Denisovans in Wallacea?

Science Magazine

In Science, Cooper and Stringer discuss the distribution of Denisovan ancestry. Before I say anything else – their map is great.

We don’t see any Denisovan ancestry in populations living in mainland Southeast Asia, or Indonesia, or indeed in any population west of Wallace’s Line [which marks the eastern border of placental mammals, while Lydekker’s line marks the western border of the marsupial fauna: Wallacea lies between].  It could be that later population movements have diluted the Denisovan taint, but it is apparently undetectable even in existing populations that are thought to represent earlier strata, such as the Andaman islanders. Nor has it been seen in ancient DNA from modern humans on the Asian mainland.

Cooper and Stringer suggest that Denisovans may have settled Wallacea before modern humans arrived, and that they may have accounted for a higher ancestry fraction there because the number of modern human settlers was very small: rafts and canoes can’t carry many.  Which would mean that Denisovans had some seagoing technology. That tech had to be in a fairly narrow range: good enough to reach Wallacea (and the Philippines), but not good enough to reach Sahul (Australia/New Guinea).  I wouldn’t say that is impossible, but it’s a bit unlikely.

If Denisovans had arrived in Australia, they would have left a mark – extinct giant tortoises, if nothing else.  Considering that the entire Australian fauna seems to have gone to school on the short bus,  even Neanderthal-class hunters would have caused a mass extinction.  And in fact, the Custodians, whose technology was Neanderthal-class until a few thousand years ago, managed to do just that.  So the Denisovans never made it that far.

Let me suggest another model.  Back in the day, Indonesia was a big peninsula, called Sundaland.  Let us assume that there were Denisovans there. In some way, Denisovan effective population density was higher there than in mainland Asia, or perhaps they were harder to displace.  For example, some of Sundaland seems to have been tropical rainforest, otherwise known as the Green Hell – maybe there were some potent tropical diseases (vector-borne) that the Denisovans had developed resistance to, while the invading anatomical modern humans had not [Cooper and Stringer were also thinking about jungle pathogens] – the same reason that Europeans or Middle Easterners didn’t replace sub-Saharan African populations.

There may be genetic hints of this: Melanesians have picked up a couple of very ancient alleles of innate immune system genes.

So modern humans expand into Sundaland more slowly, in something like a range expansion.  The further they moved into Sundaland, the more Denisovan genes they picked up.  Again, like Arabs moving into Africa.  Only not as much, because Denisovans were more different from AMH than any two existing human populations, because of much, much longer separation.

So the humans living at the eastern edge of Sundaland,  particularly in Borneo, have more Denisovan ancestry in this scenario than anyone else in Eurasia – and they would be the people who take the next step, crossing the narrow seas to the Philippines, Wallacea, and then Sahul (Australia/New Guinea), virgin lands in which the hand of man had never set foot [with the possible exception of Flores].   A quite small, unusually Denisovanized coastal population of hunter-gatherers then undergoes a vast expansion, becoming as numerous as the stars in the sky.

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40 Responses to Denisovans in Wallacea?

  1. Fascinating. What do their cold sores look like?

    • engleberg says:

      If they had cold sores on the brain, jungle rot everywhere else, in short were sufficiently riddled with smelly disease that the survivors are those who privilege a Wolverine immune system over growing a forebrain, modern humans might look Denisovan to bone collectors. Go thou Al Gore, here’s your chance to serve humanity! Go thou and be one with nature, infect yourself with every disease in Melanesia and let us see if you evolve Denisovian aspects!

  2. Sid says:

    From what I gather, the Proto-Australoids were much more gracile than modern day Australian Aborigines. Aborigines today have an unusually archaic appearance, so it’s not inconceivable that they adopted such robust features from unusually high interbreeding with Denisovans.

    • Sandgroper says:

      Doubtful – the alleged ‘archaic’ features of modern Aboriginal people are within the normal range for anatomically modern humans.

      • gcochran9 says:

        Aboriginal skulls vary, but some look much more like archaics, more like erectus, than those from any other existing population. This was noticed a long time ago, by pros like Weidenreich. Recently, skull guys in Australia have played this down. I remember an article explaining that there were no such similarities that came out maybe a week before the announcement of the Denisovan DNA in those populations.

        The similarity is as obvious as it can possibly get. Those Australian ‘scientists’ who used multivariate analysis to make it go away were lying.

        Actually, they look surprisingly archaic, considering that total archaic ancestry among Melanesians is under 10% [Neanderthal plus Denisovan]. Recent selective pressure are probably more important: it is possible that their archaic appearance is to some degree coincidence.

      • Sandgroper says:

        I’m no expert on measuring skulls, but most ‘modern’ Aboriginal skulls that I have seen (and I have seen a lot) and the features of modern living people (i.e. ‘full bloods’) that I have seen (also a lot) simply look anatomically modern, although somewhat distinctive. I mean, I would never mistake a living Aboriginal person for someone from a different geographical region. A few have somewhat archaic looking features like brow ridges, but they are very much less pronounced than say Neanderthals, and they had chins, and no occipital buns. I have seen a few pictures of skulls (rather than seeing the actual skulls) that look robust and with some archaic looking features, but in the context of the skulls I have seen, they look like real oddities, and I can’t explain them.

        I recall the article you refer to. My impression is that they were not that far off the mark, from what I have seen myself, but again, I’m certainly no expert. I’d welcome a John Hawks comment on it, if he has looked at some or at least seen some pictures, because my feeling has always been that the ‘archaic’ features have been overdone/exaggerated, and are more likely coincidence.

        If some of the really old skulls actually had a much higher archaic admixture, that wouldn’t surprise me, but we also need to factor in deliberate cranial deformation, which was practised.

        • gcochran9 says:

          I’m not an expert either, but I’m asking John Hawks and Ralph Holloway about it. Heard back: both agree that
          “The skulls of Australian Aborigines, although variable, in some ways look more old-fashioned, more like archaics, than any other existing human population.” is a fair statement. The situation is complicated by the fact that most (not all) people identifying as Aborigines have a fair amount of European ancestry at this point. So skulls from 1890s on average look different from those today.

      • Sandgroper says:

        Thanks Greg.

      • Sandgroper says:

        When I was a young boy, I went into the Yonderup Cave http://www.flickr.com/photos/wakiwi/4526547105/
        At that time, it contained the remains of men, women and children, more than 20 of them. Originally, the only access to the cave was through a hole in the roof, and it was evident that these people had been dropped down this hole into the cave, where they died. No one knows why, but it pre-dated European settlement. It is presumed to have been a form of punishment. So the skeletons could be assumed to be pre-European arrival, and a reasonable sampling of mums, dads and kids. The skulls looked pretty normal – no particularly pronounced brow ridges. But I was a young kid, uninformed about cranial morphology and lacking the knowledge and wherewithall to make detailed measurements. Having said that, if the brow ridges were even moderately pronounced, I think I would have noticed – I was reasonably interested, and not totally unaware of modern human biology at the time.

        The remains have now been removed and returned to ‘their people’ (their people actually being extinct, so read that as people who claim (incorrectly) to be their people) for cremation and disposal, so no longer available for study.

  3. Patrick Boyle says:

    “The hand of man had never set foot”?

  4. Patrick – an old mixed metaphor, surely intentional. I think the first reference is 19th C “It was as if the Hand of God had stepped in.”

    I guess it’s not unusual that I was just reading about this today at John Hawks or Mermaids Tale or something, because it’s new and rocketing about the anthropologosphere. But I’m not usually anywhere near the cutting edge of that myself, so it seems jarring.

    If I’m getting this right, the Denisovans must have covered a lot of territory at one point, then been replaced, either by AMH, or in two waves, the second being AMH. They persisted longer at some of the outer edges of this range, not their original strongholds. That seems odd to us, but really, it isn’t. Once a population of honeybees occupies Kentucky through Idaho, it doesn’t matter where they started out, does it? They could face competition from any sector.

    Still, we’d except something more, or at least something that fit some more usual explanation, But the genes just aren’t where they should be. Have I got that right?

    Maybe it’s my ignorance, but an initial nescience of what happened to displace that population seems normal, not concerning, to me. Theories should be floated, but not having an explanation this week doesn’t seem problematic to me. Cochrane’s seems to account for a fair bit of the data, but if it goes down in flames in a decade, so what?

    I get the impression that “so what?” are the two words anthropologists consider most deeply heretical, though.

  5. dave chamberlin says:

    We wouldn’t know about the Denisovans if one of them hadn’t died in a cave way up yonder in Siberia where the constant year around temperature of 1 degree above freezing allowed for the exceptional preservation of one Denisovan’s genes in a single finger bone. We know that there range was all the way up to Siberia at one time but they were driven to extinction in all areas on the mainland side of the Wallace line almost without any interbreeding. But low and behold the present day population on the west side of the Wallace line is up to 7% Denisovan. Cochran posits that their survival in present day populations west of the Wallace line is due to a genetic advantage the Denisovans held in resistance to the many deadly tropical diseases. This has to be at least part of the explanation. This same advantage sure worked well for Subsaharan Africans. I’m inclined to think at this point that survival of any percentage of Denisovans was due to the slowing of the rapid expansion of moderns to a trickle because of two barriers, the Wallace line and the difficulty of living in tropical jungles. Very slow expansion allowed for interbreeding, rapid expansion meant complete replacement of the original population.

    • gcochran9 says:

      Actually, we could and did detect something odd in Melanesian genetics without a Denisovan comparison, but having a reference genome sure helps.

      Latest hot poop is that the Denisovan individual we found has noticeable amounts of Neanderthal and smaller amounts of something unknown and old, maybe homo erectus. I think we’re talking a border with the occasional mixed marriage. Not surprising since we’ve found Neanderthal bones in the same cave.

    • dave chamberlin says:

      Regarding mixed marriages I find it really strange that Denisovans were displaced from the entire Asian continent without ANY trace of mixed marriages. Without going into the lurid details men have a very long history of not being too picky as to whom they make the beast with two backs. We know they could interbreed and we know men at least would have been willing, why didn’t they? The best guess I can come up with is the Denisovans carried pathogens which were to the advancing moderns what small pox was to the amerinds. If this was the case then a really strong taboo probably would have developed towards any comingling with the Denisovans. The moderns wouldn’t have any understanding of pathogens of course, they would have feared the Denisovan’s as witches or some such superstition and killed them as quick as possible from as far away as possible.

  6. feministx says:

    I wish we could resurrect a bunch of these critters jurassic park style. It would surely be much more potentially dangerous to us than resurrecting dinosaurs, but observing them would provide a fascinating comparison group for creatures with advanced consciousness.

    But seriously, it is truly perplexing how a group that leaves only a finger and two teeth as fossil record ends up as 6% of the genome of modern humans anywhere. The denisovans must have been

    Why are there so few denisovan remains? There are way more fossils of neanderthals, who ended up no more than 4% of the genome for some populations.

    It is possible that they were most populous in indonesia or something. I think it makes sense to say that they were not settled in Australia because their descendants range all over malenesia. Do we have evidence that any immigration wave took place from Australia to the negritos in southeast asia? That would have needed to happen if humans mixed with denisovans first in Australia. To me, it seems much more likely that the mix took place in southeast asia and then spread to Australia.

    Still, how do you end up 6% of the genome of malenesians and leave almost no trace of ever existing?

    Is it possible that significant parts of the genomes of native americans and other groups come from unknown archaic humans that we have no evidence of? Is there a way of knowing that without ever finding an actual fossil?

    • Anonymous says:

      Unlikely that any mix with archaics happened in America, the archaics would have left some fossils in that quite-searched-for-fossils place, more tools, and perhaps a megafauna more suspicious of Homos. Perhaps Bigfoot was a bear, too. I too don’t quite get why people think of “Denisovans” rather than “late Erectus” or something… I mean, what are the odds that we find ancient DNA of something without having found many fossils?

      • Sandgroper says:

        What if most of their range was humid/poor for preservation? For the period in question, China and South East Asia were always humid. Absence of evidence is not evidence of absence, as they say. But then, you could use that to conjure up all manner of imaginary beasties. Occam’s Razor suggests that you shouldn’t seek an explanation more complex than is required to explain the evidence you have, rather than the evidence you don’t have. (But let’s not ignore the really very recent discovery of floresiensis – that set the scientific world back on its heals, whatever it was – no one had even remotely thought that such a creature might have existed.) I don’t have an argument with you, though – we are all late erectines, aren’t we? Unless we think they were a separate branch that did not evolve into all of Neanderthals, Denisovans and anatomically modern humans.

        I suspect the next piece of evidence on Denisovans, if there ever is one, is going to settle a lot of questions.

        In answer to feministx, so far as I know, you can only do it by comparing genomes – no one knew with any certainty that there was any Neanderthal admixture in anatomically modern humans until a Neanderthal genome had been typed, and no one has found any genomic evidence that there is archaic admixture in Native Americans that does not exist in other geographic populations. And ‘they’ still don’t know whether there is any Neanderthal admixture in sub-Saharan Africans, only that there is comparatively 1-4% more N admixture in non-sub-Saharan populations.

      • feministx says:

        “What if most of their range was humid/poor for preservation? ”
        That’s a fair point, but there are many homo erectus remains in southeast asia and asia. If denisovans separated from neanderthals and homo erectus enough to be a different group, then they somehow left a fraction of the fossils that erectus left but still managed to become 6% of the malenesian genome. I don’t know if there is proof that erectus is part of us now.

    • David Benson says:

      “Still, how do you end up 6% of the genome of malenesians and leave almost no trace of ever existing?”

      The Denisovans ARE Homo Erectus. That is the explanation.

    • The Lithuanian says:

      Interesting, I have 4.4% Denisovan, plus Neanderthal, and my modern history shows 50% Northern European. I am guessing this will change in the future with new algorithms.

      • Toddy Cat says:

        Is this typical for Lithuanians? If so, maybe this explains why Baltic girls are so attractive. I remember a NOVA show from a few months back where they tested a college class for Neanderthal ancestry, and it turned out the two most attractive people in the room had the highest Neanderthal quotient. Obviously, this proves nothing, but still, it makes you wonder. Maybe a Neanderthal-Denisovan-AMH cross would be smokin’…

        • The Lithuanian says:

          I have no idea. I am the only person I know that has done this. I am sure the researchers have a better perspective if they can see all these data. From my empirical observations most of the women in the extended family are best described as “thick”, in line with my grandfather’s noted occupation on his Russian Empire internal passport of “peasant”. I, however, am devastatingly handsome.

      • Toddy Cat says:

        “I, however, am devastatingly handsome.”

        Well, there you are, then…

  7. Weltanschauung says:

    For New Zealand read New Guinea. I don’t think New Zealand comes into this story.

  8. athEIst says:

    gone to school on the short bus,
    HA HA, Hadn’t heard this one before
    BUT surely someone will be offended.

  9. Sandgroper says:

    Marsupials have notably small brains compared to parallel evolutionary convergent placentals. If the diprotodontidae moved with the alacrity of wombats, even if you scaled up the speed with size, catching them would not have been a problem,and koalas spend most of their lives asleep. Both wombats and koalas are almost comically slow. Catching kangaroos by stalking and trying to run them down definitely is a problem, they are extremely wary, have excellent hearing and are very fast, despite being small brained, and capable of despatching fast hunting dogs in various unpleasant ways. It needs projectile weapons capable of being used accurately over long distance. Armed with an accurate rifle and assisted by a very clever sheepdog, I have bagged a few for my dinner, but I would be useless with a hardwood spear and a woomera. Of course there is spotlight shooting, but no self-respecting kangaroo hunter resorts to such despicable methods, which the first Australians didn’t have anyway. I don’t know anything about the habits of the extinct giant kangaroos, but a group of guys with spears and woomeras setting fire to the bush to flush them out should not have had too much of a problem. I assume they dealt with Megalania and Thylacoleo carnifex the same way.

  10. Anonymous says:

    Caleb Finch says modern humans were able to cope with the infectious diseases that go with living in civilisation, where they were suddenly crowded together in large groups, because they had already evolved a massive increase in their ability to fight infection with INFLAMMATION.
    The ability to generate massive inflammation was required to adapt to pathogens in the diet of meat, and the broken bones hunters sustained from prey animals. So, maybe the archaic alleles for innate immunity from Denisovans (that got hurt a lot while hunting and were be strapped with super inflammation capacity) became more useful in civilisation.

  11. dearieme says:

    “Christopher Badcock explains …”: oh dear, oh dear – another cheerer-upper on a dull afternoon.

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  14. Dale says:

    The post mentions “Custodians”. Who are they? I can’t find any reference to that as an ethnonym.

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