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NEW YORK (GenomeWeb) – Former humans living outside of Africa have met more than once with Neanderthals, according to a new analysis.
The human populations of East Asia and modern Europe have different origins according to the Neanderthal ancestry – East Asians have between 12 and 20% more as Europeans – and there are various theories, including a dilution model, to explain how this came about.
Two researchers from the University of Temple have now modeled various scenarios on how the ancient human-Neanderthal mix could have occurred and compared these findings to the Neanderthal ancestry captured by the 1000 Genomes project . As they reported in Nature Ecology and Evolution today, they have uncovered evidence of a mixture that occurred prior to the divergence of lineages from East Asia and Europe, as well as from Later events involving only the lineage of East Asia or the European lineage.
"These results indicate a longer and more complex interaction between humans and Neanderthal than previously thought," wrote Fernando Villanea and Joshua Schraiber.
According to a previous analysis, the genus of Ust & # 39; – Ishim, 45,000 years old and originally from Siberia and related to both Asians and Europeans, has the same Neanderthal heritage as some Eurasians. today, although longer stretches of DNA. The researchers noted that this was consistent with a mixing event that occurred between 52,000 and 58,000 years ago.
However, to explain the differences of Neanderthal descent between Europeans and East Asians, some scientists have suggested that Europeans diluted it by interbreeding with another population devoid of Neanderthal descent. On the other hand, another hypothesis suggests that different demographic or selective forces have been exerted on East Asians and Europeans, which has affected their level of Neanderthal ancestry. Alternatively, there may have been more than one add-on event.
To deepen this question, Villanea and Schraiber have developed a joint Fractional Frequency Spectrum Matrix (FFS) from calls on the Neanderthal ascending origin identified in the 1000 Genomes Project dataset. . Because of this, they found that East Asians had an average Neanderthal ancestry proportion of 19.6% higher than Europeans, as they expected. They then developed analytical models to determine the appearance of FFS in various demographic models.
Neither the one-pulse mixture nor the dilution model corresponded to what is observed in modern populations. Instead, the researchers found evidence to support a two-pulse model.
They also applied a maximum likelihood approach to their models and again found evidence of the two-pulse model in both East Asian and European populations. This led them to conclude that there was a complex history of mixing between ancient humans and Neanderthals.
With a supervised automatic learning approach, the researchers also studied the impact of changing different demographic parameters on common FFSs between Europe and East Asia. . The researchers found that the most suitable models for the data were the most complex: a three-pulse mixing model and a three-pulse mixing model with dilution. These later mixing impulses affected the East Asian and European populations separately, leading to an increase in Neanderthal origins among East Asians.
As Villanea and Schraiber have noted in their article, the best-adapted model involves multiple cases of miscegenation between Neanderthals and humans. This miscegenation was ongoing, they said, although intermittent and likely to occur in a small geographic area.
However, they added that there were some limitations in their analysis. Namely, their model assumes that Neanderthal ancestry was neutral, whereas it was probably deleterious. In addition, some Neanderthal ascendants among the East Asian populations may have been misclassified and may belong to another group of ancient hominids, the Denisovans.
Yet the scenario described by the researchers is consistent with "the emerging vision of complex and frequent interactions between different groups of hominins," wrote Fabrizio Mafessoni of the Max Planck Institute of Evolutionary Anthropology. in a related comment. He added that there is evidence that Denisovans have crossed with humans at least twice.
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