Mitochondrial ancestry of African Americans

Antonio Salas and colleagues have a paper in the October American Journal of Human Genetics concerning the mtDNA affinities of African Americans within today's African populations.

The paper starts with a relatively large set of ~1100 West and Southwest African mtDNA samples, and compares this set with the mtDNA from a similarly large sample of African Americans from the U.S. The goal is to see if it is possible to determine the point of origin for individuals of African descent, at least along the exclusively maternal line.

Normally I don't go in too much for papers like this; although it is valid enough to use mtDNA for these recent comparisons, it is really informative only about a very limited part of an individual's ancestry. The maternal ancestry of many African Americans can be dated to Africa between 200 and 400 years ago; a period of around 10 to 20 generations. Any person has 1024 possible ancestors in the tenth generation in the past (possible ancestors, because later inbreeding may cause some of these people to be the same). Thus, mtDNA is informative about only around a tenth of a percent of someone's ancestry.

The promise of using mtDNA has been that its abundant variation causes strong geographic structure. If people didn't move around too much in their population of origin, the mtDNA type might be specific to a small area, or even a single village. It might tell about only a small proportion of ancestry, but that small proportion might actually be able to be placed with great geographic accuracy.

The current study finds that such accuracy is not possible, at least with the present information. I found the last two concluding paragraphs very informative:

We conclude that mtDNA variation allows us to trace the maternal ancestry of African Americans to broad geographic regions of Africa, with results that are closely concordant with historical studies that now encompass documentation for between two-thirds and three-quarters of the estimated total voyages made during the course of the Atlantic slave trade (Eltis et al. 1998). We have previously raised the possibility of whether, with larger data sets and extensive phylogeographic analyses, more-specific reconstructions will be possible (Salas et al. 2004). However, even with this substantially augmented data set, we note that it is still not possible to go further at this stage. Even with greatly improved geographic coverage, it remains the case that many mtDNAs are very widely distributed throughout the African continent, most likely as a result largely of the Bantu dispersals (Salas et al. 2002), but no doubt also as a result of both earlier and more recent movements, including those that are due to the Atlantic slave trade itself (Salas et al. 2004). This problem will continue to hamper the allocation of African American mtDNAs to narrower geographic locations in Africa, even if the resolution of the molecular analyses is increased from the first hypervariable segment (HVS-I) to complete mtDNA genomes.
Considerable caution is therefore warranted when dealing with claims in the popular media (such as the acclaimed and prestigious BBC television documentary Motherland: A Genetic Journey, first shown in the United Kingdom in 2003) and those made by genetic ancestry-testing companies about their ability to trace the ancestry of certain American (or, for that matter, European) mtDNAs to a particular locale or population within modern-day Africa. Our analyses stand as a warning to unsuspecting members of the public who may be seduced by such promises (Salas et al. 2005:679, citations in original).

A good caution to follow; one that I <a href="weblog/topics/race/race_testing_penn_state_2005.html"">certainly endorse</a>.

References:

Salas A, Carracedo A, Richards M, Macaulay V. 2005. Charting the ancestry of African Americans. Am J Hum Genet 77:676-680. Full text online