Tuesday, November 07, 2006

Did Modern Humans Get a Brain Gene from Neandertals?   posted by Razib @ 11/07/2006 05:37:00 PM

Read about it here. Here is the paper, Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage:
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 {approx}37,000 years ago and swept to exceptionally high frequency ({approx}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 {approx}1.1 million years ago and introgressed into humans by {approx}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.

The full paper is free. Read it, bitches. More to come soon.... (commentary, plus papers)

Update by Darth Quixote: The D haplogroup is the same version of MCPH1 that has nearly fixed in many human populations outside of sub-Saharan Africa, as reported at Gene Expression last year (Bruce Lahn's brain on ASPM and MCPH1). Here is the Cliffs Notes part of the new paper's Discussion:

In this study, we investigate the origin of the microcephalin D allele in modern humans. We show that the D allele is unlikely to have arisen within a panmictic population. Instead, our data are consistent with a model of population subdivision followed by introgression to account for the origin of the D allele. By this model ... the lineage leading to modern humans was split from another Homo lineage, and the two lineages remained in reproductive isolation for ~1,100,000 years. During this period of reproductive isolation, the modern human lineage was fixed for the non-D allele at the microcephalin locus, whereas the other Homo lineage was fixed for the D allele. These two alleles are differentiated by a large number of sequence differences accumulated during the prolonged isolation of the two populations. At or sometime before ~37,000 years ago, a (possibly rare) interbreeding event occurred between the two lineages, bringing a copy of the D allele into anatomically modern humans. Whereas the original D-bearing Homo population has since gone extinct, this introgressed copy of the D allele in humans has subsequently spread to exceptionally high frequency throughout much of the world because of positive selection.

Update by Jason Malloy: Post up @ MetaFilter.