The amazing talking Neandertals19 Oct 2007
This week, Johannes Krause and colleagues from the Max Planck Evolutionary Anthropology institute announced that they had tickled FoxP2 out of two Neandertal specimens from El Sidrón, Spain. The bones were excavated in sterile (clean-cave?) conditions, immediately frozen and then shipped to Leipzig, where extracts were taken in clean-room conditions.
Here's an FAQ about what they found.
Why is this paper really important?
Isn't it obvious? It's important because it demonstrates that more than one Neandertal is suitable for nuclear genome recovery. We will know about genetic variation in Neandertals, sooner rather than later. These two bones come from different individuals, because the Leipzig group found two different mtDNA sequences in them. Together with the Vindija Vi 33.16 specimen in the original Neandertal genome papers, this makes three nuclear genome Neandertals. There will be more.
It also shows the possibility of probing ancient skeletons for specific genes. Here, they went in looking for Y-DNA, X-DNA and particular sites on FoxP2, and they found them. That is definitely the way to go if you want to test a biologically significant hypothesis fast -- otherwise, you just have to wait until the sequence comes up in your genome project.
However, I question the value of probing for individual genetic variants in this way. Every probe takes a bit of sample, which might be more efficiently used in whole-genome sequencing. We have 25,000 genes, and every one is potentially interesting. Every small sample used to assay only one of those genes may destroy many sequences from the others. It would be one thing if samples were trivial and easily replaced, but they obviously aren't.
Still, we will certainly see additional probes for genes that are of particular interest. I wouldn't be surprised to see MC1R results soon, to probe whether there were pigmentation variants in Neandertals. The same has already been done for woolly mammoths.
So, Neandertals had the human-specific FoxP2 form. Did they talk?
I think the genetic observation leans toward that direction, but doesn't really change our understanding. Consider:
Neandertals have a hyoid bone with humanlike anatomy, as did the Atapuerca people at more than 300,000 years ago, even though A. afarensis did not. So something related to vocalization evolved in humans by the Middle Pleistocene. Although Neandertal vocal tracts may not have been identical to recent humans, there is nothing about them that would preclude speech. The only paleoneurological observation about language puts a developed Broca's area on the KNM-ER 1470 endocast, Homo habilis.
Like other Middle Paleolithic/MSA people, their technology required more information to learn than earlier, Lower Paleolithic industries, leading to regional differentiation and more task-specific facies. Late Neandertals made use of some technology otherwise used only by Upper Paleolithic modern humans. Their hunting methods must have required cooperation and may have been impossible without a more sophisticated communication strategy than used by other primates.
All of these things argue for some kind of Neandertal language irrespective of FoxP2.
Then again, most of the arguments against humanlike language facility in Neandertals also have nothing to do with FoxP2, either. The slow technological progress, limited collection strategies, the rarity of any artistic or symbolic expression, their high mortality rate, and -- of course -- the fact that they no longer exist have all been considered as evidence that Neandertals lacked some essential aspect of "behavioral modernity." If language is a prerequisite for the modern human pattern of behavior, then Neandertals may not have talked, at least not in the way we do.
I think the FoxP2 story has really confused people much more than necessary. But in this case, the confusion is the same that results from every other gene study: when the press says we've found a gene "for" something, what it ought to say is that we've found an allele that affects something.
No macromutation happened. Language did not spring full-formed into the mind of some ancient African. All members of Homo used communication systems including some (possibly minimal) elements of language, and the evolution of the human brain, along with technological changes throughout the Paleolithic, reflect the evolution of communication. Human language evolved -- like all things -- over a long time, and like all complex phenotypes it required a series of mutational changes. Many of these mutations became fixed during recent human evolution, some may still be changing in frequency today. Language evolution is probably a continuing process.
That means that it must have involved many more genes than FoxP2 -- which after all experienced only two amino acid substitutions in all of human evolution. I would imagine the number of genes involved in language evolution is more than 500, and I wouldn't be surprised if it were much more. In that context, it seems quite silly to say FoxP2 is the "critical" evolutionary change for anything.
Then you agree with Language Log. They told me that FoxP2 isn't a "language gene."
The case is strong that the two FoxP2 coding substitutions in humans were selected because of their role in language. The gene sequence is strongly conserved in most mammals, and shows similar changes in some other species with unusual vocal adaptations, such as echolocating bats (Li et al. 2007). Its expression pattern delineates areas related to vocalizations in both humans and birds, and the pattern itself differentiates between song-learning versus nonlearning bird species (Haesler et al. 2004, Teramitsu et al. 2004, Webb and Zhang 2005). And of course, mutations to FoxP2 can result in specific language impairment (SLI) in humans.
Still, that case is only circumstantial. We know that FoxP2 was under selection, that it became fixed in humans, probably during the Late Pleistocene, and that breaking the gene changes brain development and damages language skills. But we don't know what a human would be like with the chimpanzee form of the protein. We don't know whether both of the human-specific amino acid substitutions have a different effect than one. Most important, we don't know what other genetic changes may have been necessary backgrounds for selection on FoxP2.
This means Neandertals were really modern humans, right?
This study should put an end to the "sudden mutation" model of modern human origins.
There was not a single mutation that made the critical difference in the ancestry of today's people. There was no cognitive Rubicon leading to modern human evolution. I would analogize the process as a slow-motion avalanche: at first a few small sands began to tumble, and then selection on a large number of genes became inevitable. FoxP2 is one of those genes, and as yet we don't know whether it was near the beginning or near the end of the process.
But it is clear that the process began before the Neandertals were gone. Some aspects of behavioral complexity did begin to evolve rapidly sometime after 70,000 years ago. This rapid evolution was multiregional in context -- it was not limited to a single human population. In particular, it was not limited to Africans: the last Neandertals clearly manifested technological and behavioral strategies otherwise defined as "behaviorally modern" (d'Errico 2003). There's a reason why the Neandertal-produced Châtelperronian industry of France and Spain was historically considered the first Upper Paleolithic industry.
But we have undergone light-years of change since the last Neandertals lived. This is not a question of "modern human origins" anymore. We can now show that living people are much more different from early modern humans than any differences between Neandertals and other contemporary peoples. I think that "modern humans" is on its way to obsolescence. What matters is the pattern of change across all populations. Possibly that pattern was initiated by changes in one region but the subsequent changes were so vast that the beginning point hardly matters.
We all know that the Neandertal genome is riddled with contamination from modern humans. Isn't the null hypothesis that we have a modern human sequence here because it is a modern human?
Well, as you know, I'm not all that convinced that contamination explains the interpretive discrepancies between last year's genome papers. But still, this study has done some things to address the problem of contamination.
It is notable that Green et al. (2006) found 25% modern human mtDNA in one of the El Sidrón bones: this shows that even "sterile" excavation, immediate freezing and extraction under clean-room conditions cannot exclude contamination. There is at the moment nothing more that can be done. We will always have the problem of a contamination fraction in ancient Neandertal skeletons. So we have to judge each study by the extent to which we can exclude contaminants with statistical analysis.
For this study, Krause et al. (2007) developed a test of nuclear DNA contamination: they identified seven gene variants that differ between the recovered Vindija Vi 33.16 nuclear genome and all known living humans. In other words, these are human-derived mutations that are absent from the only known Neandertal nuclear genome. Then, they probed the El Sidrón bones for these sites. They found only the ancestral form in their extracts of both bones -- presumably because no human contaminants were present in their samples.
That seems like a pretty good indication that the other sites in their sample represent the true gene variants of the ancient Neandertals. I wouldn't go so far as to say that contamination is ruled out, but it seems like these are good results.
Did FoxP2 introgress into Neandertals?
It sure looks that way to me. Let's consider the evidence:
FoxP2 recently fixed in humans. According to Enard et al. (2002:871):
Under a model of a randomly mating population of constant size, the most likely date since the fixation of the beneficial allele is 0, with approximate 95% confidence intervals of 0 and 120,000 years.
Now, Enard et al. (2002) noted that human populations have grown over time, and that they are not randomly mating, so that this date estimate might be too recent. Allowing for population growth since "10,000--100,000 years ago," they asserted that fixation of FoxP2 must have happened "during the last 200,000 years of human history." But this is not quite accurate. Unlike genetic drift, positive selection can and often does fix genes rapidly in a growing population. It simply doesn't matter that the human population has been rapidly growing: FoxP2 may still have just become fixed yesterday.
Last year, Green and colleagues (2006) considered that the Neandertal-modern population divergence time might have been quite recent, depending on the ancestral population size. According to the estimates of Wall and Kim (2007), the Green et al. data are consistent with a Neandertal-modern population divergence time as recent as 30,000 years ago. Of course, that date would predict substantial admixture between contemporary Neandertal and non-European populations -- they would have been exchanging genes up to the very lifetimes of the last Neandertals. According to those data there would be nothing surprising about Neandertals and living people sharing the human-derived FoxP2 allele. But as mentioned above, Wall and Kim (2007) used the recent divergence estimate as evidence that the Neandertal genome data from Green et al. must be contaminated.
So, if we cannot trust the data, then we have to fall back on some other estimate of the divergence date. Noonan and colleagues (2006) estimated a divergence date between Neandertals and modern populations between 170,000 and 570,000 years ago. If we accept that, then the confidence intervals of the Neandertal-human divergence and the FoxP2 selective sweep might barely overlap. Might. But I will note that a minimal overlap between the 95% confidence intervals of two point estimates does not mean that they are not significantly different. Only if the expected value of one estimate falls within the 95% confidence interval of the other do they fail to be significantly different. It is pretty unlikely that the most recent FoxP2 sweep is older than 170,000 years ago and the Neandertal-modern population split is as recent as 170,000 years.
That is, unless the “split” time reflects widespread genetic introgression. </p>
The current paper (Krause et al. 2007) goes to some contortions to try to establish that the FoxP2 sweep could really have been older than 300,000 years ago (where they place the lower confidence limit on the N-M split):
The third scenario is that the selective sweep started before the divergence of the ancestral populations of Neandertals and modern humans around 300,000-400,000 years ago
Let me just say that I was surprised to read this explanation in a paper from this group. One of the main arguments they have been posing as a scientific value of the Neandertal genome sequencing is that conventional methods don't detect selection at 300,000-400,000 years ago. But here, they consider such an ancient mutation to be the most likely hypothesis. This seems like quite a shift just to avoid the unpleasant idea of Neandertal introgression. Ooooh -- can't have those Neandercooties!
In reality, there is no reason to think the fixation of FoxP2 happened as early as 300,000 years ago, and indeed the very high frequencies of the linked derived alleles (over 97% for six of the linked alleles) suggest strongly that the sweep probably happened within the last 100,000 years -- otherwise, subsquent genetic drift should have caused these linked derived alleles to show more dispersion in their current frequencies. The same features that make the inference of selection so strong at FoxP2 -- it is far (>286 kilobases) from the nearest gene and it includes many high-frequency derived alleles in addition to reduced polymorphism -- make it very unlikely that the selective sweep was ancient.
So, considering that the El Sidrón samples both share the human-derived amino acid substitutions on the same haplotype as modern humans, complete with all the high-frequency derived SNPs, it seems almost certain that the gene introgressed into Neandertals from modern humans.
Or, there's one other option. One of the El Sidrón bones includes a relatively rare (in humans) ancestral SNP allele at one of those linked sites where the derived allele is at very high frequency in humans. One explanation: the selected mutation arose in Neandertals and introgressed into other humans. That would explain why this Neandertal didn't have a SNP variant on its FoxP2 haplotype that later became very common in humans: Neandertals had the new FoxP2 first.
What about that Y chromosome thing?
The El Sidrón bones both tested positive for the Y chromosome site assayed in the study. That means they were both male (duh!). But more important, the Y chromosomes of both individuals lacked the human-specific derived mutation that the researchers tested for. Since all human males yet surveyed have this human-derived mutation, this means that a Y chromosome variant has fixed in modern humans that Neandertals did not have. Since the entire nonrecombining portion of the Y chromosome is completely linked, we can infer that the entire modern human Y chromosome has undergone at least one fixation not shared with the ancestors of these Neandertals.
Here's the text (from page 2):
Both Neandertals yielded products for Y chromosomal primer pairs, indicating that they were males. Strikingly, all 15 Y chromosomal products for the five assayed positions show the ancestral allele. This includes two polymorphisms that define the deepest split among current human Y chromosomes (Y2 and Y4, Figure S1) as well as two polymorphisms that cover less common African Y chromosomes (Y3 and Y5, Figure S1). These Y chromosome results must derive, then, either from Y chromosomes that fall outside the variation of modern humans or from the very rare African lineages not covered by the assay (Figure S1). For our purposes, this result shows that neither the maternally inherited mtDNA nor the paternally inherited Y chromosome shows evidence of gene flow from modern humans into Neandertals or of subsequent contamination of their mortal remains.
That's not such a big surprise. Already we knew that the fixation of the human Y chromosome was very recent -- probably within the last 70,000--100,000 years, and possibly even more recently. Every man on earth shares recent Y chromosome mutations that were completely absent in Middle Pleistocene humans. That is one radical recent evolutionary change.
The paper elsewhere suggests that this absence of the human-derived Y chromosome in Neandertals as evidence that they did not contribute other genes to us. I could not disagree more.
The very recent fixation of the Y chromosome in an expanding human population is extremely unlikely to have resulted from genetic drift. Drift does not eliminate rare variants as quickly in an expanding population. Instead, one or more Y chromosome mutations must have been positively selected, resulting in the fixation of the entire NRCY in recent humans.
In that context, the Neandertal result is quite expected: they had an earlier Y chromosome lacking one or more mutations later selected in the other ancestors of living people.
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