October 02, 2008

Human mtDNA expansions in Africa

An interesting paper in the context of the Afrasian/Palaeoafrican hypothesis

Modern human origins and dispersals are often seen in terms of an African emergence of our species and its subsequent migration to the four corners of the world. 

What is often forgotten is that the major episode in our species' history, the "Out of Africa" migration was part of a related "Deeper Into Africa" episode that preceded it. It is the East African "Afrasians" who spread their mitochondria to the rest of the continent, as well as to the rest of the world. While non-Africans are descended (at least mitochondrially) exclusively from the early Afrasians, Sub-Saharan Africans are descended from non-L3 "Palaeofricans" who were the pre-existing humans on the continent, and whose mitochondrial origin stretches to the dawn of human anatomic modernity, ca. 200kya.

According to the authors:
The fact that L3 is the only haplogroup with descendants outside Africa and shows a clear growth signal 8–12 kyr prior to the emergence of its non-African descendants strongly suggests 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.
Interestingly, the most common Sub-Saharan haplogroup L2 has a TMRCA of 73–127 kyr ago but seems to have expanded 12-20kyr ago. This underscores the importance of post-Out-of-Africa events shaping the present-day African genetic diversity.

Proceedings of the Royal Society B. doi: 10.1098/rspb.2008.0785

Bayesian coalescent inference of major human mitochondrial DNA haplogroup expansions in Africa.

Atkinson QD, Gray RD, Drummond AJ.

Past population size can be estimated from modern genetic diversity using coalescent theory. Estimates of ancestral human population dynamics in sub-Saharan Africa can tell us about the timing and nature of our first steps towards colonizing the globe. Here, we combine Bayesian coalescent inference with a dataset of 224 complete human mitochondrial DNA (mtDNA) sequences to estimate effective population size through time for each of the four major African mtDNA haplogroups (L0-L3). We find evidence of three distinct demographic histories underlying the four haplogroups. Haplogroups L0 and L1 both show slow, steady exponential growth from 156 to 213kyr ago. By contrast, haplogroups L2 and L3 show evidence of substantial growth beginning 12-20 and 61-86kyr ago, respectively. These later expansions may be associated with contemporaneous environmental and/or cultural changes. 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.



Kosmo said...

"The fact that L3 is the only haplogroup with descendants outside Africa..."

It was brought up in comments several months ago that Mungo Man might be part of an earlier pre-L lineage to leave Africa. I do understand the mtDNA evidence is still being contested though and may be the result of contamination--but as far as I know, the jury is still out on that.

This link, anyway, is still standing:

Dienekes said...

I was speaking of the present. With the exception of lineages representing more recent African admixture, Eurasians are exclusively N and M. So they have exclusive Afrasian ancestry, with a little bit of more recent African admixture, but no evidence of non-African mtDNA (due to Neanderthal etc. introgression)

If Mungo Man's reported lineage is authentic (which I doubt as per my previous comments), his lineage didn't contribute to Eurasians.

Kosmo said...

You're right, certainly. Like you, I have my doubts about the lineage's authenticity, but I just wanted to throw the asterisk out there on the idea that only L3's left Africa. (*)

* (unless Mungo's MtDNA is accurate)

But either way, only L3's are represented today.