Africans aren't pure humans either

By Razib Khan | September 5, 2011 3:38 pm

Last year when discussing the possible admixture of Neandertals with the ancestors of modern non-Africans I joked that Sub-Saharan Africans were “pure humans.” This was tongue-in-cheek in part because the results from the Neandertal genome shifted my assessment of the probability of archaic admixture within Africa as well. In other words, there may never have been a pure “human” type which expanded and assimilated archaic ancestry on the margins of its range. Species Platonism may be very misleading for our particular lineage. Rather, what it means to be human has always been in flux, a compromise between extremely different ancestral components.


For years some groups of researchers have been arguing that there is population structure within Africa itself which hints at admixture events before (or after?) the “Out of Africa” event. Genome blogger Dienekes Pontikos has been discussing this possibility for several years as well. With the possibility of archaic admixture outside of Africa it was inevitable that people would revisit their earlier exploration of ancient African admixture and the modern patterns of variation which that might explain. Finally one of the groups working on this has come out with something in PNAS, Genetic Evidence for Archaic Admixture in Africa. Unfortunately it’s not on the website, and I’m not privy to the embargoed copy, so I can’t say much. ScienceDaily and Nature have lengthy write-ups. The details are pretty straightforward. The authors infer using computational methods that there is a 1-2% admixture in Africans of a population which diverged from the mainline of the human ancestral tree ~700,000 years ago. The hybridization occurred on the order of ~40,000 years before the present. The proportions are highest in Central Africans. I assume that this means Pygmies. And I would further bet that the admixture is highest in the Eastern Pygmy populations, such as the Mbuti. The lead author also cautions that this may not be the last word on admixture. No doubt. There are other groups breathing down his neck.

If this is true then a assimilation model of the expansion of H. sapiens sapiens looks more and more plausible. The time period of admixture is pretty much what other scholars are estimating for Neandertals, and presumably Denisovans. I’m not smart enough to figure out how this could be a statistical artifact, but perhaps that explains the congruence? Otherwise, if this is true then you had several repeated events of expansion of one particular lineage (what I term “Neo-Africans”) which demographically swamped the indigenous populations, but still retained a faint, but discernible stamp of their distinctive genetic content. But this may not be exceptional. It may have happened before the emergence of Neo-Africans, and I believe it happened after them (e.g., the rise of agriculturalists). It’s possibly one instance of a rather banal dynamic in the evolution of Homo.

  • Adrian

    Just yesterday I was watching this TED talk by Svante Paabo where he comments on the possibility of archaic admixture within Africa. If you haven’t seen it here is the link:

    http://www.ted.com/talks/svante_paeaebo_dna_clues_to_our_inner_neanderthal.html

  • Milford Wolpoff

    why not just come to the understanding that all human populations have mixed ancestry?

  • http://genealogyreligion.net/ Cris

    I am starting to think that Professor Wolpoff’s suggestion should be the null hypothesis. The deluge of data to this effect aside, it certainly corresponds with what everyone “simply knows” about human sexuality.

  • Charles

    I think that now is not a good time to bring this up. I’m a white guy and this is offensive to me, and I am sure offensive to many people of African blood. We are all amixtures of various human species. I do not buy into the ides that there is any ape involved. A human is a human and has always been a human. Period!

  • Jimmy

    Hey everybody! Shut down science, Chuck is getting offended.

  • Andross

    Charles, you didn’t read any of the articles linked and prematurely jumped to conclusions. Nowhere did anyone claim there were any pure humans; quite the opposite, it has been found that there is no such thing as a “pure” human, as the various populations, particularly in the north, all have traces of neanderthal.

    And where did you get ideas about apes? Archaic humans are descendants from the homo erectus as well, and are genetically close enough to the modern human to procreate with them.

    Curiously, the difference between a native African and native Asians/Europeans are large enough to classify them both as different subspecies.

    One more thing, being “offended” has little regard to the research being conducted here. It’s dangerous to let political correctness dominate the findings, as that’s a guaranteed way to obscure the truth.

  • http://mengbomin.wordpress.com/ Meng Bomin

    I have to chuckle a bit at Charles’ righteous indignation. Obviously, this is an area of fuzzy terminology, but his last two statements:

    I do not buy into the ide[a] that there is any ape involved. A human is a human and has always been a human. Period!

    only make sense in a Creationist worldview. I’m generally skeptical of criticism on the grounds of offensiveness, but when it’s further based upon a faulty model of reality, it’s pretty easy to ignore.

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

    i thought the offended individual above was joking?

  • ElamBend

    Is Chuck calling us monkeys!? I refuse to be downgraded from great ape!

    @andross: I cAn’t remember but aren’t ‘europeans’ closer to Africans than to east Asians.

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

    I cAn’t remember but aren’t ‘europeans’ closer to Africans than to east Asians.

    no.

  • ISayPorter

    @ElamBend Only geographically.

  • http://washparkprophet.blogspot.com ohwilleke

    The notion that Dienekes and others have sometimes advanced that we will keep on finding new admixted populations that add to the mix fails to appreciate how the methodology of the African study differs from the Neanderthal and Denisovian one. The later two involved ancient DNA comparisons with modern genomes. The African study looks at parts of modern genomes that look due to their arrangement ancestral. Thus, the African study should be a pretty comprehensive view of all of the ancient admixture we should expect to find in Africans as a whole, however many source populations they derive from.

    It is looking like the African ancient admixture was confined to two or three kinds of archaic hominins at most, at levels a bit lower than seen in the Neanderthal case during that admixture period and a lot lower than seen in Melanesians with Denisovian admixture.

    Given the time scale of modern humans Out of Africa, the similarity of Eurasians genetically, and the archaeology, it would be very surprising to find more than one more Eurasian admixing population (and there might not be any more). And, in the African case, it seems as if all or all but one of the ancient admixtures happened with relict subpopulations that are not ancestral to Eurasians or even to the vast majority of Africans to any great degree. And, since modern humans have lived so much longer in African than anywhere else, and the African methodology is not hominin species specific in the same way, there probably isn’t a whole lot of room for surprises there either. I could imagine one or two very low frequency relict populations showing signs of a new hominin species or two admixing with AMHs there beyond what this study with a somewhat limited diversity of African sample shows, but probably not much more.

    Indeed, for all of the admixture events that have been documented, very few people in the world seem to have more than two traces of ancient admixture – Neanderthal for all Eurasians and possibly one hominin species that made its way into an African subpopulation that ultimately thrived. The remaining events seem confined to Khoisan, Melanesians, Pygmies, and people who trace some ancestry from one of those populations. There isn’t even that much overlap between European and Asian Neanderthal inheritances.

    The “admixture era” in modern human history seems to have been rather brief, and it also remains the case that not one of the modern mtDNA or Y-DNA lines appears to have archaic origins. In the Khoisan and Pygmies it seems like a few millenia in the time frame ca. 75kya +/- 25ky was probably the key one. In the Neanderthal and Denisovian cases the overlap was probably in some part of the time frame 50kya to 30 kya, perhaps a few millenia. The evidence of older or younger admixture eras seems pretty weak.

  • chris w

    ElamBend: I cAn’t remember but aren’t ‘europeans’ closer to Africans than to east Asians.

    Razib: no.

    Me: Looks like slightly but not much?http://upload.wikimedia.org/wikipedia/commons/a/a9/9_Cluster_Tree.png

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

    #13, i’m pretty sure he meant that europeans form a clade with africans vs. east asians. they don’t. but genetically europeans are closer to africans than east asians are to africans. but europeans and east asians are closer to either than either is to africans.

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Gene Expression

This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

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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