Thursday, September 08, 2011

R.I.P. Out-of-Africa Replacement Model?

"When two groups of people meet, they may fight but they will always mate." 
-J. Lawrence Angel

It now seems that the last bastion of uniqueness for modern humans as a species is crumbling.  If true, this is huge news.  Science Daily is reporting that even after their origination in Sub-Saharan Africa, modern humans may have interbred with archaic Homo sapiens.  They write:
In a paper published in the Proceedings of the National Academy of Sciences (PNAS), a team led by Michael Hammer, an associate professor and research scientist with the the University of Arizona's Arizona Research Labs, provides evidence that anatomically modern humans were not so unique that they remained separate. "We found evidence for hybridization between modern humans and archaic forms in Africa. It looks like our lineage has always exchanged genes with their more morphologically diverged neighbors," said Hammer, who also holds appointments in the UA's department of ecology and evolutionary biology, the school of anthropology, the BIO5 Institute and the Arizona Cancer Center.
This wasn't done by comparing modern DNA with ancient DNA, however.   They write:
First, the team sequenced vast regions of human genomes from samples taken from six different populations living in Africa today and tried to match up their sequences with what they expected those sequences to look like in archaic forms. The researchers focused on non-coding regions of the genome, stretches of DNA that do not contain genes, which serve as the blueprints for proteins. "Then we asked ourselves what does the general pattern of variation look like in the DNA that we sequenced in those African populations, and we started to look at regions that looked unusual," Hammer said. "We discovered three different genetic regions fit the criteria for being archaic DNA still present in the genomes of sub-Saharan Africans. Interestingly, this signature was strongest in populations from central Africa." The scientists applied several criteria to tag a DNA sequence as archaic. For example, if a DNA sequence differed radically from the ones found in a modern population, it was likely to be ancient in origin. Another telltale sign is how far it extends along a chromosome. If an unusual piece is found to stretch a long portion of a chromosome, it is an indication of being brought into the population relatively recently.
Aside from the assumptions that go into such research, it will likely send shock waves through the discipline, and will certainly require the revision of some cherished models concerning the origins of our species. It certainly rules out the idea that there was a speciation event in a cladistic sense and stretches our species back much further than we had originally thought.  Look for these results to be challenged soon.

No comments:

Post a Comment