January is mid Summer in Argentina, holiday season so I have taken some time off to read and relax. And as usual I found an interesting article: Alan R. Rogers and Ryan J. Bohlender Bias in Estimators of Archaic Admixture, 23.Dec.2014. I will not bore you with the technical matters, I will go straight to the point:
Rogers and Bohlender take a look at the methods and assumptions used in estimating admixture of ancient humans (Denisovans and Neanderthals) in modern H. sapiens. They noticed that these methods "are sensitive to “ghost” admixture, which occurs when a population receives archaic DNA from sources not acknowledged by the statistical model". They detail different estimators and take the cases of Europeans and Melanesians for Neanderthal and Denisovan admixture to check their hypothesis.
They do find that " Such comparisons indicate that archaic gene flow into Europe came primarily from Neanderthals and support the view that archaic populations were much smaller than those of early modern humans, which supports their theory but, when it comes to Melanesians: they expose an inconsistency, and point out that "Although this inconsistency may be a statistical artifact,it could also result from an incorrectly specified model. For example, Homo erectus may have contributed genes to populations of Denisovans (Prüfer et al., 2014, p. 48) or to modern humans in Melanesia (Mendez et al., 2012)."
This is a very interesting conclusion, I don't recall having read anyone suggest that H. erectus mingled with Denisovans or with modern Humans in Melanesia... but it is, and always has been the best explanation.
How did the genes of a group of Denisovans living in Altai get into the lineages of a coast-hugging migration Out Of Africa on its way to Melanesia? If the ancestors of Melanesians did not cross the Himalaya and other mountain ranges to reach Altai then the Denisovans had to do the same, in the opposite direction... and this does not sound reasonable.
The simple explanation: a group of humans (H. erectus) who have been living in South East Asia and China for over one million years were the common link between Denisovans and Melanesians. They were directly in the path of the migrating humans. Ergo they admixed, humans and erectus.
And this is exactly what I suggested over a year ago in my post of Dec. 21, 2013 Denisovans interbred with Homo erectus after reading Prüfer et al. paper.
I had not seen, however, the other paper mentioned by Rogers and Bohlender:
Mendez, F. L., Watkins, J. C., Hammer, M. F., 2012. Global genetic variation at OAS1 provides evidence of archaic admixture in Melanesian populations. Molecular Biology and Evolution 29 (6),1513–1520.)
So I looked it up and read it, and to my surprise, they (Mendez, Watkins and Hammer) reached the same conclusion outlined above:
Another unanswered question concerns how gene flow occurred given the large geographic distance between the Denisova archeological site in the Altai Mountains and the likely route of migration of Melanesian ancestors. It is unclear whether individuals carrying Denisova-like sequences in the Altai region contributed directly or shared some genetic variation with the population that contributed to the ancestry of Melanesians.
Then they try to identify the source: " we suggest that the archaic ancestor contributing the deep lineage to Melanesians and the specimen from Denisova were members of genetically differentiated populations. Interestingly, the coalescent time of the human reference and deep lineage of 3.3 Ma is much older than the population divergence time of ∼0.3 Ma for humans and Denisova suggested by Reich et al. (2010).... These observations present the intriguing possibility that this deeply diverged region of OAS1 may have introgressed into the common ancestor of Denisova and Melanesians via admixture with an unsampled hominin group, such as Homo erectus. In fact, the introgression of a more archaic form into the ancestors of Denisova was also considered by Reich et al. (2010) to explain some archaic morphological features of the Denisova molar. However, without DNA from this “ghost” population or evidence from other loci showing a similar pattern of extreme haplotype differentiation, such a scenario is speculative.".
So what we need is DNA from H. erectus, for comparison, something which may be very unlikely, but who know, maybe a relatively recent H. erectus fossil may be found with viable DNA for sequencing... After all if the H. floresiensis survived until 17 kya, maybe H. erectus did so too... after all, they had to be there, in person, to introgress with humans.
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