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Human Population Replacement as Megafaunal Extinction

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51pgrGlsM4L._SX327_BO1,204,203,200_ About ten years ago a standard model of the understanding of the peopling of the world by modern humans was that ~50,000 years ago a massive demographic swell out of eastern Africa overwhelmed, to elimination, all other human populations. With a few exceptions, such as the New World, these modern Africans quickly settled down, and the extant distributions of genes, generally mtDNA and Y lineages, reflected the long equilibration between then and now (the recent changes in the New World being an exception to that). Human genetic variation then could be understood as having been shaped by a rapid pulse expansion, and then a subsequent stabilization where genetic variation was maintained by geographic barriers across founding populations, and diminished by gene flow governed by isolation-by-distance. To a great extent this is the story you’ll find in Stephen Oppenheimer’s Out of Eden: The Peopling of the World.

To a large extent that story was wrong. Ancient DNA in particular, though not exclusively, has reshaped our understanding of the past (see Toward a new history and geography of human genes informed by ancient DNA). The ubiquity of population discontinuity and admixture suggest that it was naive to assume that modern genetic variation in any way reflects the founding stock which initially arrived in the first wave in many regions of the world. Additionally, the existence of archaic admixture in most modern lineages attests to the fact that the chasm between “them” and “us” was not perhaps quite as great as some might have claimed.

The ubiquity of population replacement is the reason I recent predicted that the first Aurignacian genome would show no relation to modern Europeans. (I was correct for what it’s worth) That is, modern humans in Europe have no special relationship to the first modern humans that settled Europe 45,000 years ago. The work on ancient DNA does suggest that modern Europeans have hunter-gatherer ancestry…but how deep does this go? I hazarded that perhaps the Gravettians are the earliest candidates for being the direct ancestors of the “Western Hunter-Gatherers” (WHG), who contribute a substantial portion of their genes to modern Europeans through Mesolithic hunter-gatherer populations. But, I wouldn’t be surprised if the genomic character of European Mesolithic hunter-gatherers was determined after the Last Glacial Maximum, ~20,000 years ago.

A new paper in Current Biology, seems to tip toward the latter conclusion. Pleistocene Mitochondrial Genomes Suggest a Single Major Dispersal of Non-Africans and a Late Glacial Population Turnover in Europe. In particular, using a confluence of “best of breed” phylogenoomic methods and archaeological dating the authors contend that there was a major turnover in the mtDNA heritage of Europeans ~14,500 years ago, during the Bølling-Allerød interstadial, a relatively mild and warm period of the Pleistocene before the sharp and harsh regression of the Younger Dryas. The big result is that some very old (pre-LGM, Gravettian) belong to mtDNA haplogroup M. This is one of the two major groups common outside Africa, but it is absent in Europe today (the Roma harbor M because of their South Asian heritage).

u5 The lineage that to a great extent has been canonical as that of European hunter-gatherers, U5, seems to have increased in frequency only late in the Pleistocene, during the above warm period. Because of the nature of random genetic drift we do expect lineage to go extinct over time. These are mtDNA, direct maternal lineages, so only one locus in the genome (though mtDNA is copious, so tends to be low hanging fruit for any new extraction technique). The combination of low long term effective population sizes and meta-population dynamics on the Eurasian fringe might mean that these are not unexpected results. But as suggested in the paper there is also a great possibility that the disruption of the interstadial resulted in some advantage to a particular subset of Pleistocene Europeans, who expanded rapidly, replacing their competitors. Many of the hunter-gatherers of the Mesolithic have relatively low genetic diversity in comparison to modern populations, suggestive of the small population sizes on the European frontier.

The expansion of U5 at the expense of other lineages though around ~14,500 years ago does seem to not be attributable purely to chance according to the models tested within the paper. Then what? One hypothesis is that the climate change resulted in extinction of many populations ill equipped to adapt to climate change, and these were later replaced by newcomers. Another, not exclusive, model is that there was conflict between different groups, and the climate change opened up opportunities for one subculture. There is an allusion to megafaunal extinction in the paper around this period…perhaps we should think of humans as just another megafauna for the super-predator cultures?

One way to look at geological process is that it is uniformitarian, not catastrophic. But it strikes me that with human demography catastrophic pulses are quite common on a geological scale. Why? Likely because cultural evolution is not quite so gradual and continuous, but that innovative revolutions and rapid sweeps of inter-group competition “thin the field,” so to speak.

The main caution I would add is that though we know a lot more than we did, we still no little. What was the ancient population structure in prehistoric Europe? Really we don’t know much, as the sampling is thin at best. That is changing.

• Category: Science • Tags: Genomics

13 Comments to "Human Population Replacement as Megafaunal Extinction"

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  1. So instead of out of Africa 50,000 years ago we have out of the Levant 50,000 years ago and a 150,000 year divide between Africans and non-African humans where both anatomical and behavioral modernity emerged out side of Africa.

    If Omo 1 is declinated as “proof positive” of anatomical modernity, then the picture becomes crystal clear of anatomical modernity originating in the Levant circa 100kya and piecemeal introgressing into Africa from a northeast to southwest direction, and not quite going to completion. Behavioral modernity is a 60 to 45 kya phenomena.

    Note that these world-beaters are all sapien/neanderthal hybrids, whereas Africans are sapien / hominid x hybrids which some suppose to be erectus or antecessor etc.

    I would like to see red deer cave DNA to see if there is another as of yet unknown hominid that accounts for the unique features of east Asians relative to other groups.

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  2. Those early Gravettian Czechs with U5 might have been ‘WHG’ but not necessarily the same WHG we are looking for. It does seem very likely though that the ancestors of WHG were somewhere in the Gravettian sphere of influence or close to it.
    I noticed quite a few people on genetics forums were upset with the idea that WHG might not be from Western Europe. It might have been a convenient theory but not a very good one.

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  3. Oase 1 is considered part of the first population wave of Europe partly because it is found near the Iron Gates, the small valley made by the Danube through the Carpatians. It was considered a way into Europe because the Carpatians were glaciered during the ice ages.

    However, there is another pathway to the west, through Moravia. I find it noteworthy that from early on we see U5 in Bohemia. What if the LGM cut this pathway off, because the cold would make it an arctic desert? We know that before the LGM, around 40.000 ya during the Hengelo interstadial Germany was a tundra with shrubs, fit for hunting.

    Now assume the following scenario: A population extending from the Don to Bohemia, of K14-like characteristics. A population in the south sort of similar of Oase 1 with mtDNA N and M. In the west they meet. During LGM the northern pathway is closed. The western refuge expands afterwards, but the Moravian pathway opens up too and more come in.

    Basques seem to have a form of U8a, according to Wikipedia.

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  4. U8a seems very rare, though, even among the Basques.

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  5. Omo is not evidence for modernity outside Africa, as it is in Ethiopia (remains from ca 195-104,000 bc., before the earliest proposed African-non African split).

    Many modern behaviors are first seen in Africa:
    The oldest projectiles(though throwing spears and not yet bows) occur ca. 270,000 bc Ethiopia (likely associated with some form of early sapiens-transitional sapiens/heidelbergensis)

    Evidence of arrowheads dates 60-70,000 in South Africa

    Middle stone age cultures of southern Africa ca 150-70,000 bc show modern behaviors , including the use of bone tools, shell beads, the trade of minerals across substantial distances ,the making of fat-based paints from ochre, seasonal fishing and shellfish use,engraved ornaments, and upper paleolithic-style stone points.
    sites; blombos, Howiesons Poort, and Pinnacle point
    e.g.:the preparation of stone for making microliths by precise heat treating to increase its flakeability at pinnacle point SA. ca 100-70,000 bc.

    A Howiesons Poort tradition of engraving ostrich eggshell containers dated to 60,000 years ago at Diepkloof Rock Shelter, South Africa

    Bone harpoons from central Africa ca. 90,000 bc.

    Modernity did not diffuse north to south, nor likely did the upper paleolithic package specifically.
    The late stone age (upper paleolithic technology) appears in South Africa at about the same time as in Eurasia(40-50,000 bc.).

    Border Cave and the beginning of the Later Stone Age in South Africa

    I am not sure what is meant by “not quite going to completion”. All cultures of Africa are fully behaviorally modern, including hunter-gatherers; Pygmies and Khoisan etc.

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  6. edit: “Many modern behaviors are first seen in Africa:”

    many of the behaviors considered most indicative of modernity are first seen…

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  7. Two minor points, 1 purely pedantic, the other harder to characterize.

    Pedantry warning The work on ancient DNA does suggest that modern Europeans have hunter-gatherer ancestry…but how deep does this go? I cannot imagine that there are any modern humans who do not have hunter-gatherer ancestry. I suspect that what is meant here is that modern Europeans can trace some of their ancestry back to hunter-gatherers who were living in (what is now) Europe when they were doing their hunting and gathering.

    But it strikes me that with human demography catastrophic pulses are quite common on a geological scale. Why? Likely because cultural evolution is not quite so gradual and continuous, but that innovative revolutions and rapid sweeps of inter-group competition “thin the field,” so to speak. OMG: are we talking about punctuated equilibrium? Sounds like it to my innocent ears (although I have to admit that it is applied to something quite different than what Gould and Eldredge thought).

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  8. Mesolithic marine resources exploitation may have involved working partly immersed in water (which chills about 15 times faster than air). Maybe there was mtDNA selection for working in the cold and wet, or even the most efficient use of fish oil for energy. Doggerland’s abundant resources are a good bet for an expansion, and maybe there was an invasion of the sparsely populated higher ground as Doggerland sank. Someone extirpated the Motala HG DNA, and hard times make hard people. “Most historical examples pertain to colonial representatives mounting the skulls of murdered natives”

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    The paper itself is largely an account of the detective work involved in pinning down a specific mutation which has been positively selected for in a Siberian population living in the Arctic. The same mutation is also present in non related groups inhabiting the Arctic areas of northern America. The mutated gene is very common and frequently homozygous. It puts a leucine in the place of a proline in CPT-1a, the core enzyme for getting long chain fatty acids in to mitochondria. Putting a leucine where there should be a proline means the protein is basically f*cked. The mutation is linked, not surprisingly, to failure to generate ketones in infancy and can be associated with profound hypoglycaemia, potentially causing sudden death.

    From the evolutionary point of view we have here a mutation which is significantly lethal at well below reproductive age, so it should have been weeded out because affected individuals are less likely to live long enough to pass on the gene. But it has been highly positively selected for in several populations, the common factors being cold climate and minimal access to dietary carbohydrate. It’s a paradox.

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  11. “Note that these world-beaters are all sapien/neanderthal hybrids”

    Also, the “world-beaters” of Africa were not such hybrids, as they began in one region of the continent spread (over/within the continent), replacing its other hominins (with minimal admixture).

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  12. Razib, can you suggest a more recent book than Oppenheimer’s for a non-specialist, or is the state of knowledge changing so rapidly that I should wait a few years?

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  13. The revolutionary implications of Mr. Khan’s take on culture and population are highly reassuring to me personally. Wish I had more time to comment lucidly on the matter, but for now I’ll just say that it’s really nice to see someone with some quantitative knowledge of the ebb and flow of humanity assert that the status quo is ephemeral. I’m so tired of the biblical “Out of Eden” bias that has plagued anthropology for decades that it makes me despondent at times.

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