Tuesday, October 30, 2012

Latest on Human/Neandertal hybrids

You probably recall the interesting evidence of hybridization between humans and Neandertals initially discovered when a (very) rough draft of the Neandertal genome was published.  I believe this evidence of hybridization has significant consequences for the Christian debate over origins, especially for those who believe that Adam and Eve were Homo sapiens, to the exclusion of other species.  If you took that position, you now have four basic options:

1. Deny that humans and Neandertals are separate species, which strikes me as possible but quite difficult given the morphological, developmental, and genetic differences between the two.
2. Abandon your insistence that humans are only one species and allow for human speciation (either within a human "created kind" or as macroevolution from non-human ancestors).
3. Maintain your belief in a single human species separate from Neandertals by affirming the possibility of offspring from bestiality, a position that I critiqued in a recent essay.
4. Deny the evidence of interbreeding or offer a different interpretation of that evidence.

As I noted here, the fourth option was explored in an article published in PNAS, but another study supported the initial diagnosis of hybridization using a method of exploring "linkage disequilibrium," which can tell you whether two populations have been hybridizing or not.  Now there's a new study from Sánchez-Quinto et al. published in PLoS ONE that further supports the hypothesis of hybridization with an intriguing twist.

Here's how it works:  The initial study looked at genes from Asians, Europeans, and sub-Saharan Africans and noted that the Eurasian genomes had an excess of sequences shared between Neandertals but not sub-Saharan Africans.  Since Neandertals had a Eurasian distribution, it made sense that any interbreeding would leave Neandertal genes in Eurasian populations but not sub-Saharan African ones.

The new twist is the question of what happened to African populations from north Africa.  Since Neandertal fossils have been found in modern Israel, could Neandertals have interbred with ancestors of north Africans?  According to the new paper from Sánchez-Quinto et al., the answer is yes.  Could this evidence of interbreeding with north Africans be the result of recent interbreeding with Eurasians?  That's definitely possible, but Sánchez-Quinto et al. controlled for that by selecting specific cultural groups that are known to have lived in north Africa for a very long time, and they have an excess of Neandertal sequences when compared to other north Africans.  These results add further support to the contention that Neandertals interbred with the ancestors of modern humans.

Read all about it.  PLoS ONE is free.

Sánchez-Quinto et al. 2012. North African Populations Carry the Signature of Admixture with Neandertals. PLoS ONE 7(10): e47765.

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