Non-zero sum transitions in the human past

By Razib Khan | August 1, 2011 3:56 pm

A few people have pointed me to the recent paper in Science, Tenfold Population Increase in Western Europe at the Neandertal–to–Modern Human Transition. The basic result is obvious, and not totally revolutionary: anatomically modern humans may simply have demographically absorbed the Neandertals (the word “absorbed” has many connotations here obviously). The results are clear in this figure:

This is not surprising, even though I have only a glancing familiarity with the guts of paleontology I was aware that there’s a lot of inferential evidence that Neandertals were not as efficient at extracting resources from any given piece of territory as modern humans. In The Dawn of Human Culture the paleoanthropologist Richard Klein offered a straightforward biological explanation for why and how the neo-African populations so rapidly marginalized Neandertals: some sort of macromutation which allowed for language and so the protean flexibility of human culture.


Implicitly this was the conventional wisdom until the likelihood of Neandertal admixture was discovered, and earlier the fact that Neandertals seemed to share the derived FOXP2 variant (the “language gene”). The earlier inferences of human demography using mtDNA, and later ancient DNA extractions from mtDNA, also seemed to align well with the proposition that there was a clean replacement of Neandertals across their range by modern populations (and the Y and autosomal results were supportive of this model too).

In hindsight it may be that these earlier genetic methodologies simply lacked the power to smoke out low levels of autosomal admixture on the scale of a few percent. Uniparental lineages have a tendency to fluctuate and go extinct fast, so the lack of the Neandertal lineages in modern populations may be a specific instance of a broader tendency whereby the vast majority of ancient lines of ancestry are extinguished.

So what smoking gun did the neo-Africans have which allowed them to be fruitful and multiply? The demographic assimilation by massive numbers is clear all across the world; there’s a reason that the “Out of Africa” model has found broad support in both morphology and genetics. Not only did the neo-Africans swarm over the northern and eastern landscapes dominated by other hominin lineages, but they were moving into the territory of others. This is quite the mean feat.

But today I think we need to revisit both the idea that Neandertals were qualitatively differently from us in a deep species-level sense, and that there need be one smoking gun to explain it all. That’s because I think we have many cases of more recent replacements and assimilations on the scale of that of the Neandertals. In the New World Europeans and Africans have replaced and assimilated the indigenous population in many regions which were ecologically suitable. In places like the Dominican Republic indigenous ancestry does persist at low levels, especially in the mtDNA, but it is not longer salient or culturally relevant in a concrete (as opposed to symbolic) sense. There were major biological differences between these Old World populations and the indigenous ones, mostly having to do with susceptibility to disease. Still, we can not separate biological advantages of the new populations from their cultural context. Malaria resistance amongst Africans became prevalent only with the rise of agriculture, as broad swaths of wilderness were cleared and transformed into farmland which was a superior environment for the mosquitoes which transmitted the pathogen. Similarly, the various infectious agents to which Europeans were inured spread via long distance contacts, which could exhibit a scale in Eurasia unmatched in the New World thanks to the emergence of a genuine ecumene.

The Columbian Exchange looms large in part because it is well documented and concerns Europeans, but genetic, linguistic, and archaeological data from Southeast Asia strongly implies that the ancient hunter-gatherers of both the mainland and the maritime zones have been assimilated by successive waves of agriculturalists issuing from the margins of southern China. There is also now evidence of massive population shifts in Europe and India due to the spread of agriculturalists. If an alien archaeologist examined the data I do think they might posit that were a biological speciation events which might explain this, as new traits arose which allowed the farming population to expand and replace the hunter-gatherers. Some of this is actually straightforwardly plausible. Consider the spread of lactase persistence or the ability of farming populations to digest amylose. But neither should we ignore the possibility of biobehavioral changes over time. Last year reading about the cultural history of Australian Aborigines I was struck by accounts of missionaries as to the different experiences in Polynesia, Melanesia, and Australia. In the two earlier cases the common strategy had been to convert “big men,” which immediately brought over whole populations. This simply did not yield in Australia, where missionaries had to target on a much finer-grain, as members of an individual band were just as liable to assume that their leader had gone crazy if he converted to Christianity than not. The leaders of these bands did not have the social capital to enforce group level cultural change. In contrast the populations of Polynesia and Melanesia had very different structures and organizations for thousands of years (e.g., agriculture) which were much more top-down than those of Australia. And it may be that not only did deeply embedded values differ, but the average personality profile may have shifted due to cultural selection upon the extant standing variation in the trait.

If there was a great leap forward to behavioral modernity ~40,000 years ago, then I think one should logically assert that there was another “great leap forward” ~10,000 years ago in the Middle East with the first farmers. There was also another “great leap forward” ~5,000 years ago ago with the invention of writing. There was another “great leap forward” ~300 years ago in Western Europea with the crystallization of a genuine scientific community.

I’m not actually suggesting that what happened 10,000 years ago was a speciation event. What I’m suggesting is that the near past may be more similar to the distant past than we imagine. This makes the near past more exotic, and the distant past less exotic.

  • http://abitmoredetail.wordpress.com Randy McDonald

    The question, then, is what technological innovation allowed homo sapiens sapiens to flood out of Africa and to assimilate the territories and gene pools of all the other hominins. Ideas?

  • dave chamberlin

    Richard Klein wrote “The Dawn of Human Culture” back in 2002 when it was generally assumed, incorrectly as it turns out, that there was no interbreeding between modern humans and Neanderthals. Now that we know that we are 1% to 5% Neanderthal it seems highly likely that we hybridized, we benefited with increased abilities in key mental areas and this created “the great leap forward” not some macromutation. Not only were Neanderthals different than us, we were different than us, before we benefited from their influx of genes. We interbred with Neanderthals who quite obviously had their own bunch of intelligence enhancing genes and we benefitted from them. This line of reasoning pisses some people off because Africans south of the Sahara don’t have any Neanderthal genes. It isn’t sly racism, we would have replaced the Africans if we were superior to them, but we didn’t so we are not.

  • Torbjörn Larsson, OM

    The broad view seems predictive, this is good stuff.

    Neandertals seemed to share the derived FOXP2 variant (the “language gene”)

    And before that I believe there was a Neanderthal hyoid bone that predicted language use.

    Not only did the neo-Africans swarm over the northern and eastern landscapes dominated by other hominin lineages, but they were moving into the territory of others. This is quite the mean feat.

    I have a problem here, probably of understanding as I’m no biologist and have to resort to handwaving.

    As I understand it, one of the more lasting Neanderthal contributions were in traits that adapted carriers to local climate (genes for fair skin) and disease (immune system genes). In fact, I believe I read it here.

    Assuming the genes survived after introgression by being selected for (so no later bottleneck fixation), the climate genes looks like a fairly easy win. You wouldn’t die from lack of D vitamin, so it is compatible with the given population scenario.

    But the immune system genes would come with the spread and survival of presumably nasty local disease.

    That seems like a “mean” handicap to overcome, especially without loosing the population advantage. All the while presumably having a larger population density, frequency of groups meeting, undergoing migration, et cetera, which should promote disease.

    What am I missing? Is it perhaps the larger population that made this more selective as larger population makes selection more effective? And the new diseases were not in general “nasty”?

  • Torbjörn Larsson, OM

    @ Randy McDonald:

    FWIW, I think the question may be the exact opposite of the “no smoking gun” hypothesis offered.

    Anyway, why does it need to be the same reason? Migrations would happen if you travel. Anyone with two feet can travel, there is no innovation necessary for that.

    For the later assimilations, plenty of ideas were offered in the article.

    Perhaps the question is, why were there clusters of migration? IIRC, offered explanations have been climate and population size barriers for long periods.

    @ dave chamberlin

    This line of reasoning pisses some people off because Africans south of the Sahara don’t have any Neanderthal genes. It isn’t sly racism, we would have replaced the Africans if we were superior to them, but we didn’t so we are not.

    You could take that further. They are genetically more diverse, so evolutionary more robust.

    Unless you can make a case that the bottlenecks of out-of-Africa selected useful genes. Maybe it is the old “potato, potatoe”.

  • melqart

    The problem with the idea of two commenters above that Neanderthal genes added something cognitive to the genome is that there is no demographic event ensuing from Neanderthal hybridization – no demographic surge that seems to be related to hybridization. All we know is that independent Neanderthal lines die out. That’s a pretty weak basis for saying we gained something from it. The point of Razib’s post is that these repeated replacement events probably reflect strengths of the replacing populations, not strengths of those that got replaced.

  • Sandgroper

    Yes, there is no evidence for any of those things.

    The great leap forward was not real.

    http://www.youtube.com/watch?v=C2cMG33mWVY&ob=av3e

  • Sandgroper

    Seriously.

  • dave chamberlin

    #5 and 6) I’m no great enthusiast for the “great leap forward” either, that is why I put the concept in quotes. It seems from our present day view that there was a great leap forward at least from the standpoint we seemingly went from art that was nothing more than patterned scratches on an ostrich eggshell to beautiful cave paintings that humbled and inspired Picasso and stone tools that went from scrapers and choppers to a veritable tool kit, but we don’t know what happened or when it happened because the evidence is so scarce. All we know, and it admittedly isn’t very much, is that 1% to 5% of our 30,000 genes derived from Neanderthals. That is a very significant number of new genes and it is highly probable that we significantly benefited from at least some of them. As of now that is just an unproven hypothesis, it is too damned bad we are discouraging rather than encouraging brilliant scientists like Bruce Lahn who found the gene Microcephelin(MCDH1) that hopped into the human gene pool 37,000 years ago and in all probability derived from Neanderthals and increased in frequency in modern humans too rapidly to be comparable with neutral drift. I suppose it could be a coincidence that the first signs of modern human behavior first occurred in Europe in those locations where we were replacing Neanderthals, but I doubt it. I understand that our interbreeding with Neanderthals had to have happened earlier in the middle east for the rest of humanity north of the Sahara to possess Neanderthal genes, but I find it highly unlikely that we did not benefit intellectually from the influx of new genes from neanderthals.

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

    some minor points

    1) the few pigment genes known are different. probably functionally similar, but not the same as the european ones around today (also, loss of function toward depigmentation isn’t that hard).

    2) the immune stuff is very new, and not even published. so don’t stand on that too much.

    3) the idea of introgression and picking up locally favorable alleles seems plausible.

    4) last i checked microcephalin was not introgressed from european neandertals. the neandertals had a different variant.

    anyway, it’s a unsettled field. kind of hard to say anything stupid since so little is known with solidity IMO.

  • Wonks Anonymous

    dave chamberlin, it was my impression that the methods used to detect Neandertal ancestry by their nature were unable to determine if sub-saharan Africans have any. They looked at those genes found only among non-Africans, checked to see if Neandertals had them, and that way were able to conclude Neandertals were the source. For genes found among both Africans and Neandertals, we can’t tell how each group got them.

    I could be misrepresenting the research, but I recall “The 10,000 Year Explosion” saying that Africans did probably also have Neandertal genes that filtered their way back and underwent selection.

  • John Roth

    There’s a recent hypothesis that rather neatly explains the “great leap forward.”

    Grandparents.

    The article is a puff piece from the Telegraph that mentions an article in Scientific American.

    http://www.telegraph.co.uk/science/science-news/8647686/Few-grandparents-until-30000-years-ago.html

    This moves the question from “intellectual leap forward” to “longevity,” an entirely different issue. Unfortunately, since it’s from the Telegraph they don’t give a pointer to the actual article. At first blush though, the science and the inference seem sound.

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

    i have the original article. i’ll review it tonight.

  • dave chamberlin

    10)Wonks. Since “The 10,000 Year Explosion” was written the Neanderthal genome has been decoded enough to conclusively tell us that all of humanity north of the Sahara has 1% to 5% Neanderthal ancestry but Africans south of the Sahara have no Neanderthal ancestry.

  • Sandgroper

    Dave – “the first signs of modern human behavior first occurred in Europe in those locations” – that isn’t true. There is really rather sophisticated Australian Aboriginal rock art of recognisable animals, including likely megafauna, that is considerably older than the oldest cave art in Europe (the oldest documented and reliably dated is c. 40,000 yBP).

    Unfortunately, many Australian rock art sites are not well curated or even particularly well documented (there are really a lot of them, many in very remote locations). Locations are also kept secret to try to avoid vandalism, which is distressingly all too common.

  • Eurologist

    @John Roth:

    “There’s a recent hypothesis that rather neatly explains the “great leap forward.”
    Grandparents. ”

    That’s actually a fairly old argument, although, admittedly, it was rather qualitative and the new work improves on that. Longevity fosters longevity, since it reduces childhood and teenage mortality through experience such as knowledge of medicine and climatic fluctuations, and promotes risk-aversion (via painful memories of loved ones lost), among other reasons.

    @ dave chamberlin:

    I generally agree with your sentiment, but I think the “useful Neanderthal gene introgression” was of much longer-duration and wider spatial scope. In reality, AMHs were able to leave Africa ~120,000 – 130,000 years ago, but the apparent quantum leap did not take place until ~40,000 – 50,000 years ago. So, there was plenty time and space for admixture within N and NE Africa, the Levant, and later the wider West Asia and the Indian subcontinent.

    Also, I believe admixture outside the immediate Near East was with more generalist Heidelbergensis-like people, not with highly-specialized Neanderthals.

    Finally, substantial gene flow between Europe, West Asia, and North Africa until about ~300,000 years ago appear likely based both on climate and fossil studies. As such, North Africans likely had “benefited” from admixture even before AMHs arose (and vice versa).

  • dave chamberlin

    @Eurologist
    You have to be right that the useful Neanderthal gene introgression was of longer duration and wider scope. If I left the impression that it happened quickly in one place than I’m glad you corrected me. It is also an excellent argument against strict adherence to the great leap forward idea.
    @Sandgroper
    You can accuse me of subjective opinion on this one but I think the artwork of similar age in the caves of France and that of rock art in Australia are not comparable. The work in several locations in France is of incomparably greater detail from a realist point of view and from an aesthetic point of view knocks the socks off of professional artists even today. Maybe if Australia had been so lucky as to have their master artists work sealed up and preserved up to this century, like happened in France, I would change my opinion. But your point is well taken, not only were the Australians of this time period making wonderful art they were incredibly competent hunters. Driving to extinction 90% of the megafauna in a very brief time 40,000 plus years ago clearly indicates modern competence. Maybe I should amend my earlier comment to say the first signs of modern human genius as we recognize it today occurred in Europe in those locations, but who knows, time has erased so much and left so little.

  • Sandgroper

    More likely burning than hunting – or fire stick hunting. Whatever.

    “the first signs of modern human genius as we recognize it today occurred in Europe” – Remind me again of the incomparably greater detail with which human anatomy was depicted in the >5,000 year younger cave paintings in France than in the rock art in Australia, would you Dave? Put yourself in my place – I’m in outback Australia 40,000 years ago, and I develop appendicitis. What do I need, some sensitive genius painting realistic depictions of animals, or some Aboriginal guy with a tool kit of very sharp rocks and a wall chart depicting the internal organs of humans?

    In any case, this is not a pissing contest between Europe and Australia – the point really is that by 40,000 years ago, cave and rock art were already highly developed in both continents on opposite sides of the world, from much earlier starting points that already revealed behavioural modernity and symbolic thought. I look at that and see progression over a long period of time, not some rapid cultural explosion. What encourages belief in the great leap forward is absence of evidence, which is increasingly no longer absent.

    There could be something in the latest Caspari paper to explain some sudden cultural leap, but I don’t have access.

    To associate that with Neanderthal introgression is pure speculation, as is pale complexion and greater brain power, and the evidence is looking weak or non-existent. The question is then well what did we get from them that was advantageous, if anything? I think we want some pretty hard evidence about this, not pleasing speculation to ponder, when such evidence as there is suggests that there appears to be nothing real in it so far.

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

    To associate that with Neanderthal introgression is pure speculation,

    come now, that’s just a rhetorical stab. a lot of scientific hypotheses differ from pure speculation only insofar as you think it might be true or not, evidence (or lack of) be damned.

  • Insightful

    Papuan New Guineans and Australian Aborigines also have Neanderthal gene introgression and they are dark-skinned. There is so much rancor speculation out there about what these supposed gene contributions meant to Africans coming out of Africa that it is bordering on fantasy. I wish folks like Torbjörn Larsson would study the facts before muddying the water even more than it already is..

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

    #19, minor note, you mean admixture. introgression usually implies specific genes which are decoupled from the overall genome. so for example the east asians who may have denisovan immune alleles are good candidates for introgresssion; the total genome isn’t reflective of mixing, but at specific genes there may be. so you have very little admixture overall, but a lot of introgression on specific loci. the neandertal admixture story was more about the total genome.

  • Sandgroper

    It seems a reasonable hypothesis that the surviving Neanderthal genes conferred some kind of advantage by reason of their survival, i.e. an attempted explanation for an observed phenomenon, but beyond that currently seems to me more in the realm of speculation, in that the phenomena exist only if one is being selective in observation, e.g. ignoring the existence of Australian Aborigines, or Amerindians, or whatever. If someone wants to hypothesize that admixture yielded an intellectual advantage, or pale skin, or disease resistance, at the very least they would need to observe that these things actually exist in people with Neanderthal admixture but not in people without it. Surely to have a scientific hypothesis you need at least an observational reason to believe something might be true, but as a non-scientist I defer to others who know better.

  • dave chamberlin

    I plead guilty to speculating Sandgroper, to me it is a fascinating subject and a very important subject for a variety of reasons. Speculation masked as fact is what should make you upset, but I added to all of my comments statements like but who knows, time has erased so much and left so little. I think I have profited from reading GNXP for the last seven years or so to become a better critical thinker, to mute my speculations to those grand questions on the cutting edge of science. Our lost past is beginning to emerge and I love speculating about it. Too many people here and elsewhere make disagreements personal, I don’t understand that. We should enjoy hashing out our opinions and I look forward to further discussions with you on subjects that Razib has weighed in on.

  • Sandgroper

    Dave, no, nothing personal.

    AFAIK, Aborigines never performed appendectomies using sharp rocks – that was me being erm rhetorical :)

    Apart from the Noongar, they did perform fairly comprehensive male circumcision using sharp rocks though. Ouch.

    One of the things that interests me is that in the whole of Australia, the Noongar were the only people who did not practise male circumcision, and they resisted all attempts by neighbouring people to convert them to the practice. Question is why. There is evidence that the south west of Australia, Noongar country, was inhabited pretty early after entry of humans into Australia, which is not necessarily what you would expect, because it is far distant from possible entry points. Anyway, that’s tangential and irrelevant.

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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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