A new paper shows how the mtDNA evidence is inconsistent with Neandertal admixture, by looking at ancient Homo sapiens, Neandertal, and modern human mtDNA. The basic idea is simple, that if admixture with Neandertals did take place at the levels predicted (~4%), then we would expected to see some Neandertal-like mtDNA in either ancient or modern humans, but we don't.
There are some alternative explanations for the lack of Neandertal-like mtDNA in modern humans that I have considered:
- Modern human-Neandertal crossings were female-male exclusively.
- Neandertal mtDNA sequences were weeded out by natural selection, perhaps because Neandertal mtDNA was well-adapted to extreme cold and became maladaptive in modern human bodies, as the latter had better cold-protection technology (tight clothes, for example), and the glaciers retreated anyway.
I had a certain degree of appreciation for #2 until recently. If Europeans had Neandertal admixture, and Neandertal mtDNA was maladaptive to a postglacial technological environment, then selection is a reasonable hypothesis.
However, if Neandertal admixture affected all Eurasians equally, as the current theory holds, then there are plenty of people in North Asia that continued to live in glacial-like temperatures and with predominantly meat diets until the present. Why would Neandertal mtDNA be maladaptive in them?
At present I retain my agnosticism on the subject of Neandertal admxture: the Neandertal genome studies have certainly proven one thing: that archaic humans are not irrelevant to the human story. How they are relevant remains to be seen.
AJPA DOI: 10.1002/ajpa.21569
No evidence of Neandertal admixture in the mitochondrial genomes of early European modern humans and contemporary Europeans
Silvia Ghirotto et al.
Neandertals, the archaic human form documented in Eurasia until 29,000 years ago, share no mitochondrial haplotype with modern Europeans. Whether this means that the two groups were reproductively isolated is controversial, and indeed nuclear data have been interpreted as suggesting that they admixed. We explored the range of demographic parameters that may have generated the observed mitochondrial diversity, simulating 3.0 million genealogies under six models differing as for the relationships among contemporary Europeans, Neandertals, and Upper Palaeolithic European early modern humans (EEMH), who coexisted with Neandertals for millennia. We compared by Approximate Bayesian Computations the simulation results with mitochondrial diversity in 7 Neandertals, 3 EEMH, and 150 opportunely chosen modern Europeans. A model of genealogical continuity between EEMH and contemporary Europeans, with no Neandertal contribution, received overwhelming support from the analyses. The maximum degree of Neandertal admixture, under the model of gene flow supported by nuclear data, was estimated at 1.5%, but this model proved 20–32 times less likely than a model without any gene flow. Nuclear and mitochondrial evidence might be reconciled if smaller population sizes led to faster lineage sorting for mitochondrial DNA, and Neandertals shared a longer period of common ancestry with the non-African's than with the African's ancestors.