For the last few years it’s been pretty much received wisdom that African populations shared only a tiny proportion of their genes, if any, with Neanderthals. In contrast, other non-African sapiens populations had a small but significant admixture of Neanderthal genes. The underlying reason for this, it’s been assumed, is that Homo sapiens and neandertalensis only bred with each other in Europe and Asia.
However, a new study (Chen, Wolf, Fu, Li & Akey, 2020) seems set to change that understanding. The researchers employed a technique that used DNA sequences from a Neanderthal who’d lived in Denisova cave (in the Altai mountains) as an ‘archaic’ reference sample, and compared that with genomes from 2504 modern humans. The technique allowed them to identify sequences that had entered a population’s gene pool via introgression, finding that “African individuals carry a stronger signal of Neanderthal ancestry than previously thought” – about 0.3% of their genome, compared to around 2% in Europeans and Asians. Some of those genes may have been selected for. They also found a lot of overlap between Africans and non-Africans: “of the Neanderthal sequence identified in African samples, more than 94% was shared with non-Africans.”
How to explain this? After careful analysis, the team concluded that there were probably two events involved. The first: Neanderthal genes were carried into the African gene pool by Homo sapiens individuals migrating back into Africa, after picking up Neanderthal sequences as a result of hybridisation. Interestingly, this seems to have happened after European and East Asian sapiens populations diverged, and involved individuals from Europe, not Asia. In addition, their modelling suggested a certain amount of human -> Neanderthal gene flow resulting from migrations at least 100,000 years ago, preceding the main Out-of-Africa diaspora.
Students looking at previous years’ Schol Bio exam papers will have heard about the Jebel Irhoud fossil, an ancient sapiens cranium and jaw that’s a mosaic of ancient and modern characteristics. At least some authors have suggested that this mix of features could be evidence of introgression of Neanderthal genes, perhaps as a result of those back-migrations from Europe into Africa. I think this is very much a case of “watch this space”.