There’s a new paper out, Partial genetic turnover in neandertals: continuity in the east and population replacement in the west. The primary results are above. Basically, using 13 mtDNA samples the authors conclude that it looks as if there was a founder effect for Neanderthals in Western Europe ~50 K years ago, generating a very homogenized genetic background for this particular population before the arrival of modern humans. Perhaps it’s just me, but press releases with headlines such as “European Neanderthals Were On the Verge of Extinction Even Before the Arrival of Modern Humans” strike me as hyperbolic. I’m also confused by quotes like the one below:
Hominin increase in cranial capacity, courtesy of Luke Jostins
A few years ago a statistical geneticist at Cambridge’s Sanger Institute, Luke Jostins, posted the chart above using data from fossils on cranial capacity of hominins (the human lineage). As you can see there was a gradual increase in cranial capacity until ~250,000 years before the present, and then a more rapid increase. I should also note that from what I know about the empirical data, mean human cranial capacity peaked around the Last Glacial Maximum. Our brains have been shrinking, even relative to our body sizes (we’re not as large as we were during the Ice Age). But that’s neither here nor there. In the comments Jostins observes:
The data above includes all known Homo skulls, but none of the results change if you exclude the 24 Neandertals. In fact, you see the same results if you exclude Sapiens but keep Neandertals; the trends are pan-Homo, and aren’t confined to a specific lineage….
Last August I had a post up, The point mutation which made humanity, which suggested that it may be wrong to conceive of the difference between Neanderthals and the African humans which absorbed and replaced them ~35,000 years ago as a matter of extreme differences at specific genes. I was prompted to this line of thinking by Svante Pääbo‘s admission that he and his colleagues were searching for locations in the modern human genome which differed a great deal from Neanderthals as a way through which we might understand what makes us distinctively human. This sort of method has a long pedigree. Much of the past generation of chimpanzee genetics and now genomics has focused on finding the magic essence which differentiates us from our closest living relatives. Because of our perception of massive phenotypic differences between H. sapiens and Pan troglodytes the 95-99% sequence level identity is thought by some to be perplexing. Therefore models have emerged which appeal to gene regulation and expression, or perhaps other forms of variation such as copy number, to clear up how it can be that chimpanzees and humans differ so much. Setting aside that the perception of difference probably has some anthropocentric bias (i.e., would an alien think that chimpanzees and humans are actually surprisingly different in light of their phylogenetic similarities? I’m not so sure), it doesn’t seem to be unreasonable on the face of it to plumb the depths of the genomes of hominids so as to ascertain the source of their phenotypic differentiation.
But can this model work for differentiating different hominin lineages? Obviously there’s going to be a quantitative difference. The separation between chimpanzees and modern humans is on the order of 5 million years. The separation between Neanderthals and modern humans (or at least the African ancestors of modern humans ~50,000 years B.P.) is on the order of 500,000 years. An order of magnitude difference should make us reconsider, I think, the plausibility of fixed differences between two populations explaining phenotypic differences.
Even if the odds of successful interbreeding were just 5 percent, Neanderthal genes would make up the majority of the human genome today. As it is, a lack of viable sex explains why none of the Neanderthals’ mitochondrial DNA made its way into modern humans, and why so little of their main genome did.
Currat and Excoffier suggest that either modern humans and Neanderthals didn’t have sex very often, or their hybrids weren’t very fit. They favour the first idea. According to their model, it would only have taken between 197 and 430 liaisons between ancient humans and Neanderthals to fill 1-3 percent of modern Eurasian genomes with Neanderthal DNA. Considering that they two groups probably interacted for 10,000 years or so, it would have been enough for one human to sleep with one Neanderthal every 23 to 50 years.
From what I gather in the comments this is due to the fact that if there was a wave of advance very small levels of admixture per unit of advance can build up rather rapidly. I think this is easy to express in temporal rather than spatial terms.
For example, let’s imagine a population of modern humans expanding into a population of Neandertals. The original source population doesn’t receive any more contributions after the initial push, so you have a series of admixture events over time. Assuming 5% admixture per generation, this is the dilution of the “original ancestry” which would occur over 30 generations, or 750 years:
Steve Hsu points me to a piece in The New Yorker on the science and personality of Svante Pääbo. The personality part includes references to Pääbo’s bisexuality, which to me seemed to be literally dropped into the prose to spice it up. Of course it was the science which I found interesting. There are many more bisexuals than there are heterodox scientists. And yet like many researchers of yore it seems that Pääbo is out to find the genes which make humanity distinctive as we understand it (if the reporting is accurate, which I don’t take as a given). There are some interesting tantalizing clues littered about; some genes implicated in autism seem to exhibit Neandertal vs. modern human differences (with the Neandertals carrying the autism-implicated variants).
Ewen Callaway has a good survey of what’s been going down in ancient human genomics over the past year in Nature, Ancient DNA reveals secrets of human history. It’s not paywalled, so read the whole thing. Most of it won’t be too surprising for close readers of this weblog, but this part is new:
By comparing individual DNA letters in multiple modern human genomes with those in the Neanderthal genome, the date of that interbreeding has now been pinned down to 65,000–90,000 years ago. Montgomery Slatkin and Anna-Sapfo Malaspinas, theoretical geneticists from the University of California, Berkeley, presented the finding at the Society for Molecular Biology and Evolution meeting in Kyoto, Japan, held on 26–30 July.
Slatkin says that their result agrees with another study presented at the meeting that came from the group of David Reich, a geneticist at Harvard Medical School in Boston, Massachusetts, who was involved in sequencing both the Neanderthal and Denisova genomes. The dates also mesh with archaeological finds bookending early human migrations out of Africa to between about 50,000 and 100,000 years ago. Reich’s team is now developing tools to find signs of more recent interbreeding that might have occurred after humans arrived in Asia and Europe.
Remember that the Neandertal admixture seems present in all non-Africans. That pegs the admixture event very early, before the diversification of modern human populations. I wouldn’t put too much stock in any one value presented at a conference with a large confidence interval. From what I hear there will be much more on statistical genetic inferences of admixture timing over the next year, but if there’s one thing that the enormous yield from genomes constructed from ancient DNA has convinced me is that we should be really cautious of results which we can’t cross-check easily because of their time depth. I read a lot of papers by high-powered teams before 2010 on how the genomic evidence implied no admixture between modern human and archaic lineages, and gave them great weight. There’s only so much power in working back to the past from the present.
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.
Recently something popped up into my Google news feed in regards to “Neanderthal-human mating.” If you are a regular reader you know that I’m wild for this particular combination of the “wild thing.” But a quick perusal of the press release told me that this was a paper I had already reviewed when it was published online in January. I even used the results in the paper to confirm Neanderthal admixture in my own family (we’ve all been genotyped). One of my siblings is in fact a hemizygote for the Neanderthal alleles on the locus in question! I guess it shows the power of press releases upon the media. I would offer up the explanation that this just shows that the more respectable press doesn’t want to touch papers which aren’t in print, but that’s not a good explanation when they are willing to hype up stuff which is presented at conferences at even an earlier stage.
A second aspect I noted is that except for Ron Bailey at Reason all the articles which use a color headshot use a brunette reconstruction, like the one here which is from the Smithsonian. But the most recent research (dating to 2007) seems to suggest that the Neanderthals may have been highly depigmented. This shouldn’t be too surprising when one considers that they were resident in northern climes for hundreds of thousands of years.
But there are some new tidbits, from researchers in the field of study:
Update: John Hawks’ lab is working in the same area, and he disagrees with the specific results presented here. Always reminds you to be careful about sexy results presented at conference! (someone should do a study!)
So claimed Peter Parham at a Royal Society meeting last week, Human evolution, migration and history revealed by genetics, immunity and infection. You can actually listen to the talk by pulling down the mp3 file. To get the part about human evolution and introgression, jump to 24 minutes in.
Here is the general sketch: It looks like ~50 percent of the HLA Class I alleles in Europeans derive from Neandertals, ~70-80 percent of HLA Class I alleles in East Asians derive from Denisovans, and that and ~90-95 percent of HLA Class I alleles in Papuans derive from Denisovans. If you recall, ~2.5% of the total genome content of non-Africans seems to be Neandertal, while ~5% of the total genome content of Papuans seems to be Denisovan. The total genome content proportions are rough estimates, there may be some wiggle room in there. But you can see that the HLA allele admixture estimates from these ancient Eurasian lineages is greater by an order of magnitude. Why?
Mr. James Winters at A Replicated Typo pointed me to a short hypothesis paper, Neanderthal-human Hybrids. This paper argues that selective mating of Neandertal males with females of human populations which had left Africa more recently, combined with Haldane’s rule, explains three facts:
- The lack of Neandertal Y chromosomal lineages in modern humans.
- The lack of Neandertal mtDNA lineages in modern humans.
- The probable existence of Neandertal autosomal ancestry in modern humans.
If you don’t know, Haldane’s rule basically suggests that there’s going to be some sort of breakdown in the heterogametic sex. In humans females are homogametic, XX, and males are heterogametic, XY. The breakdown need not be death (or spontaneous abortion). It could be sterility (e.g., some mutation or genetic incompatibility which results in the malfunctioning of the flagella of sperm would do it).
So you have a scenario where only Neandertal males are interbreeding with the intrusive groups from the south. The hybrids from these pairings would then lack Neandertal mtDNA, since mtDNA is passed only from mothers. But the male offspring would have Neandertal Y chromosomes. This is where Haldane’s rule kicks in: these males in their turn would not reproduce. Therefore only the female hybrids would pass on their genes. These females obviously don’t pass on a Y chromosome. And, they would pass on their non-Neandertal mother’s mtDNA.
Mitochondrial DNA from 147 people, drawn from five geographic populations have been analysed by restriction mapping. All these mitochondrial DMAs stem from one woman who is postulated to have lived ab7out 200,000 years ago, probably in Africa. All the populations examined except the African population have multiple origins, implying that each area was colonised repeatedly
And so was published in the year 1987 the paper which established in the public’s mind the idea of mitochondrial Eve, which gave rise to a famous cover photo in Newsweek. This also led to the Children of Eve episode on the PBS documentary NOVA. Here is the summary:
NOVA examines a controversial theory that traces our ancestry to a small group of women living in Africa 300,000 years ago.
As Milford Wolpoff has complained it is probably accurate to characterize the documentary as not particularly “fair & balanced.” Mitochondrial Eve may have been controversial, and subsequently plagued by issues of molecular clock calibration as well as spurious interpretations of the cladograms, but the tide of history was on its side, and PBS was telling that story. And the story was not just the primary science, rather, one had to understand the controversy in light of the debates among paleontologists and between paleontologists and molecular biologists. A group of researchers, spearheaded by Chris Stringer argued for the recent origin of modern humans from Africa on the basis of fossils alone. They were challenged by an established school of multiregionalists who argued for deeper roots of modern human populations, which derived from local hominins which diversified after the the migration of H. erectus out of Africa. The argument of the multiregionalists was that selective sweeps across the full range of the human populations gave rise gradually to modern humanity as we know it, a compound of specific ancient local features and trans-population characters which unified us into a broader whole. Stringer and company presented a simpler model where anatomically modern human being arose ~200,000 years ago in Africa, and subsequently expanded to other parts of the world, by and large replacing the local hominin populations. In the multiregionalist telling Neandertals became human beings, while Out of Africa would imply that Neandertals were replaced by human beings.