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    Neanderthal Genome Teaser
    By Michael White | November 2nd 2009 01:33 PM | 42 comments | Print | E-mail | Track Comments
    About Michael

    Welcome to Adaptive Complexity, where I write about genomics, systems biology, evolution, and the connection between science and literature,

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    This is a little old, but Thomas Mailund, who writes on of my favorite blogs, has posted a video interview with Svante Pääbo, the scientist leading the neanderthal genome project.

    At one point Pääbo addresses the question everyone's asking: did humans and neanderthals have sex? Pääbo says of course they did (why wouldn't they during the 10,000+ years they lived together), but the question is whether they produced hybrid offspring. He hopes to answer the latter question with the neanderthal genome sequence.







    Read the feed:

    This is a little old, but Thomas Mailund, who writes on of my favorite blogs, has posted a video interview with Svante Pääbo, the scientist leading the neanderthal genome project. At one point Pääbo addresses the question everyone's asking: did humans and neanderthals have sex? Pääbo says of course they did (why wouldn't they during the 10,000+ years they lived together), but the question is whether they produced hybrid offspring. He hopes to answer the latter question with the neanderthal genome sequence.







    Read the feed:

    Comments

    Gerhard Adam
    ...did humans and neanderthals have sex?
    I always find it interesting the way this is phrased, because it clearly suggests that neanderthals weren't human.
    Mundus vult decipi
    jtwitten
    Human is a specific species. Hominid is a group of species-es. Does this rephrasing help?

    Did individual Homo sapiens (humans) and individual Homo neanderthalensis (Neandethals) have sex?
    My understanding of the term "human" is that it refers to any member of the subfamily Homininae (including such genera as Homo, Australopithecus, Ardepithecus, etc.) in the family Hominidae (humans and great apes).

    Modern man and neaderthals may or may not have had sex, but they were both humans.

    jtwitten
    In terms of translating the intent of this sentence, your understanding is neither here nor there. Mike's intent is what matters. Biologically, the question is clearly stated. The term "human" refers specifically to H. sapiens. To have "human" equate to anything other than H. sapiens would make the question really stupid and Mike is not really stupid. He may be a degenerate biochemist, but he is not stupid.
    adaptivecomplexity
    We're getting excessively pedantic here. When we speak in casual conversation, 'humans' or probably better, 'modern humans' vs neanderthals is fine.
    And don't underestimate the differences between neanderthals and modern humans. Neanderthals were a lot smarter than they used to be given credit for, but, based on evidence of their tool use, lack of art, style of burials&other cultural elements, mode of hunting, it's clear that if you dressed one up in a suit and set it down somewhere in Chicago, it would not pass for a modern human.  

    Whether considered a separate species or subspecies, neanderthals in most of their traits were well outside the range of modern human variation.
    Mike
    Gerhard Adam
    ...neanderthals in most of their traits were well outside the range of modern human variation.
    I suppose it depends on what you mean by "well outside", since the point was interbreeding then they couldn't be too far off in variation.  After all, producing successful hybrids would still require some adult commitment, a social infrastructure, and something that ensures that both members are accepted within their respective social groups.

    So if the variation is too great, then it is highly unlikely that any successful hybrids were born.  The sticking point wouldn't be genetic compatibility but rather social acceptance.
    Mundus vult decipi
    adaptivecomplexity
    After all, producing successful hybrids would still require some adult commitment, a social infrastructure, and something that ensures that both members are accepted within their respective social groups.
    I don't think that's true at all - look at human history. No commitment is necessary for people to have sex. I also don't agree with your point about genetic variation - a mule is very different from a horse, genetically (probably even more different genetically than a Neanderthal and a modern human) and you can produce hybrids. Same thing with tigers & lions. You can have two species/subspecies that very different, and they may still on occasion produce hybrids. A neanderthal raised in a 21st century home would not be able to pass for a modern human.
    Mike
    Gerhard Adam
    Sex is not interbreeding.  Similarly your other examples don't seem to occur in the wild, so that also begs the question.  Even if we consider the case of the recent polar bear/grizzly hybrid, there is no evidence that indicates that they can reproduce and the social structure of bears ensures that the female can operate on her own.  Even under those conditions it is obviously a quite rare occurrence.   Most cases of hybrids in nature are between relatively closely related species.

    For homo sapiens and neanderthals to interbreed requires successful reproduction or it becomes a tremendous resource drain on the tribe.  I can't imagine any tribal society committing its resources to supporting offspring that are incapable of advancing the tribe.  In addition, a pregnant female without a corresponding support infrastructure has approximately zero survival value for her offspring in such a society.   Other problems could've occurred if the female was homo sapien, by increasing the difficulty of birth if the child should have had more neanderthal characteristics like an increased cranial size.

    Without considering all the nuances of ancient tribal society I can still be pretty confident that successful reproduction involves far more than the trivial act of having sex.  Since the only way to determine if they had sex is to examine any offspring, then it stands to reason that there must've been sufficient value to the pairing to commit to raising it.  Anything less is pretty much a dead end and not particularly interesting.


    Mundus vult decipi
    jtwitten
    I can't imagine any tribal society committing its resources to supporting offspring that are incapable of advancing the tribe.
    That may just be a failure of your imagination. Personally, I can think of some great advantages to having a friendly neanderthal or human-neanderthal around.
    Gerhard Adam
    Such as?
    Mundus vult decipi
    jtwitten
    Opening pickle jars.
    Gerhard Adam
    No doubt, leading to the evolution of more sophisticated tool usage.
    Mundus vult decipi
    While I generally agree with this, I don't think it is as simple as social acceptance vs genetics here.

    A tiger is well outside the phenotype of lions but lions and tigers can produce offspring. Just not offspring that has much of a chance of producing offspring. They are less fit than both ancestor species. Google ligers.

    You can easily have different phenotypes -- each "well outside" the variation of each species -- combined in a hybrid. The hybrid can be born and all. With an increased fitness or a decreased fitness. If the fitness is decreased it can be social or through other means.

    Of course, if the difference between the ancestral species is too large, this wont happen. If a Grand Dane male mates with a Chichaua female, I would expect an explosion over a hybrid. But this doesn't mean that you cannot expect a hybrid born of species with very different phenotypes. A German Shepard and a Labradore have very different phenotypes, but their offspring is just fine.

    If we don't seen an signs of hybrids between H sapience and H neanderthalensis, it could be because hybrids were just not possible, or it could be that they were less fit (for social or other reasons; but essentially those would still be genetic).

    Gerhard Adam
    A German Shepard and a Labradore have very different phenotypes, but their offspring is just fine.
    But those aren't different species. 

    In general, the point isn't that hybrids are possible or not, but whether they would be successful enough to show up as anything other than rare anomalies.  If the social environments between the two groups was different enough, then it may be quite unlikely for such a hybrid to ever find a mate, so they would be a dead-end regardless of how viable their genetics might be.
    Mundus vult decipi
    That those aren't different species is part of my argument :) Sorry if I am not expressing myself so well, I've just gotten back from the pop and my thinking is less then clear :D

    I'm just pointing out that phenotypic variation and the possibility of hybrids is not all that clear. You can have two "species" (or populations if you prefer; the species and population concepts are equally wholly defined) that differs significantly in phenotype that can produce hybrids.

    But when you say that the hybrids are dead-ends, and that is not due to genetics, what do you mean? Sexual selection is very much genetics, after all...

    adaptivecomplexity
    If we don't seen an signs of hybrids between H sapience and H neanderthalensis, it could be because hybrids were just not possible, or it could be that they were less fit (for social or other reasons; but essentially those would still be genetic).
    This will be the hard thing - in the end, if we don't see any evidence of hybridization, we'll only be able to speculate why. I don't think I have a good feel for the a priori probability. From a biological plausibility standpoint, it's probably very reasonable to believe hybrids were possible, and it's not unreasonable to think that fertile hybrids were possible. But beyond that - it's hard to say. Without a population genetics model, I don't have a good feel for how much viable hybridization you would need to have a reasonable chance of getting introgressed genes.
    Mike
    This is tricky indeed. If we get any strong positive evidence, we have a conclusion, but no evidence for hybrids is not evidence for no hybrids, of course.

    We can work out the chances for having no evidence from population genetics, of course, and give you estimates of how rare hybrids must have been, but there are just so many unknown parameters that it is damn near impossible to say anything conclusively.

    Anyway, we are working on it :)

    I very much look forward to seeing what they got out of the first Neanderthal genome paper; I'm expecting the draft soon now. The next step is proper population genetics analysis, but for that we need deeper coverage and that requires some improvements in the sequencing technology.

    adaptivecomplexity
    Anyway, we are working on it :)
    I'm looking forward to seeing it. And of course I was speaking tongue-in-cheek when I said human-neanderthal sex is the real reason we want the Neanderthal genome sequence. I'm anxiously awaiting its release. And BTW, I think you write a great blog - I've been following it for awhile now. Everyone reading this thread should follow your link to it.
    Mike
    Well, the human-neandertal romance is part of the story after all :) I would love to find out if we have any ancestral human genes.

    Happy to hear you like the blog :) I quite enjoy this one as well.

    Mine is a bit slow now, though. I'm pretty swamped with some genome projects and don't have much time to blog after work these weeks...

    jtwitten
    Or, they could have been so rare that the probability of introgression is very small, which is just vaguing up your "not possible". The point is that we get into talking about absolutes. We get caught up in the did they or didn't they argument about hybrids, when the real question is whether any hybridization made a significant contribution to the pool of narrow-sense human (hs-yeah, i changed the notation slightly, but that is my perogative) variation.

    Human history would certainly suggest that our ancestors would have tried to hybridize with neanderthals.
    Becky Jungbauer
    if you dressed one up in a suit and set it down somewhere in Chicago, it would not pass for a modern human.
    Clearly you haven't spent much time in Chicago.
    I thought Gerhard made a good point, that's all.
    What taxonomist doesn't enjoy a good pedantic volley!
    I better just stick to beetles.

    jtwitten
    The question of how to define "human" is certainly interesting to some (not me, but that is personal taste). The pedantry, however, did nothing to illuminate the question of hybrids.
    Of course it didn't, since that was not the intent of my comment. But if illuminating the question of hybrids is, as you suggest, a requisite for commenting on this post, please help me understand how your Ad Hominem Abusive claims of pedantry and failure of imagination have contributed to the discussion.

    Here's my view – it's possible, but nothing produced yet strongly suggests that hybridization occurred with any regularity. The evidence to the contrary, on the other hand, is considerable. Neanderthals and moderns coexisted for some time in the Levant, but the former disappear almost immediately with the development by moderns (i.e., modern humans) of Upper Paleolithic technology. The subsequent spread of moderns into Europe also coincides with an immediate and rapid decline in Neanderthal sites. While there were several thousand years of overlap in Europe as a whole from the time moderns first entered Europe until the last Neanderthal is recorded, the shift from Mousterian to Upper Paleolithic at individual sites occurs rather abruptly with no intermixing of cultural artifacts. This pattern is seen so consistently across sites that whatever contact there was between Neanderthals and moderns in Europe could not have been prolonged. Further, significant differences in skeletal morphology, as well as the substantial cultural, behavioral, and cognitive differences mentioned by Mike above, strongly suggest specific distinction between Neanderthals and moderns. This makes the production of successful hybrids even more implausible, an assumption that none of the recent molecular work has effectively challenged.

    I'm sure that whatever encounters there were between Neanderthals and moderns were by and large not pleasant ones. Considering our own well-documented history of waging war against and enslaving members of our own species, it's hard to imagine that moderns were any less ruthless in their treatment of the cognitively handicapped Neanderthals. The only "intermixing" I can even imagine is the ambush of Neanderthal camps by moderns, who killed the males and captured the females for carnal recreation. If, as the morphological and cultural evidence so strongly suggest, Neanderthals and moderns were specifically distinct, then no significant number of viable offspring could result from such a scenario. Even if the two were only subspecifically distinct and able to produce viable hybrids, it seems highly unlikely that such offspring would receive the level of nurturing required to reach reproductive adulthood.

    But, that's just my opinion.

    jtwitten
    Your view is plausible. Perhaps even likely.

    The imagination quip was funny, even Gerhard appeared to have turned on his sense of humor for that one.

    I was not aware that "pedantry" was derogative. A great deal of space was being used to debate the objective definition of "human" in relation to Mike's question to which the objective definition (if such a thing exists) is/was/will continue to be irrelevant, since we know the way he intended to use the word in the question about hybrids, or dirty, cross-species nooky. Since Mike likes the phrase, which is a bit creepy.
    jtwitten
    Touche. Well played. Well played, indeed.

    I will defend my long windedness with two points:
    1. The initial clarification of Gerhard's admitted confusion (http://www.scientificblogging.com/comments/25727/Re_Neanderthal_Genome_T...) has led to a on point discussion of hybridization.
    2. It made Mike laugh, which is important because biochemists, like clowns, are usually crying on the inside.
    adaptivecomplexity
    It made Mike laugh, which is important because biochemists, like clowns, are usually crying on the inside.
    That's because when you're a biochemist, it makes you sad to be surrounded by so many inferior, non-biochemists.
    Mike
    adaptivecomplexity
    Although a lot of the neanderthal genome publicity involves speculation on whether any modern human genes were neanderthal introgressions, I'll put myself on record now with the prediction that we won't find any. I agree with this:

    he only "intermixing" I can even imagine is the ambush of Neanderthal camps by moderns, who killed the males and captured the females for carnal recreation.


    But not with this:
    If, as the morphological and cultural evidence so strongly suggest, Neanderthals and moderns were specifically distinct, then no significant number of viable offspring could result from such a scenario.


    Very distinct animals (like horses and donkeys) can produce viable (as in, surviving) hybrids. We're closer to Neanderthals than horses are to donkeys. I don't think it's biologically implausible that non-sterile hybrids could have been produced, and over 15-20k years, it must have happened multiple times.

    It doesn't seem likely to me that any such hybrids passed on neanderthal genetic variants that later rose to high frequency in the ancestors of modern Europeans, for the reasons people have mentioned here. But I haven't actually worked through the population genetics math - someone like John Hawks might have a better answer.
    Mike
    Very distinct animals (like horses and donkeys) can produce viable (as in, surviving) hybrids.

    I didn't mean to suggest that cross-species hybridization resulting in viable offspring can't happen, as it is certainly possible between even not particularly closely related species (such as horses and donkeys). But mules are not fertile, and if Neanderthal/modern hybrids were also sterile than their occurrence is of little significance except as a curiosity. I suspect that Neanderthals and moderns may not have been as closely related as many people assume - nearly half a million years is a good chunk of the average temporal existence of most known hominid species. But, as Gerhard has pointed out, even if they were related enough to produce fertile offspring, there still would have been considerable anthropological obstacles to the offspring reaching adulthood and reproducing themselves. Moreover, there is growing evidence of considerable impediments to gene flow even among species that are only subspecifically distinct - hybridization occurs with regularity along a zone of contact, but genes still do not then flow into the general population. I'll reiterate my point about the apparently very limited opportunity for contact - despite the several thousands of years across Europe as a whole that the two species existed, replacement on a site by site basis occurred abruptly and permanently. The two species occupied Europe together, but that doesn't mean they were encountering each other with any great frequency.

    What gives? I finally get a tip of the hat from Josh, and my comment gets deleted?

    adaptivecomplexity
    I can't figure out why your comment got deleted - I didn't delete it. I generally don't delete any, unless they are spam.
    Mike
    At best, I would consider "human" to refer to the Homos, not including Ardi or Lucy. In general, I think "human" refers to modern H. sapience though. If we want to be technical, we have the species names, but when you hear "human" do you really think "everything from habilis to today"?

    Gerhard Adam
    Yes, the rephrasing makes much more sense.  However, it should also be pointed out that neanderthals are also sometimes classified as a subspecies of Homo sapiens (Homo sapiens neanderthalensis).

    While I realize that there is a strong suggestion that this subspecies diverged about 500,000 years ago, it illustrates just how closely related they may have been.  For all practical purposes, if Homo sapiens and Homo neanderthalensis did interbreed, it would imply that they produced viable offspring, indicating that perhaps they were more closely related as a subspecies rather than as a separate species.
    Mundus vult decipi
    jtwitten
    The same people who do that extend Homo sapiens to Homo sapiens sapiens. They are still called "humans". There has been an incredible amount of verbage spilled over what is a very straightforward understanding of Mike's question about dirty, cross-species nooky.
    adaptivecomplexity
    dirty, cross-species nooky.
    Because that is the primary reason for sequencing the neanderthal genome!
    Mike
    The species concept is a bit of a hack that doesn't actually work when confronted with real data. If you want modern humans and neandertals to both be Homo sapiens you are saying that they are sub-species, but the distinction between sub-species and species is very much dependent on your definition of species in the first place.

    Gerhard Adam
    I'm sorry, but it seems that the considerations of sex versus interbreeding are being taken far too casually contrasted to the severe survival risks that such activity actually entailed.

    Risks of pregnancy associated with childbirth could decimate a tribal group if the women died, so I suspect that any group would be far more protective of its females than engaging in casual encounters.  Similarly the idea of raiding other villages makes more sense when they are of the same species than to venture across species.

    If any offspring were produced, then the resource commitment to feeding and raising them becomes a huge impact that may be a waste of time if they turn out to be sterile.  Bear in mind that many of these tribal groups may not have numbered more than 10-15 (including children) so we aren't talking about a lot of excess manpower.  If such an event occurred once or twice (within the same group) it would likely mean the end of that tribal group.

    When this is considered with the possibility that the two groups may not have seemed attractive to one another, it creates another obstacle towards considering the viability of producing any offspring.

    In all likelihood the indications are that human ancestors were simply more prolific and mobile that neanderthals, including the probability of higher interactions with other human groups.  This would've tended to isolate the neanderthals, who seem to have been less interactive with other neanderthal groups and had a longer reproductive/maturation cycle.  With their larger cranial capacity, it may have still presented a problem in childbirth resulting in a disproportionate number of women dying.  All these factors could have resulted in the steady demise of neanderthals over time.

    While the reproductive biology may be possible, I don't see how it would work anthropologically.  It is unlikely that women would've spent much time not pregnant.   The only scenario I could see that could fit, is the opposite of what was suggested previously (that of neanderthals attacking moderns in raids).   If this was an attempt to acquire women, then it could have occurred resulting in sterile offspring which would have doomed the neanderthals.
    Mundus vult decipi
    We are talking about a time frame much longer than recorded history, so weird stuff will have happened. Attractiveness of the other species is not an argument. Today we will jump anything we can find if we are desperate enough, so we would have as well 30K-100K years ago as well. I am just not buying that argument.

    There are plenty of scenarios where hybrids are not possible, though, and I find those easier to believe.

    Gerhard Adam
    Today we will jump anything we can find if we are desperate enough, so we would have as well 30K-100K years ago as well. I am just not buying that argument.
    Once again you're confusing sex with raising offspring.  Whatever their sexual proclivities may have been is irrelevant.  The only thing that matters is whether they would have been committed enough to rear offspring.  If not, then it is irrelevant who had sex with whom.
    Mundus vult decipi
    My comment was about the "attractiveness" argument. That is what I'm not buying. If we are not seeing hybrids it is not because we didn't have sex, it is because it did not provide viable offspring!

    Gerhard Adam
    My comment was about the "attractiveness" argument.
    Depends on what you mean by "attractiveness".  I can't think of too many "higher" animals that don't discriminate who they mate with.  I would expect it would be even more stringent across species (which is why many of the hybrids mentioned don't occur naturally).
    Mundus vult decipi
    The "species" concept and "who they mate with" are very closely related, to the point where they don't make sense independently, really.

    Anyway, considering that we will have sex with pretty much any farm animal today if we get the chance (we as a species, not any particular individual) do you find it unlikely that we ever had sex with a neandertal in 100,000 years? That is 50,000 longer than the time since the beginning of our current era.

    I can buy that we could have had hybrids but didn't because we never had sex with neanderthals if we had very few chances, but over the time range where we lived in Europe together my bet is that we had plenty of chances and I am just not buying that we "just said no"!

    Gerhard Adam
    Anyway, considering that we will have sex with pretty much any farm animal today if we get the chance (we as a species, not any particular individual) do you find it unlikely that we ever had sex with a neandertal in 100,000 years?
    I don't think the type of promiscuity you're describing is found when it occurs outside of a species or social group definition.  A horse and donkey will breed under domesticated conditions, but the likelihood of a donkey having the opportunity in a horse herd is another matter entirely. 

    My point regarding "attractiveness" is that it would consist of all the elements that a part of the sexual selection process that females engage in.  While a male may be able to force a sexual encounter, it will depend on a social group (when considering humans) to ensure that any offspring sired are raised to adulthood.  Therefore we have to consider that any offspring would have to be either "more" attractive as possible mates, or "less" attractive.  If more, then we would expect to see a merging of the species into a completely new form since they would clearly have the advantage in attracting mates (i.e. fitness).  If less, then they would tend to be non-viable and it would be a hard argument to make that they experienced any degree of success that would allow them to be detected in the subsequent genome.

    All this is assuming that they were capable of reproducing.  Failure to reproduce would be a serious mark against such a coupling since neaderthal or H. sapiens tribal groups didn't have an abundance of resources that they could arbitrarily provide to a non-essential member of such a group. 

    While sex may have occurred, I would argue that natural selection would operate against hybrid offspring as being too risky (in terms of resource commitment for questionable benefit), so that the "appeal" of such cross-species couplings would have been minimal. 

    While we don't know the internal social workings of such tribal groups, it is unlikely that either group would have willingly cooperated making females available, except under the duress of raids or skirmishes.  Given the risks of childbirth and the future needs (i.e. members) of such a tribe, it would seem highly unlikely that casual coupling occurred.

    When one considers how long it takes humans (and neanderthals) to raise a child, the close social proximity would become much more significant, so once again we are left with either a merger of the two groups, or an alienation between them.

    Mundus vult decipi