Theory One: Ancient African Population Structure
It could be due to a shared common ancestor, Homo heidelbergensis, which could have split into Neanderthal population in the North and early modern humans in Africa around 350,000 to 500,000 years ago. In this scenario the differences in the amount of apparent Neanderthal admixture between non-Africans and Africans was due to population structure in Africa, with the Out of Africa population drawing on a different part of the genetic diversity of modern humans at the time than the population which in ancestral to most Africans today.
Keep in mind that much of the population diversity present at the Out of Africa moment in Africa was found in populations that subsequently left no descendants or are survived only by relic populations like the Pygmies and Khoisan today, or only have ancestors who slightly introgressed into other, more successful African populations.
Different African and Eurpean hominid populations around 100,000 years ago could have had different levels of relatedness to each other, presumably on a North to South cline, with the populations closest to Europe being most similar to Neanderthals and the populations most distant from Europe being least similar. If many of the intermediate hominin populations descended from Homo heidelbergensis between Northeastern African modern humans who were necessarily the ancestors of non-Africans on one hand, and the Southern African or Western African modern humans, on the other, subsequently went extinct, the genetic trace that would be left in modern humans would look a lot like an admixture model, but without the hot interspecies sex. Likewise, hominin populations intermediate between Northeastern African modern humans (pre-Out of Africa), and Neanderthals, in the Levant, would also have left no genetic traces. Fans of Jean M. Auel's Clan of the Cave Bear series wouldn't be vindicated after all.
This is the essence of the argument made by Andrea Manica and Anders Eriksson in a paper in the Proceedings of the National Academy of Sciences, which was acknowledged by viewed a less parsimonious in the seminal Neanderthal admixture paper. (Archaeogenetics blogger Dienekes is a fan of this theory.)
Theory Two: Neanderthal Admixture
Or, it could it be that the sexier theory that Homo neanderthalensis interbred in small numbers with some of the earliest ancestors of all non-African modern humans is correct?
This is the proposal of Svante Pääbo "who led the sequencing of the Neanderthal genome in 2010 and has championed the idea that modern humans interbred with Neanderthals[.]"
Pääbo has co-authored a paper, which is yet to undergo peer-review, to further support his thesis that humans and Neanderthals did in fact interbreed. "We find that the last gene flow from Neanderthals (or their relatives) into Europeans likely occurred 37,000-86,000 years before the present, and most likely 47,000-65,000 years ago," he writes. "This supports the recent interbreeding hypothesis, and suggests that interbreeding may have occurred when modern humans carrying Upper Paleolithic technologies encountered Neanderthals as they expanded out of Africa."These Admixture Dates, If Recalibrated, Fit An Early Out of Africa In The Levant That Lived
If the dates are calibrated to fit archaeological suggestions of date that make sense, to be longer by a factor of 1.6, as I have advocated in previous posts (the difference mostly reflects a probably inaccurate mutation rate used to develop the estimate of the number of years ago that genetic populations became distinct), one gets a date range of 75,200 to 104,000 years ago.
This neatly coincides with the earliest time period in which there is archaeological evidence of Neanderthal and modern human co-existence in the Levant, after which modern humans reappear in the Levant about 50,000 years ago.
The adjusted date range would be powerful evidence that the first Out of Africa period in the Levant was not "Out of Africa that failed", but instead, was the source of the population that is the dominant ancestral population for all non-African populations today. In this scenario, rather than dying out, after which Eurasia was recolonized by a second wave of modern humans leaving Africa, the early Out of African Levantine population was dispersed around 75,000 years ago from the Levant to refugia (probably in a wetter interior of Arabia, in the less inundated Persian Gulf, in Iran, and/or in South Asia) that later rebounded and ultimately populated the rest of the Earth.
Why Don't Europeans Have More Neanderthal Admixture Than Asians?
How can one explain the similarity in levels of African admixture between Europe and Asia in an admixture theory, despite the fact that Neanderthals co-existed with Cro-Magnon modern humans in Europe for many thousands of years, but in Southeast Asia and East Asia for far shorter time periods?
Did the propensity of modern humans to interbreed with Neanderthals decline in the Upper Paleolithic? I don't think that it did.
The more likely scenario, I think, is that Neanderthal co-existence with modern humans in the Middle East ended far sooner than it did in Europe, and that most of that ancestry of Europeans today comes not from the Cro-Magnon population that had a prolonged period when there could have been admixture, but from Middle Eastern populations with less Neanderthal exposue. These peoples repopulated Europe after the Last Glacial Maximum (around 20,000 years ago, long after the Neanderthals had gone extinct) and in subsequent waves of population expansion around the beginning of the Neolithic Revolution in Europe (plus or minus a few thousand years) and with Indo-European language expansion. This would dilute the amount of excess Neanderthal admixture in Europe.
Since the level of Neanderthal admixture would have been modest even in populations with long periods of co-existence with Neanderthals relative to the non-African baseline, even in more admixed populations, it could be that these elevated Neanderthal admixture levels from Cro-Magnon populations are almost indistinguishable from background random noise in modern populations.
For example, suppose that later waves of populations from the Middle East provided 80% of the autosomal genetic source for modern Europeans (one number that has been seriously proposed based on assumptions made about uniparental genetic markers) and 20% of the autosomal genetic source for modern Europeans was Cro-Magnon. Then, assume that the Middle Easterners were 4% Neanderthal admixed and that the Cro-Magnon were 8% Neanderthal admixed (half due to their remote Middle Eastern source ca. 43,000 years ago, and the other half from further admixture in Europe). The end result would be a modern European population with 4.8% Neanderthal admixture on average, a percentage that has a standard deviation in Europe on the order of 0.5 percentage points in the data so far used to derive that figure.
More realistic numbers are something like this:
* The level of Neanderthal admixture may be somewhat higher in the minority of modern European populations, probably not more than 10% of all Europeans overall (mostly in the far Northeastern part of Europe), with the largest genetic contributions from the last European hunter-gatherers to convert to herding and farming, who might have, perhaps as much as 20%-30% of ancestry in these populations traceable to Cro-Magnons.
* Don't forget that post-last glacial maximum relict European hunter-gatherer populations, even before the demic impact of Neolithic and metal age migrants, would not have been 100% Cro-Magnon. Almost all of Northern Europe was 99.99% depopulated of modern humans because it was under glaciers (there may have been a warm protected valley or current warmed coastal area in a few isolated spots that could support a village or two), leaving relict populations mostly in Iberia, Italy and far Southeastern Europe. When the glaciers retreated, some of the repopulation probably had its source not from these Cro-Magnon refugia, but from the Middle East. By the time that the herders and farmers arrived, these source populations would have had at least several thousand years to reach something close to
* In contrast, most European populations, who make up perhaps 90% of all Europeans, might have on average 10% Cro-Magnon ancestry or less.
* The level of Neanderthal admixture in Cro-Magnon populations might have been 50% more than in Middle Easterners (i.e. Cro-Magnon populations might have been 6% Neanderthal), rather than twice as great.
* This would suggest an expected average level of European Neanderthal admixture of 4.23% relative to a baseline of 4% +/- circa 0.5%, and the existence of population structure in Europe would mean that the choice of sample individuals from which you made the estimate could skew the sample far more than the excess of the European average over the shared non-African baseline percentage of Neanderthal admixture. Thus, the excess Neanderthal admixture would be almost invisible.
Why Is There No Neanderthal mtDNA or Y-DNA in modern humans?
There are also plausible models that can account for the absence of Neanderthal mtDNA or Y-DNA in any modern humans.
In these models, a principle called Haldane's law insures that almost all of the fertile Neanderthal-modern human hybrids were probably female in the first generation, accounting for the lack of Neanderthal Y-DNA. Haldane's law says that mixed sex chromosome interspecies hybrids (XY males in the case of hominins) tend not to be born, or to be infertile if they are born, relative to homogeneous sex chromosome interspecies hybrids (XX females in the case of hominins).
Neanderthal mtDNA's absence is explained by the theory that hybrid children grew up with a tribe made up of the species of the mothers rather than the fathers, and that the hybrid children born into Neanderthal tribes died out with the rest of the Neanderthal population, while the hybrid children born into modern human tribes have descendants who are alive today. In this scenario, there could have been just as many hybrid children with Neanderthal mothers as there were with modern human mothers, and yet have no Neanderthal mtDNA in modern humans.
Of course, in both the Y-DNA and mtDNA cases, isolated exceptions to the rule could wash out of the gene pools due to genetic drift.
6 comments:
"Or, it could it be that the sexier theory that Homo neanderthalensis interbred in small numbers with some of the earliest ancestors of all non-African modern humans is correct?"
To me that is the more logical explanation. The idea than modern humans descend from a single subset of a wider population, with no subsequent mixing, owes its hold to several older views of biology. Firstly the Genesis belief of everything being descended from a single pair on an ark. Secondly to the Victorian economic belief in 'survival of the fittest. Thirdly because it fits early twentieth century ideas a 'racial purity'. Fourthly because it is simple.
The idea doesn't make sense at the most basic level. If any single subset of an existing species becomes genetically isolated it almost always becomes extinct through inbreeding. To survive any species needs at least some level of hybrid vigour.
"In these models, a principle called Haldane's law insures that almost all of the fertile Neanderthal-modern human hybrids were probably female in the first generation, accounting for the lack of Neanderthal Y-DNA. Haldane's law says that mixed sex chromosome interspecies hybrids (XY males in the case of hominins) tend not to be born, or to be infertile if they are born, relative to homogeneous sex chromosome interspecies hybrids (XX females in the case of hominins)".
I agree with your comments regarding the European scenario and the paucity of Neanderthal DNA. However I don't think Haldane's Law applies in this instance. I doubt that Neanderthals were as different from us as to suffer reduced male fertility. Although genetic difference is not closely correlated to time generally mammal species separated by less than a million years are fully inter-fertile.
"Neanderthal mtDNA's absence is explained by the theory that hybrid children grew up with a tribe made up of the species of the mothers rather than the fathers"
That was probably true regardless of fertility levels. Children are usually brought up by their mothers after all. And many ancient tribes were quite possibly matriarchal. We find that situation during the Polynesian migration out into the Pacific. And In East Asia societies appear to have been matriarchal until the last thousand years or so.
"the hybrid children born into Neanderthal tribes died out with the rest of the Neanderthal population"
Very likely indeed.
"the hybrid children born into modern human tribes have descendants who are alive today".
The expanding tribes are likely to have been largely closed shops, accepting only limited numbers of 'foreigners' and then usually females who were greatly outnumbered. That seems to be the situation with the migration to America. It was proably led bt Y-DNA Q and perhaps mt-DNA X, but along the way the tribes picked up eastern mt-DNA haplogroups.
"many ancient tribes were quite possibly matriarchal"
The only direct evidence we have on point - a family of Neanderthals that was together when a cave collapsed on them where it was possible to conduct DNA tests upon the remains showed patrilocality. Obviously, this one data point doesn't govern what is possible.
But, the genetic evidence from Neanderthals suggest that there were only two to perhaps four subpopulations of them and it would be plausible to expect that a given subpopulation would share rules on patrilocality.
Also, the greater gender dimorphism in Neanderthals than in modern humans is indirect evidence of patrilocality.
And, patrilocality is by far the more common situation in ethnographically studied modern human societies even though there are absolutely modern human cultures in which matrilocality prevails.
In my view, the answer is that Neanderthal-modern human hybrids were generally the product of short term sexual encounters rather than long term marriage-like relationships in which both parents would live together in the same tribe (there is no archaelogical evidence of even a single tribal band that had both Neanderthals and modern humans in it at the same time, although there are a handful of skeletal remains that are arguably admixed) Instead, these were probably cases of single mothers staying in a tribe of their own species. Thus, general societal rules of patrilocality in long term marriage-like relationships wouldn't apply to these cases.
One doeesn't need to resolve whether these short term sexual encounters were rapes, were flings, or were genuine romantic relationships as long as a few months while the father's tribe and the mother's tribe were close to each other (perhaps, for example, at a major crossroad for game at a certain time of year, both a Neanderthal and modern human tribe would have been near each other and there might have been room for a hunting season relationship). Any relationship length shorter than the gestation period (i.e. shorter than nine months or so) ought to produce the same result.
An alternative to Haldane's law would be infanticide and selection (reproductive and otherwise). Neanderthal men were more different from modern human men than Neanderthal women were from modern human women (due to greater sexual dimorphism).
Infanticide of non-cute babies continued to be widespread at least through the era of ancient Rome. And, even if not the subject of infanticide, a hybrid boy may have been seen as a poor mate by people in a modern human tribe were hunting methods and styles were adapted to one or more unspecified abilities that modern humans had and Neanderthals lacked. Perhaps hybrid women were better able to fit into their gender roles in modern human tribes than hybrid men and hence were not seen as comparably unfit as mates.
John Hawks has a post about high Neanderthal admixture in the frozen Otzi individual. This tends to support the dilution by later migration model that I suggested in the original post.
"John Hawks has a post about high Neanderthal admixture in the frozen Otzi individual".
Thanks for drawing my attention to that. This bit from his abstract sums it up for me:
"The genome of this Neolithic-era individual shows a substantially higher degree of Ne- andertal ancestry than living Europeans. This comparison suggests that early Upper Paleolithic Europeans may have mixed with Neandertals to a greater degree than other modern human populations".
"This tends to support the dilution by later migration model that I suggested in the original post".
I think you are absolutely correct.
"The only direct evidence we have on point - a family of Neanderthals that was together when a cave collapsed on them where it was possible to conduct DNA tests upon the remains showed patrilocality".
We have evidence from China that surnames there were passed through the mother until relatively recently. This has been taken as evidence of matrilocality in that region. Matrilocality is certainly not unknown in human societies and so was probably present in ancient times as well. In most mammals the females stay with their mothers and the males move, often forming bachelor groups until they are able to conquer a group of females for themselves.
"Also, the greater gender dimorphism in Neanderthals than in modern humans is indirect evidence of patrilocality".
Gender dimorphism in primates is usually an indication of single male/several females. So perhaps that was the neanderthal social organisation.
"there is no archaelogical evidence of even a single tribal band that had both Neanderthals and modern humans in it at the same time, although there are a handful of skeletal remains that are arguably admixed"
I think it is fairly obvious that those skeletons are of admixed individuals.
"One doeesn't need to resolve whether these short term sexual encounters were rapes, were flings, or were genuine romantic relationships as long as a few months while the father's tribe and the mother's tribe were close to each other"
I think it not unlikely at all that members of each species would be accepted on occasions into the other species. We have examples of indigenous people being accepted into incoming societies as well as the more common example of incoming individuals being accepted into indigenous societies in more recent times. Of course it is possible to argue that in the modern case we are talking of definitely the same species but in many cases the indigenous population was initially regarded as as a separate species.
"Infanticide of non-cute babies continued to be widespread at least through the era of ancient Rome. And, even if not the subject of infanticide, a hybrid boy may have been seen as a poor mate by people in a modern human tribe were hunting methods and styles were adapted to one or more unspecified abilities that modern humans had and Neanderthals lacked. Perhaps hybrid women were better able to fit into their gender roles in modern human tribes than hybrid men and hence were not seen as comparably unfit as mates".
I think you have become committed to seeing Neanderthals and modern humans as extremely different from each other. I see them as basically the same and limitation of modern survival of Neanderthal genes as much because of 'dilution by later migration' as from mutual revulsion.
"I think you have become committed to seeing Neanderthals and modern humans as extremely different from each other. I see them as basically the same[.]"
The kinds of key points where I see distinctions are: (1) Neanderthals had a more of a big game predator subsistance strategy, (2) modern humans have much more and more sophisticated evidence of fishing and tool making with e.g. bone rather than stone, (3) the modern human tool set showed signs of technological advance about ten times as frequently as Neanderthals and with more regional variation, (4) there was notably less sexual dimorphism in modern humans than Neanderthals, (5) modern humans show indications from the start of greater use of more sophistcated art (Neanderthals were not entirely without funeral ceremonies and art, but it was much, much less), (6) modern humans show indications from the start of greater use of crafted clothing and crafted structures, (7) modern human population density appears to be somewhat greater from the start, (8) Neanderthals have more physical adaptations to cold environment that were absent in modern humans and probably had light pigmentation and possible red hair although with different genes coding those phenotypes than are found in modern humans.
If I were to sum up the different species strategies, modern humans were less morphologically specific but were culturally more plastic and innovative. Neanderthal culture is very static from the start of the Mousterian until the very period after contact with modern humans, which is a really long time over a quite broad geographic area.
Also, the evidence of trade or cooperation in an organized way between Neanderthals and modern humans is scarce, the duration of co-existence within contact range of each other in any specific location doesn't appear to be any longer than 1,000 years and may have been shorter (the resolution of the dating isn't capable of much finer distinctions), and there is no evidence of mixed pure types in a single tribe - archaeological evidence, for example in Italy, pointing that way has been ruled out by more careful re-examination of the evidence. The relative low levels of overall admixture support this view.
And, for some reason, when the Augnacian came around, the Neanderthals spiralled into extinction while the modern humans didn't.
I don't think, for example, that it is likely that any Neanderthal would have been capable of learning a modern human language in anything approaching fluency.
I don't disagree that Neanderthals may have been equally competent or even better big game hunters in their ecological niche than modern humans and may have had some kind of significant smarts and may have had loving intratribe relationships (and may have seen their own babies as cute even if the modern humans didn't and visa versa). But, I think that they were probably much more hard wired than modern humans, and that it is likely the Homo Erectus was even more hard wired than the Neanderthals were.
'Also, the evidence of trade or cooperation in an organized way between Neanderthals and modern humans is scarce"
That is probably the most telling point. However it also seems to hold for the inter-Neanderthal situation. Modern humans display far wider social interactions than do Neanderthals. But that is as much a demonstration of social development as it is genetic change. The same seems applicable to the first 7 points of difference you raise. They indicate social evolution rather than genetic separateness.
"(8) Neanderthals have more physical adaptations to cold environment that were absent in modern humans and probably had light pigmentation and possible red hair although with different genes coding those phenotypes than are found in modern humans".
I agree totally with that. In fact I wouldn't be surprised if it is eventually shown that northern Neanderthals were blond. However, again, I don't think the level of genetic difference indicates any greater divergence than that between subspecies. Several modern human groups are genetically adapted to particular ecological conditions but perfectly able to form fully fertile offspring with other human groups. I am sure that other explanations than genetic incompatibility are necessary to explain the very small level of neanderthal genetics in modern humans. Social separation is a far more likely explanation in my view. Perhaps similar to the racism separating Europeans and many indigenous groups during the 18th and 19th centuries.
"And, for some reason, when the Augnacian came around, the Neanderthals spiralled into extinction while the modern humans didn't".
I strongly suspect that modern humans, with their much wider social circles, were more able to survive inclement conditions than were Neanderthals with their smaller groups (more likely to suffer extinction during hard times).
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