Neandertal introgression, genetic-style

The paper by Patrick Evans and colleagues, from Bruce Lahn's lab, is now live (and free) at PNAS. There is a short news report by Michael Balter at ScienceNOW, and the Howard Hughes press release is admirably clear.

If you've been hearing a lot about the word "introgression" lately, this is why. At least, the first of the reasons why.

Here's the abstract:

At the center of the debate on the emergence of modern humans and their spread throughout the globe is the question of whether archaic Homo lineages contributed to the modern human gene pool, and more importantly, whether such contributions impacted the evolutionary adaptation of our species. A major obstacle to answering this question is that low levels of admixture with archaic lineages are not expected to leave extensive traces in the modern human gene pool because of genetic drift. Loci that have undergone strong positive selection, however, offer a unique opportunity to identify low-level admixture with archaic lineages, provided that the introgressed archaic allele has risen to high frequency under positive selection. The gene microcephalin (MCPH1) regulates brain size during development and has experienced positive selection in the lineage leading to Homo sapiens. Within modern humans, a group of closely related haplotypes at this locus, known as haplogroup D, rose from a single copy 37,000 years ago and swept to exceptionally high frequency (ca. 70% worldwide today) because of positive selection. Here, we examine the origin of haplogroup D. By using the interhaplogroup divergence test, we show that haplogroup D likely originated from a lineage separated from modern humans 1.1 million years ago and introgressed into humans by ca. 37,000 years ago. This finding supports the possibility of admixture between modern humans and archaic Homo populations (Neanderthals being one possibility). Furthermore, it buttresses the important notion that, through such adminture, our species has benefited evolutionarily by gaining new advantageous alleles. The interhaplogroup divergence test developed here may be broadly applicable to the detection of introgression at other loci in the human genome or in genomes of other species.

I'm starting a second post with Q and A regarding the paper.

Here, I want to note some news:

  1. I have my own paper on introgression (with Greg Cochran) that will be coming in PaleoAnthropology, so it is a topic that to which we've devoted a lot of consideration.
  2. There will be more Neandertal news in the next week. These are busy Neandertal times!
  3. Because we've been working on this topic, I've been avoiding it. But now that some of this stuff has come out, I will point out that there is now a substantial literature on genetic introgression from archaic humans. More in the Q and A.

UPDATE (11/8/2006): My colleague, Greg Cochran, has a post at GNXP discussing introgression and microcephalin further:

If this pans out the way we think it will, introgression from Neanderthals (and maybe with other archaics) may have been one of the two fundamental patterns underlying recent human evolution.

One of the two.


Evans PD, Mekel-Bobrov N, Vallender EJ, Hudson RR, Lahn BT. 2006. Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage. Proc Nat Acad Sci (early edition) DOI link