“We started with a very strong bias against mixture”

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So says David Reich, and he was hardly alone. Why? It was always likely, in fact almost inevitable. I can’t think of a human expansion where there wasn’t some admixture with the locals. I’m serious: why? I’ve certainly heard arguments to that effect, but they were all silly. Intersterility was quite unlikely, if you look at mammalian hybridization. Lack of Neanderthal mtDNA in moderns meant nothing much, since it could easily have been lost due to selective disadvantage or by chance. The argument that humans were simply too picky to have ever mated with Neanderthals is plain ridiculous: at worst, they were a lot more human than sheep. Even a small amount of admixture would have allowed introgression of alleles with an advantage: we’re seeing a higher level of admixture than that bare minimum but nothing particularly surprising.

So, why?

24 Comments

  1. I think you hit the nail on the head when you mentioned the lack of even a trace Neanderthal mitochondria in modern human populations. With that in mind, it wasn’t really clear where to apply Occam’s razor. Was is simpler that Neanderthals and humans couldn’t breed for some reason, or that they could, and did, but the mitochondrial evidence was for some reason conveniently absent? I guess now we have the answer. Looking back now, it does seem like the second choice was the more likely all along.

  2. one issue isn’t scientific: when data is thin on the ground we adhere to very elegant stark models. i.e., out-of-africa vs. multi-regionalism. i used to follow the debate in the 80s and 90s, and even if the multi-regionalist side didn’t really believe it, the impression one always got was parallel and peaceful gene flow as all humans marched toward sapiency. the out-of-africa model of a major shock to this model, a disruption, was right. it was just taken way too far.

  3. Another possible reason, at least for some people: a lot had been invested in mtDNA studies that assumed neutrality. Maybe the idea of selective advantages or disadvantages in mtDNA was unwelcome?

    On the other hand, in most of the strong-bottleneck models, losing mtDNA variants with an initial frequency of 2% was the likely outcome, even with neutrality.

  4. Not that I have much knowledge about genetics but if Neanderthal women were somewhat uglier due to the their nucleus DNA, in the long run this may prove a selective pressure strong enough to remove their mtDNA from the gene pool even it it is perfectly neutral or only slightly advantageous.

  5. Or simple only modern women find Neanderthal guys sexy and masculine. Sapiens males just do not find Neanderthal females with exagerated bones that attractive. If modern day interracial mating dicrepancy mean anything to you……….

  6. i believe y chromosomes also imply out-of-africa, fwiw.

  7. “i believe y chromosomes also imply out-of-africa, fwiw.”

    But Y chromosomes can be seperated from other chromosomes. If male hybrid fail produced another male, y is over. Certainly mtDNA can break lineage if female neanderhal hybrid failed to produce femal offsprin in next generation.

  8. Especially such mixture happen at very low percentage level like 1-4%. The odd of y chromosome or mtDNA loss is quite high over such long time since these gene would not go through routine gene swap or exchange with paired chromosomes. It only needs once.

  9. There was no positive genetic data of admixture, even after the first analyses of nuclear loci, and you can make non-silly justifications about why there wouldn’t be any (maybe hybrids had very decreased fitness, for example–even an advantageous locus has to last through the first few generations when it’s linked to the rest of the introgressed genome).

    Or maybe they just convinced themselves there wasn’t any admixture so they could enjoy the surprise of finding it :)

  10. Perhaps the finding of no traces of human mother’s mitochondrial DNA among Neanderthals is due to no or very few Neanderthal females having gotten inseminated by human males. Instead perhaps only the reverse rarely but sometimes happened. For one thing Neanderthal females were probably more than a match one on one for human males. Humans probably generally defeated Neanderthals by better human war band co-ordination with their advanced language, not one on one. As well she’d likely be unattracted to puny and wimpy looking human males. In contrast the human female would turn Neanderthal males on often enough (if secretly), being more neotonous, lithe and gracile than his own females. While individual loser males may sometimes f*ck sheep, humans don’t band together to bring a prized sheep into their camp and screw her while their mates physically restrain her. Screwing crude ugly looking and huge Neanderthal females was probably a huge taboo, and not proud making. Not something to let the human girls see you doing. So a human male would need to do it on the sneak tip, with the Neanderthal female’s size and strength thwarted. A Neanderthal male in contrast could easily subdue a human female he caught alone, and possibly his overwhelming size even though he was thought dumb might turn some human females on.

    Human mothers kept their Neanderthal babies they conceived after being jumped a ways from camp and away from a band of humans that could gang up to ward off the Neanderthal male. Neanderthal males after the act just left the grown human female his own group would never accept. Human females birthed their mixed babies in their human band (though they probably usually died in childbirth, given the size difference), and those mixed offspring remained to mix with human mates and have their own further diluted children. Except advantageous genes that tended less to kill the mother in childbirth and also positively contributed in other ways would increase w/in the human genome as generations passed.

  11. They started with a bias against finding admixture because that’s what all the previous data suggested they would find.

    I agree that without knowing anything else, I would find lack of interbreeding surprising between two closely related humans that came into contact. But we’ve got lots of other data, including lots of Neandertal DNA sequence and there was no evidence for interbreeding. Of course the expectation was not to find admixture.

  12. In relevant examples, say hybrids of chimps and bonobos, ‘very decreased fitness’ doesn’t happen. At least not in any obvious way. If someone had said that we see noticeably decreased fitness in even one cross between mammalian sister species with comparable time depth of separation, I would call that a non-silly argument. I haven’t ever seen that argument, and I don’t think that there is any example of such a cross.

    And to all the people out on the net who are speculating about all gene flow being from Neanderthal males to human females – we don’t see any Neanderthal paternal _or_ maternal markers, so stop already.

  13. eh, if you look for evidence of mixture (as they did previously, using a bunch of nuclear loci, not just mtDNA) and don’t find it, you come up with reasons why not. obviously their prior belief about the probability of admixture wasn’t 0, or they wouldn’t have even bothered looking. so maybe your prior was 90% on admixture and theirs was 10%; fair enough.

  14. And to all the people out on the net who are speculating about all gene flow being from Neanderthal males to human females – we don’t see any Neanderthal paternal _or_ maternal markers, so stop already.

    All the gene flow doesn’t need to have been male Neanderthal to HSS female for all the Neanderthal genes in modern people being from male Neanderthals. Dudes don’t get pregnant, so kids are more likely to live with mom than with dad. Neanderthal’s lost. Genes from hybrids in mostly cro-mag groups would be better represented in modern populations than genes from hybrids living with Neanderthals.

    What’s more likely, Neanderthal women joining the cro-magnon baby-daddy’s tribe, or a cro-magnon woman pregnant (by whatever means) with a hybrid giving birth in her tribe? Unlike the sapiens man, she doesn’t have to bring a Neanderthal spouse along.

    Moderns being descended from HSS-male Neanderthal female hybrids in crog-magnon populations seems like it requires more compatibility between the two populations than the opposite.

  15. This is a nugatory observation, but Reich is a co-author on an extremely visible paper making an empirical claim of admixture. So, why would he “round up” his prior probability of admixture? It’s more than plausible that he has in fact rounded down.

    I mean, we don’t exactly expect an author on a visible and somewhat-controversial result to say in print, “I have always really known deep down that it was true; it is almost an article of faith with me, and bitter would be my remaining years on this earth if it were ever disproven. Yet I was able to be ruthlessly objective despite that.”

  16. As someone said on GNXP a while back humans do animals but only domesticated ones. Doing a neanderthal would would be like doing a wild animal. If chimps encountered bonobos in the wild they would kill them and eat them. Bonobos would be far too terrified of a chimp to mate with one in the wild.

    Neanderthals would also be terrified of humans for similar reasons and any human who tried to mate with one would have to have help holding it down or risk getting their nose bitten off. IMO the only possibilty for a European Neanderthal to mate with a human would be one that was captured as an infant and kept as a pet.

  17. Neanderthals didn’t live in China. Shouldn’t Chinese have far less Neanderthal genes than Europeans if the Neanderthal genes entered the modern human gene pool over an extended period after modern humans got to Europe?

    If the 1-4% is just those which were selected for then it seems like an awful lot of the Neanderthal genes were found useful.

  18. My guess is that the experts (people like Stringer on the one hand, Paabo on the other) never checked out the literature on mammalian hybridization, never checked out what is known about adaptive introgression (either known cases or pop-gen theory). So their priors were based on nothing.

  19. “But we’ve got lots of other data, including lots of Neandertal DNA sequence and there was no evidence for interbreeding”

    Because Neandertal want their blood pure with one drop rule. Any one with Sapiens mixing would be kicked out of Neandertal tribe. lol

  20. How can Svante Paabo and associates announce in February of 2009 that they had mapped 60% of the Neanderthal genome and found no trace of interbreeding with moderns and now they have found 1 to 4%? It wasn’t all coincidentally hidden in the remaining 40%, I don’t get it. Can someone with expertise explain this to me.

  21. My spontaneous reaction to the news about neanderthal genes was:
    “Oh no, the trolls raped her!”

  22. Something else never passed the smell test for me which supports admixture. I’ve heard talk over the years that the great leap forword was caused my one momentuous mutation. That kind of insults the complexity of the human brain and the as yet uncounted number genes that influence it. It always made a lot more sense to me at least that it was more likely caused by an influx of new genes from a hybridization event. The results of new inventive behavior would not be immediate, because the optimum combinations may take tens of thousands of years to sort out.

  23. “The argument that humans were simply too picky to have ever mated with Neanderthals is plain ridiculous: at worst, they were a lot more human than sheep.”

    I assume you get some giggles out of mentioning sheep, but it cheapens your argument and invites responses like Doug’s. Unless you were simply baiting.

  24. Sheep happen. I suppose mentioning _any_ fact cheapens my argument.

    Pretty much anything one might say invites responses from people who have no clue.

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