The other day Dienekes pointed out a paper on ancestral human population dynamics within Africa before the out of Africa migrations. The paper is very similar to one I reviewed in April, which also focuses on the diversity of the mitochondrial haplogroup L — one of the oldest mtDNA haplogroups out there.
The new paper, “Bayesian coalescent inference of major human mitochondrial DNA haplogroup expansions in Africa,” published in the Proceedings of the Royal Society B, uses coalescent theory to investigate past population sizes of each of the four major African mtDNA haplogroups (L0-L3). 224 different mitochondrial genomes were analyzed and the comparison yielded some similar results to the previous paper I mentioned. But remember, the last paper investigated the time of the emergence of each haplogroup. This paper focuses on effective population sizes.
Anyways, for starters, the results show that three distinct demographic histories can be seen from the underlying the four haplogroups. Two of the oldest haplogroups, L0 and L1, show exponential growth from 213,000 to 156,000 years ago. The previous paper suggested that the L0 and L1 split about 200,000 years ago. Soon after this split, one of the the paleoafrican branches L0 established what we now consider sub-Saharan Khoisan peoples.
L1 split up into the L2 and L3 branches sometime around 127,000 to 72,000 years ago, again consistent with the previous paper. The L2 and L3 branches show two exponential growth periods, one around 86,000 to 61,000 years ago and another around 20,000 to 12,000 years ago. The authors observed a distinct expansion of the L3 branch around 12,000 to 8,000 years ago. They suggest that,
“L3 did not simply spill over into Eurasia, but was driven as part of an expansion that had begun in sub-Saharan Africa thousands of years earlier.”
While this date is a bit later than the one suggested in the previous paper, both indicate that there were deep African migrations within Africa. The later expansions of L2 and L3, coincide with environmental and cultural changes, such as the greening of the Sahara and emergence of pastoralism. The authors write that,
“The timing of the L3 expansion-8-12kyr prior to the emergence of the first non-African mtDNA lineages-together with high L3 diversity in eastern Africa, strongly supports the proposal that the human exodus from Africa and subsequent colonization of the globe was prefaced by a major expansion within Africa, perhaps driven by some form of cultural innovation.”
- Quentin D. Atkinson, Russell D. Gray, Alexei J. Drummond (2008). Bayesian coalescent inference of major human mitochondrial DNA haplogroup expansions in Africa Proceedings of the Royal Society B: Biological Sciences, -1 (-1), -1–1 DOI: 10.1098/rspb.2008.0785