Pygmy-pocalypse?

I just want to highlight this graph I came across yesterday while trying to research archaic introgression in the Igbo:

Populationsize
source

From Whole-genome sequence analysis of a Pan African set of samples reveals archaic gene flow from an extinct basal population of modern humans into sub-Saharan populations, by Lorente-Galdos et al.

There are three versions of this graph in the paper (check the supplemental materials for two of them), all showing about the same thing. It is supposed to be a graph of population size at different times in the past, and the most incredible thing is that for the past 100,000 years or so, the most numerically dominant populations in Africa were the Baka Pygmies, followed by various Bushmen (San) groups. The authors write:

To unravel the ancient demographic history of the African populations that are present in our data set, we used the Pairwise Sequentially Markovian Coalescent (PSMC) model that analyzes the dynamics of the effective population size over time [60]. We included at least one representative of each of the 15 African populations and two Eurasian samples in the analysis (Additional file 1: Figure S7.1) and considered both the classical mutation rate of 2.5 × 10−8 [61] and the 1.2 × 10−8 mutations per bp per generation reported in other analyses [6263]. The demographic trajectories of the sub-Saharan agriculturalist populations are very similar to each other; and only South African Bantu and Toubou individuals differ partly from the rest of sub-Saharan farmer samples; however, their considerable levels of admixture with other North African or hunter-gatherer populations (Fig. 2b) might explain this trend. Therefore, in order to ease visualization, we plotted a Yoruba individual (Yoruba_HGDP00936) and two Ju|‘hoansi individuals as representatives of the sub-Saharan agriculturalist and Khoisan populations, respectively (Fig. 3 and Additional file 1: Figure S7.2 considering a mutation rate of 1.2 × 10−8).

The authors note that the apparent large size of the pygmy groups could have been due to groups splitting and merging and thus getting more DNA variety than they would normally. It’s all very speculative. But still, the Baka Pygmies could have been the absolutely dominant group over central Africa for centuries.

What happened?

 

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