A paper published in Nature, by Cosimo Posth et al, published in Nature looks into the odd discrepancy in the age of the split between Modern Human and Neanderthal genomes
The date of the split between humans and Neanderthals differs when you consider their nuclear or their mtDNA:
- Nuclear DNA says that humans and Nearnderthals split some 765,000 to 550,000 years ago
- Mitochondrial DNA or mtDNA says we split 365,000 to 400,000 years ago
I believe that discrepancies are what makes science leap forward (like the problems caused by the "ether" theory that led to relativity and quantum physics). And a gap like this of several hundreds of thousands of years is a big discrepancy. Either the dating is all wrong and has to be reviewed (ie. mutation rates and so forth), or the current ideas on the migration and origin of Homo sapiens are incorrect.
The paper looks into the mtDNA extracted a "complete mtDNA of an archaic femur from the Hohlenstein–Stadel (HST) cave in southwestern Germany. HST carries the deepest divergent mtDNA lineage that splits from other Neanderthals ∼270,000 years ago..."
This date of 270 kya is actualy the average, the real divergence date between HST and all other Neanderthals (95% HPD) is between 316 and 219 kya. The Altai Neanderthals split from the other Neanderthals some ∼160,000 years ago (95% HPD of 199 - 125 kya).
This paper tries to explain the genetic incongruence of Sima de los Huesos (see my post Sima de los Huesos remains, Neanderthals, Denisovans and their nuclear and mtDNA), the remains from Sima de los Huesos in Spain which are 430 ky old, have mtDNA that resembles that of Denisovans more closely than that of the Neanderthals. But, their nuclear DNA is more similar to that of Neanderthals than to Denisovan nuclear DNA.
The authors write (Bold is mine) that the nuclear DNA (nDNA) "... from the Sima de los Huesos site... confirmed their closer affinity to the Neanderthal lineage, suggesting that at least by ∼430 ka, Neanderthals and Denisovans had already diverged. However, in contrast to genome-wide data, the Sima de los Huesos mtDNA was found to branch off with the deeply divergent Denisovan mtDNA lineage. The phylogenetic discrepancies could be reconciled if the mtDNA of early Neanderthals was indeed Denisovan-like and was subsequently replaced by a more derived mtDNA lineage.".
This "derived mtDNA" got into the Neanderthals through " a genetic introgression event from African hominins into the early Neanderthal population that gave rise to the ‘Late Pleistocene’ Neanderthal mtDNA lineage".
This introgression took place long ago, and came from " an African source, which we constrain taking place more than ∼270 ka". In other words the paper suggests an Out of Africa event over 270,000 years ago which admixed human mtDNA into Neanderthals, mtDNA which replaced the older mtDNA -i.e. Denisovan and Sima de los Huesos hominin, with a new lineage, the "African mtDNA that evolved into the Late Pleistocene Neanderthal mtDNA type". And this "new" mtDNA spread from the HST Neanderthal in Germany to all others, including Sidron in Spain and eastwards all the way to Altai. Thousands of kilometers from the Atlantic to Siberia.
Furthermore, they estimate that "if Ne (effective population) was <5,000 units, a mean temporal interval of 300 ka is sufficient for an incoming mtDNA lineage below 0.1% in frequency to drift up to fixation." in other words, a very small initial input of "African genes".
But looking at this theory with a critical eye, we must point out that Neanderthals were spread out over a very wide area, which makes it very difficult for this replacement to take place in the whole population.
Furthermore a different explanation can be found: if modern humans were already outside of Africa i.e. in Asia, and they mixed with the Neanderthals outside of Africa -no Africans involved in the introgression. They would have had plenty of time to admix over a wide area, and this would also explain why there was an Ancient gene flow from early modern humans into Eastern Neanderthals 100,000 years ago as suggested by Martin Kuhlwilm et al, in Nature, Feb. 2016. DOI:10.1038/nature16544.
So instead of suggesting an earlier migration (it actually took place 1.8 Mya when Homo erectus left Africa for Asia) why not think about H. sapiens living in Eurasia and mixing with Neanderthals?
I wonder how this ties in with the Oldest Homo sapiens remains dating back to 315,000 years ago, found in Morocco?
Sources
(1) Deeply divergent archaic mitochondrial genome provides lower time boundary for African gene flow into Neanderthals, Cosimo Posth, Christoph Wißing, Keiko Kitagawa, Luca Pagani, Laura van Holstein, Fernando Racimo, Kurt Wehrberger, Nicholas J. Conard, Claus Joachim Kind, Hervé Bocherens & Johannes Krause. Nature Communications 8, Article number: 16046 (2017). doi:10.1038/ncomms16046.
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