Human mtDNA Diversity Before Migration Out of Africa

image Yesterday, a very interesting paper was published in the American Journal of Human Genetics by the Genographic Project Consortium entitled “The Dawn of Human Matrilineal Diversity.” The results of the study, which examined the 624 mtDNA genomes from sub-saharan Haplogroup L lineages, suggests that humanity once split into two small groups with one group in eastern Africa and the other in southern Africa, and that humanity bottlenecked into a relatively small number of individuals (as few as 2,000 based on results from a previous study). Note, as always, that these are hypotheses based upon the results of this and other studies, and will require further research to support or refute.

Two mtDNA Branches

The human mtDNA tree has two main branches, the L0 branch which includes individuals concentrated in southern and eastern Africa, and the L1’2’3’4’5’6′ branch (aka the L1’5 branch), which includes the entire remainder of humanity including non-Africans (see the figure to the left). Based upon the analysis of the 624 genomes, the researchers hypothesized that the L0 and L1’5 branches diverged into two small populations around 140,000 to 210,000 years ago, with one group settling in eastern Africa (the L1’5 branch) and the other settling in southern Africa (the L0 branch). Interestingly, the results also suggest that there was little to no intermingling of these branches for the next 50,000 to 100,000 years!

The L0 branch comprises 60% of the Khoisan people (two ethnic groups named the Khoi and the San) of Southern Africa. The L1’5 branch comprises all other branches of the mtDNA tree, which includes the N and M matrilines that eventually spread out from Africa.

Population Bottleneck

The results of the study also suggest that humanity was once comprised of a relatively small number of individuals (as few as 2,000, according to another study cited by the researchers). This was suggested because there are very few matrilineal lineages present today that split during the first 100,000 years of our species’ history, likely because they they died out or never developed in the first place. If there had been many more individuals alive at that time (with descendants alive today), scientists would expect to see more different types of lineages in Africa. This is what happened during the second 100,000 years of human history, with as many as 40 different matrilines at the time that humans left Africa to spread to the remainder of the world. As many of us know now, there is a multitude of current matrilines because of our enormous population explosion in the past few thousand years. Note, however, that this hypothesis may change if researchers suddenly discover large amounts of new matrilines present in Africa which split from the main line in the first 100,000 years.

An article in the Economist did a very good job of making the article understandable. Here is a quote from this terrific article in the Economist, which I suggest you read for yourself:

Comparing Khoi and San DNA with that of other Africans shows that the first big split in Homo sapiens happened shortly after the species emerged, 200,000 years ago. Most people now alive are on one side of that split. Most bushmen are on the other. The consortium’s analysis of which DNA “matrilines” are found where suggests that for much of its history the species was divided into two isolated populations, one in eastern Africa and one in the south of the continent, that were defined by this split. However, few other matrilineal splits from the first 100,000 years of the species’s history have survived to the present day.

This suggests the early human population was tiny (so the opportunities for new matrilines to evolve in the first place were limited) and reinforces the idea that Homo sapiens may have come close to extinction (eliminating some matrilines that did previously exist). Indeed, there may, at one point, have been as few as 2,000 people left to carry humanity forward.

This shrinkage coincides with a period of prolonged drought in eastern Africa, and was probably caused by it. The end of the drought, however, was followed by the appearance of many new matrilines that survive to the present day. The researchers estimate that by 60,000-70,000 years ago, the period when the exodus that populated the rest of the world happened, as many as 40 such groups were flourishing in Africa—though that migration involved only two of these groups.

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