The origins of modern humanity: 2012

By Razib Khan | August 18, 2012 6:26 pm

Matt Ridley has a column up, Did Your Ancestor Date a Neanderthal? In it he juxtaposes the two recent papers which got some attention on the relationship of modern humans to Neandertals. To be frank I think it took too much of the “two sides/opposing views/views differ” tack. From what I can gather the scientific consensus is moving further toward admixture, rather than ancient structure. Additionally, as John Hawks states flat out the PNAS paper which casts doubt on the strength of the admixture hypothesis is not the strongest of contributions to the field(in line with David Reich and Nick Patterson’s assessments, and others who shall remain nameless).

But it’s hard to keep track of all the threads, isn’t it? One of the most tragic aspects of the death of classic “Out of Africa” is that we lost an exceedingly simple narrative which had the positives of clarity and precision. The basic outlines were reducible to one sentence: all humans today descend from a small group of modern humans alive ~100,000 years ago. At this time we do not have such an elegant replacement framework. In fact, we may never have such an elegant model which also has the merit of being true. Instead of a fully fleshed out model we have a set of disparate facts, which we can assert with various weights of strength.

First, we now have evidence for admixture with humans diverged from the main stem of anatomically modern humans. Both outside of Africa and within it. An alternative explanation is that these signals of admixture may actually be a reflection of ancient population structure within Africa, carried over to the modern era. So that the African ancestors of non-Africans happen to have been genetically closer to Neandertals even before they left Africa. The likelihood of admixture within Africa makes me willing to accept that structure is important, but the new results on dating of hybridization of Neandertals with the Africans also makes me convinced that admixture did occur outside of Africa. In other words, we may need to move beyond conceiving of this in an either/or fashion.

Second, there is the story of structure within Africa. We’re just scratching the surface, but today I’m pretty sure that the thesis of admixture with non-modern lineages will pan out (this is partially contingent on the stronger evidence outside of Africa because of reference sequences of ancient genomes). And, there is now a fair amount of evidence of “back migration.” Several groups have found evidence for Neandertal ancestry within the Masai of East Africa. For me the wild card is when this admixture with Eurasians occurred.* If most back migration to Africa is after the Ice Age, then our model is far simpler. On the other hand, if there were several ancient back migrations, then it gets far more complicated. I suspect that the Khoisan, and perhaps the Eastern Pygmies of the Congo, are keys to shining a light on these questions. If any populations are appropriate references for “pure” Africans, these would be it I would think.

A few years back a reader suggested that the Multi-Regionalists were wrong on the facts of modern human origins, but right on the process. Instead of a singular speciation event ~50-100,000 years ago, modern human as we know it may have evolved in pulses, through a process of synthesis and selection.

* This is may be an artifact of ancient structure, but that can probably be settled by looking for signals of linkage disequilibrium in the future with high coverage genomes.

MORE ABOUT: Human Evolution
  • Doug Alder

    Given that hominids, as a whole, are a pretty randy family, it has always struck me that if there was any cohabitation of a landscape the odds of there not being interbreeding seem pretty slim.

  • Contemplationist

    Is there an estimate we can use? Say 70% of modern humans out of Africa?

  • Dallas

    I second Doug’s point. I don’t find it too surprising* that with the rise of genomic analysis the simplicity of Out of Africa fell apart. Given that, on average, it takes something like 3 million years for two closely related mammal species to lose their ability to hybridize, it almost seems like introgression should be more likely than not, unless speciation is occurring via strong allopatric barriers. I know some folks who think introgression will end up being recognized as an incredibly important part of the evolutionary history of at least some groups**. It’ll be interesting to see how things turn out as more genomes get sequenced.

    *Although to be honest, before the Neanderthal genome was published, I hadn’t questioned it much. But in hindsight…
    **The folks in question studied crocodilians. For reptiles and birds, I’ve seen data that suggests it takes something like 20 million years of divergence before hybridization becomes inviable, and in some cases even with large differences in karyotypes. So, for at least non-mammals, they maybe be right.

  • AMac

    Re: Dallas #3 — The role of chromosome number in speciation seems inadequately appreciated. To me. The extant primates have widely divergent numbers of autosomes. Considering speciation, how is the individual with the novel diploid count going to reproduce; who will be her partner?

    Re: Neandertals, has their karyotype been deduced from whole-genome sequencing? If WGS is complete enough, this should be possible by counting centromeric and telomeric regions. But, due to the problems with getting a clear picture with repetitive DNA (e.g alpha-satellite repeats at the centromere), I suspect this can’t be deduced. Yet.

    A pointer to a recent overview of this subject would be appreciated.

  • martin lavin

    I am inclined to favor any messy scenario as more likely than a parsimonious narrative.
    The Virgin Mary not withstanding.
    Admittedley a personal predjudice but one that has been confirmed by my narrow non multiverse experience. If one concedes we are stil finding evidence of other “humans”, most recently Denisovians, one can pretty much assume there are many others out there to discover, and to the peril of the narrativians , probably many more in Africa that we will never discover due to the conditions. So for a Chaositician these are fine days in the AOT.


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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


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