I may have Neanderthal hair. Unless you're African, you may, too. Or not; let's just say many of the genetic sequences that determine what kind of hair we have come from Neanderthals.
That's not what most of us were taught. Long ago and far away, when I was an undergraduate studying physical anthropology, I helped restore a skull — the fragments of a skull — that had been in the university's natural history museum for many years. I used dental tools and a weak acid solution to rid the bone fragments of the caked mud in which they had been imbedded for millennia. (Using the acid, I panicked when I thought I had managed to dissolve the ancient bone, but the Ph.D. candidate in charge of my work assured me that I was only dissolving hardened mud.) The skull and others found with it were clearly from modern humans and yet ... they clearly had some Neanderthal characteristics.
The explanation seemed obvious: Humans and Neanderthals had interbred. But at the time and for many years afterward, the scientific establishment rejected the obvious: Science said that interbreeding hadn't happened. I never believed it, so I was interested but not surprised four years ago, when genetic researchers announced that those of us who didn't come from Africa carried genes from Neanderthals after all. (Neanderthal genes make up some 2 percent of our own genomes.)
And I was intrigued in January when University of Washington geneticist Joshua Akey and his graduate student, Benjamin Vernot, and a group of Harvard Medical School geneticists published papers suggesting that we incorporate perhaps 20 percent of the Neanderthal genome. (East Asians have up to 25 percent.) Both groups found that those genes aren't sprinkled uniformly across our own genetic structure. Rather, they are bunched in areas that control the development of different body parts and characteristics: We may have Neanderthal hair and skin (at least a lot of the Neanderthal genes are concentrated there) but, since none of those genes appear in areas linked to language, no trace of a Neanderthal accent — assuming there was such a thing.
Why did it take so long for the scientific establishment to acknowledge this? No doubt, religion played a part. My friend Sally suggests it was because we thought we were made in God's image — but God has turned out to be somewhat hairier than we supposed. Were Neanderthals really as hairy as we imagine them? When I sat down with Akey in his office at Foege Hall, just west of the university's health sciences complex, he said his department chairman had asked him that. He doesn't know. Maybe. But perhaps, he suggests, Neanderthals were a lot more like us than we tend to think.
Why, apart from religion, did scientists believe for so long that we were too unlike Neanderthals for mutual attraction? Personally, I tend to hear echos of old American racist rants against the horrors of "miscegenation." Akey is more charitable. "A lot of people, especially in the public at large, believe human beings stand outside of nature," he suggests. If you're all alone at the top of the pyramid, everyone else is beneath you. Besides, if you're a separate species, and one characteristic of a species is — as we were taught — that it won't breed with anyone else, that's the end of the discussion. Or not.
Akey suggests that the old idea of a species as a group that won't breed with others hasn't been especially useful. There is a species barrier, but it's permeable. He said he prefers to think of that barrier as quantitative — i.e., interbreeding doesn't happen often — rather than qualitative — it doesn't happen, it can't happen, it would be an unnatural act. In fact, he says, inter-species hybridization happens all the time; look at wolves and dogs.
Akey is OK with the old saw that "extraordinary results require extraordinary proof," but he suggests that the idea of human beings mating with Neanderthals isn't really so extraordinary. "If you look at the Internet," he says, "you see that humans will mate with" just about anything."
Away from the Internet, there's evidence that humans also mated with Denisovans — a hominid species that is known only from a pinkie bone found five years ago in a Siberian cave. Denisovan DNA makes up as much as 6 percent of the genome in some Melanesians.
How often did this kind of thing go on? Take the number with a grain of salt, Akey told me, but the genetic evidence suggests perhaps 300 matings between humans and Neanderthals — that is, 300 matings that left genetic evidence. Assuming that hybrids may have tended to be infertile — the Harvard group suggests that the offspring of male Neanderthals and human females were sterile — the actual number may have been much larger.
We are hybrids. That probably includes Africans who don't have Neanderthal or Denisovan DNA. When modern humans first left Africa, they encountered Neanderthals in the Middle East — that skull I worked on long ago had been found in what at the time was the British mandate of Palestine — and elsewhere. They presumably encountered Denisovans somewhere in central Asia. People who stayed in Africa wouldn't have met either group.
But they may have met someone else. In the future, Akey said, he'd like to look in African DNA for genes from an extinct hominid that has not yet been identified. There is no archaeological evidence of this hominid, but there is genetic evidence: Akey said that the African genome suggests the presence of genes from an unknown species that appears roughly as Neanderthal genes show up in the non-African genome. Given the lack of evidence you can hold in you hand, the "bar would be pretty high" for convincing the scientific world you'd really found something, Akey said. But he figures something is probably there. Who knows how many different kinds of people we used to share the planet with?
Now, Akey said, humans are just about unique in sharing the earth with no other species that closely resembles us. But that's not the way it was for most of human history.
Neanderthals shared the earth with us until about 30,000 years ago. The small non-human people whose remains have been found on Indonesia's Flores Island probably died out only 13,000 years ago. For most of human history, we haven't been alone.
Why didn't those other species stick around? No one knows. Neanderthals may have disappeared because we did them in or because we monopolized the big game or because we assimilated them or because of something else. The evidence suggests, Akey said, that the Neanderthals' population had started shrinking before they started mating with humans. And that population must have been small to begin with, so that any serious environmental or other problem might have pushed it over the edge.
Akey said a BBC inteviewer had asked him recently whether or not the benefits of hybridization had flowed both ways: Had Neanderthals benefited from getting some of our genes? It couldn't have helped them very much, Akey replied, because after all, we're here and they're not.