They've finally done it. Bruce Lahn's lab has an article in PNAS (review here) showing evidence for introgression of a gene from an archaic Homo species into the modern human genome. They suggest the possibility that Neanderthals are that archaic species. That's right: there are Neanderthals among us (at least in small portions of our genome).
For those of you interested in the details, here's a brief summary. The gene in question, microcephalin (MCPH1) has an interesting pattern of polymorphism. The approximate coalescence time for all MCPH1 alleles in humans is approximately one million years ago -- much longer ago than other human genes. That's because there is one haplotype that diverged from all the other alleles a million years ago and evolved independently from the other haplotypes. It then introgressed with the other alleles about 37,000 years ago, back when there were multiple Homo species walking the earth. Lahn and colleagues argue that the two groups of alleles evolved separately because they were found in different species or lineages. At least one mating event introduced the MCPH1 gene into the modern human lineage from the other lineage.
On top of the Neanderthal-modern love story is the fact that this gene plays an important role in brain development. And there is evidence that it has been subject to recent positive selection in the human genome. Some people are going to interpret this to mean that modern humans got smart by borrowing a gene from Neanderthals. While this is very good evidence for interbreeding between the modern human lineage and an archaic species, I'd be reluctant to reach any conclusions regarding the evolution of human mental capacity from these data.
Evans PD, Mekel-Bobrov N, Vallender EJ, Hudson RR, and Lahn BT. 2006. Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage. PNAS In press. doi: 10.1073/pnas.0606966103
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Any particular reason you are using the archaic spelling of Neandertal?
Because that's the spelling used by the authors of the paper.
Wow, this all makes sense ... and all this time nobody would believe me when I said there are a couple Neandrethals living down the street from us ;)
Seriously though, this is a great story. I've always suspected that we must have interbred to a certain degree ... which probably has benefited our gene pool since they did have larger cranial cavities (and likely larger brains).
..and the small fact that the rules of precedence in taxonomy mean that Neanderthal has to remain the name of the genus regardless of how Germany has since reformed its language.
I guess you could argue for Neanderthal in science and neandertal in popular usage but that sounds a bit dumb to me.
Uh ohhh, ... does this mean that those insurance commercials with the "cave-men" are more prescient than I suspected?
Well I've recently started studying Genetics and Evolution. Thats an interesting piece of information.
Why is this an example of introgression and not just common ancestry?
Why is this an example of introgression and not just common ancestry?
Technically, it's both. The reason it's introgression is that modern humans and the archaic species had split into distinct evolutionary lineages and gene pools. The MCPH1 allele from the modern human population shares a common ancestor with the MCPH1 allele from the archaic species (the one that introgressed into the modern human lineage from the archaic lineage). The issue is that the common ancestor of all MCPH1 alleles within contemporary human populations is much further back in time than the the common ancestor of the most other human alleles. That suggests that one of the the MCPH1 alleles in contemporary human populations existed in another lineage (possibly neanderthals) then was reintroduced into the modern human lineage via introgression.
I'm sorry, but I'm not following the reasoning here. I know some physical anthropologists are really skeptical of this issue, but they aren't explaining their side very clearly either.
Are you saying that at some point the MCPH1 allele was lost by humans, and then appeared again? I thought no alleles were actually lost -- just deactivated.
Otherwise, I can't figure out how this isn't just a common ancestry thing. We have common alleles with all vertebrates, down to fish, don't we?
I'm saying that the common ancestor of humans and the archaic subspecies had some set of MCPH1 alleles. Those lineages split ~1mya and evolved somewhat independently for tens of thousands of years. As they diverged, their MCPH1 alleles diverged (there were two separate gene pools). Then, ~37kya, some alleles from the archaic subspecies were introduced into the lineage leading modern humans via hybridization. We can tell that those alleles came from an isolated subspecies because they are very different than the alleles that existed within modern humans for the entire time since the divergence of the archaic subpecies and modern humans.
What makes hybridization the most likely explanation for how MCPH1 showed up again? I'm honestly curious, not trying to be rude or trollish. Other ScienceBloggers, like John M Lynch (StrangerFruit) seems to think its ridiculous.
What makes hybridization the most likely explanation for how MCPH1 showed up again?
How else would a very divergent MCPH1 allele entered the modern human lineage? Balancing selection maintaining multiple haplotypes seems highly unlikely because they have diverged with no genetic exchange. A demographic model would require absurd parameters.
Why couldn't recombination or mutation reintroduce the allele? Like a sort of regressive mutation.
I don't know what you mean by a demographic model -- how the allele spreads is different than how it got there to begin with.
Why couldn't recombination or mutation reintroduce the allele? Like a sort of regressive mutation.
Recombination can only shuffle existing variation. One of the haplotypes is so differetiated from all the other modern haplotypes that there is no plausible mutational scenario that could explain the differences. It had to exist in an isolated gene pool for a long time (~1my) and then be introduced into the modern lineage quite recently (~37kya).
I would suggest reading the actual article, as these issues are all addressed by the authors. Also, a basic understanding of population genetics are necessary before one criticizes the conclusions reached by the authors.