A couple years ago when I first started blogging here, my friend John Edvalson asked me right out of the gate my opinion on the Neandertal-sapiens interbreeding controversy. I think I skirted an answer because, though as a biological anthropologists I thought I should probably have one, I was behind the curve & didn’t feel informed enough to say anything.
Things haven’t really changed, but I’ve always been uncomfortable with this whole interbreeding-with-Neandertals scenario. It makes for good play in the media, but it feels too tidy for me. Real human stories are just never that tidy. Take for instance, as geneticist Sarah Tishkoff just pointed out in her talk with my class & her ALLELE lecture yesterday, the plethora of new Homo species we keep finding–Denisovans, floresiensis, erectus gets demoted as regional variation of ergaster. A messy playing field. Given the ring speciation we see among other genuses in relatively small areas, that just makes more sense to me & that even most people with access to planes, trains, & automobiles still don’t really travel that far from home when they grow up & marry, it seems odd that Homo sapiens got their hands on the ring of power 200 kya & just came out of Africa kicking ass & taking names away.
But I don’t see many people posing the counter-argument lately, since the Neandertal & human genomes have been published. I keep wondering, why is 2% of Neandertal DNA evidence of interbreeding? Why isn’t it just evidence of shared ancestry? In fact, I’m pretty sure I pressed John Hawks on this when he was here a few years ago, but I forgot his answer. The continued barrage of news media & even scientific party-line towing about interbreeding had pushed it from my mind. So when Jim Bindon sent this blog post around, I thought–aha! here’s someone bold enough to articulate my confusion. So I dutifully posted it on my Facebook wall, got a bevy of my biologist friends scoffing at our ignorance (but still not explaining this to me in terms I could understand), & got John to rearticulate his position, but in writing this time!
All of us have shared ancestry with Neandertals. Some have more than others. Indicates ancestry from a population more like Neandertals than any living people are. Who was that population? One possibility is Neandertals. Another is one or more ancient non-Neandertal populations that were genetically more like Neandertals than anyone living now. Length of chromosome blocks that are similar to Neandertals suggest that this similarity comes from a population that existed within last 100,000 years. So we need Neandertals, or an as-yet-unknown population that was genetically similar to them within the timeframe that Neandertals existed, and lived somewhere that sub-Saharan Africans could have mostly avoided them.
THIS makes sense to me, given the evidence. Maybe it’s just because he’s an anthropologist & knows how to speak my language. I know John is a big Neandertal supporter, & I get why (besides the fact that he’s a paleoanthropologist & geneticist who actually knows the data from the inside to a degree most of us will never have access to). H. neandertalensis has been knocked around for too long–why COULDN’T they have interbred w/ other Homo? Clearly, they were not dumb, as pop culture tended to depict them. I’m with him on that.
John impressed me during his talk at Bama with many things I hadn’t known about them, such as that our knowledge of them from European samples is probably from a later backwater of their own population, which may have been centered more in Asia, where they were probably also much more genetically diverse. But bonobos & chimps are genetically MORE similar & haven’t interbred for, what, 3 million years?
Shedding more light on this, as many of you know, are two new papers out last month in Science & Nature. One of the key points is that this 2% of shared DNA is in non-African populations. The sticking point is that, while this is compelling, we have only sampled a small proportion of the African genetic diversity yet, which constitutes 2/3 of the genetic diversity worldwide, & most of those samples come from Yoruban populations, who are connected to the Bantu expansion. This point was made clear in a great talk this past Thursday at Bama by geneticist Sarah Tishkoff.
Tishkoff’s story is great. I was going to write up a separate blog post about it, but I’ll detail a bit here. Why don’t we know more about African genetic diversity, many people ask her? She explains this by showing slides of her fieldwork. First, let me say that she has truly impressive credentials. She was trained as an undergraduate in Allan Wilson’s lab at UC Berkeley & worked with the very first PCR machine. She did her graduate work with Kenneth Kidd at Yale & did a postdoc at Penn State. Despite starting in Anthropology, it wasn’t until her postdoc that she got to experience her dream of doing fieldwork. She was lured to study the genetic diversity of African hunter-gatherers because it’s so understudied. Even at that point, because of political issues, it took her 6 years to get the permits & funding to begin the data collection. Then she had to set up a lab in the bushes, literally. Working in field settings with no electricity, they use truck batteries to power centrifuges.
Anyway, Tishkoff’s work indicates that we can see selection in the genome, & we can see admixture in the genome as a result of migration (together with linguistic, phenotypic, & archaeological data). For instance, she has found evidence of lactase persistence genes that mutated in E. Africa & are distinct for mutations associated with lactase persistence among Europeans. Those mutations show up in S. Africa but at a much lower rate, indicating migration by some E. Africans to S. Africa & some admixture (in press, J Hum Gen).
So, IMO, the jury is still out on who bred with who, but the genetics, like the fossil record can get us there. Until we collect more data, however, all the current theorizing is essentially speculation, &, I fear, the sexiest interpretation will continue to hold the spotlight until the evidence catches up. And what is sexier than actual sex?
