. Which is that human evolution is a lot more unpredictable than you might think.
Before we had sequenced the Neanderthal genome and discovered our mixed ancestry, the traditional story of how Homo sapiens came to rule the western half of Eurasia was a lot uglier. Basically, scientists believed that humans had come out of Africa and killed off every other hominin they met. Maybe they didn't actually stab them in the face — though anthropologists like Richard Klein do believe there may have been a genocide. But as evolutionary biologist Ian Tattersall has said, a more likely scenario is that humans just out-competed the local Neanderthals, using up local resources and driving their big-boned cousins to extinction.
Now that we know about the Neanderthal ancestry in Europeans, we have a third possibility, which is that Neanderthals assimilated into Homo sapiens. Sure, there may have been competition between the two groups but they had families too.
This is where the philosophical importance of Neanderthal/Homo sapiens sex comes in. We may never know the real story of what happened between early Homo sapiens and the other hominin groups they met outside Africa. Any scientist who claims to know the truth of these events is, well, a bad scientist. Like all histories, the tale of our distant forbearers is an interpretation, based on available evidence. And the story we choose to tell ourselves about that evidence matters a great deal.
Today we can look back at our history as a species and understand ourselves as far more complex than genocidal maniacs. I'm not denying that humans can be bloodthirsty — obviously we are. But we also have the ability to form true, lasting bonds with people who are very different from us. And our predilection to form those kinds of unexpected relationships apparently goes back a pretty long way.
So, why does it matter that Homo sapiens had sex with Neanderthals? It matters because now we have genetic evidence that humans as a species don't always kill and destroy what we don't understand. Sometimes, we fall in love and settle down instead.
Frankly it's nice to have scientific evidence, instead of wistful mysticism, to back up the claim that we aren't so terrible after all.
Annalee Newitz is the editor-in-chief of io9 and this is her column. She's also the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction.