"Multiregionalism vs. Out of Africa"

By Razib Khan | April 21, 2010 1:06 pm

John Hawks has a post up, Multiregional evolution lives!, in response to Rex Dalton’s reporting on Neandertal-human admixture. He notes:

These ongoing studies are concluding that present-day genetic variation is inconsistent with a simple model where a random-mating ancestral population gives rise to today’s global population by means of a staged out-of-Africa dispersal. They next look at a model with some substantial (possibly complete) isolation between ancient human populations followed by a subsequent out-of-Africa dispersal. They show that this model fits the data significantly better.
So far, so good.

For a moment, I’m going to adopt a critical perspective. Previous results haven’t yet been able to answer an important possible question: Can they distinguish the effects of intermixture outside Africa from an ancient population structure inside Africa? Increasingly it looks like population structure inside Africa may have been very important to the evolution of Late Pleistocene Africans. How can we distinguish these kinds of structure from each other?

The short answer is that maybe we can’t, yet. Human population history was not simple. If we take a simple model and add more parameters, it will fit the data better. The question is whether there may be some even better model with the same number of parameters. Population structure within Africa, selection on some loci but not others, asymmetrical migration — all these and more might be possible.

The Out of Africa + total replacement model had a clean elegance, but it might not be viable in the near future. That being said it seems to me that the old Multiregional model implied, though proponents were often careful to reject this characterization, more regional parity than was the case. I do not expect the predominant African ancestry of modern humans to be rejected for example. There are other frameworks out there, such as Alan Templeton’s Out of Africa again and again (Richard Dawkins favors this in The Ancestor’s Tale).

CATEGORIZED UNDER: Anthroplogy, Genetics, Science
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  • pconroy

    As I’ve predicted previously, modern Bantu populations are probably a product of Out Of Africa, multiregional admixture in Eurasia/America and then a Back Into Africa admixture with more Archaic Moderns remaining in Africa

  • MW

    Out of Africa is at least 98% correct – we’re only arguing over where in the 0-2% range the non-African contribution to our genetics is. I wouldn’t describe multiregionalism as being alive on this basis.

    Given that we overlapped with Neandertal by >10,000 years in Europe, it is natural to expect some introgression, however I found the review paper http://genomebiology.com/2008/9/2/206 pretty convincing in showing that there has been very little introgression.

  • pconroy

    MW,

    We may be arguing about just 2%, but remember that you are probably less than 2% different from a Neanderthal, and a little bit more from a Chimp…

    You also might be 90% similar to a mouse – so what’s your point exactly?

  • MW

    I’m talking about a different 2%: if you followed the ancestry of a given nucleotide, up to 2% of the time the ancestry would pass through a neandertal or erectus, the rest of the time it comes via the expansion out of Africa. For the well over 99% of the genome which is identical between us and neandertal and erectus, you can’t tell by sequencing which way it came, but the bases which do differ are like a random sample for which you can tell. From the bases where you can tell, you infer what proportion game by which route for the bases where you can’t tell. (Putting it another way: of the places *where the African diaspora humans differ* from non-African archaic humans, less than 2% match the non-African sequence.)

    The point being that the multiregional hypothesis in its original form (and I’d argue in any form worthy of the name), is dead in the water. http://en.wikipedia.org/wiki/Multiregional_hypothesis, first diagram. This is ‘erectus evolved into modern humans in parallel (via gene flow) in many parts of the world.’ Under this hypothesis, if you’d looked in Asia 100,000 years ago, you’d have found hominids much closer (genetically, phenotypically) to modern than to classical erectus.

    The current possibility – modern humans expanded out of Africa, and picked up a few ‘archaic’ genes by introgression – does not fit this model. Although I’m now arguing about words rather than arguing about the world, I’d say that calling this a ‘multiregional hypothesis’ rather than ‘out of Africa with a minor adjustment’ is not sustainable. It would be like a country claiming victory in a war because despite losing 90% of their territory, they got to keep their capital.

    I’ve long supported Out of Africa (you may have noticed) but I never felt that was inconsistent with limited introgression from Neandertal and just maybe erectus. It is only due to papers I’ve read in the last few months that I’m no inclining towards there being no or very minute introgression.

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    Given that we overlapped with Neandertal by >10,000 years in Europe, it is natural to expect some introgression, however I found the review paper http://genomebiology.com/2008/9/2/206 pretty convincing in showing that there has been very little introgression.

    wait a few months, there might be other data which bear upon this…. (there may be a reason that someone at max planck had some positive things to say about research presented here)

    i agree that probably out of africa with modifications is a better term. also, the non-genetics people, like chris stringer, never discounted some admixture.

  • Jeff

    Response to MW:

    Obviously the story is very complicated. We’re talking about at least 2 million years of back and forth and all around. To say that Multi-regionalism is dead in the water or that Out of Africa is the answer is to vastly oversimplify the situation. You can just as easily oversimplify to fit whatever hypothesis you favor. The facts are that both models have validity and that we still do not know enough to say much about what actually happened. As much as you favor Out of Africa the facts are that things are much more complicated than that. There are obvious facts that support Multi-regionalism both in Asia and Europe. The story in Africa is very complex and again the facts seem to point to a lot of out and back in to Africa. It now seems likely that H. erectus evolved in Asia. So we are left with the desire to know more.

    Jeff in Chicago

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    The story in Africa is very complex and again the facts seem to point to a lot of out and back in to Africa. It now seems likely that H. erectus evolved in Asia. So we are left with the desire to know more.

    i think i disagree with this. there might be a story of hominin migration back to africa, before the out-of-africa event. but to a first approximation i think out-of-africa is a good and useful simplification.

  • farang

    “i think i disagree with this. there might be a story of hominin migration back to africa, before the out-of-africa event. but to a first approximation i think out-of-africa is a good and useful simplification.”

    Now stop and think: why would hominids evolving to conditions (quite favorably, especially in S.E. Asia, migrate back into Africa?

    Makes absolutely no sense. None.

  • ed

    Some of this material is a little above my head—but I would say this from what I have gleemed from past arguments. There are some people who would argue that their was coincidental evolution of two similiar human species in Asia and Africa long after the very first hominids left Africa but long before the so called “Adam and Eve” migration about which Spencer Wells has written so much. From what I can see the evidence just doesn’t support this coincidental evolution of our species. I simply don’t believe that the genetic variation within the human population would be so small if coincidental evolution was an accurate theory.

  • http://jqjacobs.net/anthro/paleo/index.html James Q. Jacobs

    Brief summations of three important articles in the modern human origins debate are fond on my web site. The third pole in this discussion is the demographic perspective. A bottleneck in human population occurred and must be accounted for in any model/hypothesis. Ambrose’s article places the two camps in a new perspective.

    The mathematical modeling of evolutionary shifts offers valuable perspective on use of present genetic traits to infer past genetics. Understanding demographic mathematics while comprehending the time depth offers new perspective on how limited our understandings are. The great time depth involved results in weak inferences at best. Archaeological evidence is less subject to equivocation while also extremely limited, but at least not limited to just present-day evidence. I suspect, new evidence will continue to fuel this debate and add omplexities for a long time before it resolves the question.

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About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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