African-American Admixture
December 22, 2009, 03:54 PM
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Over the weekend, I started thinking about a 2002 article I wrote called "How White Are Blacks? How Black Are Whites?" about Penn State geneticist Mark D. Shriver`s research that came up with an estimate of self-identified African Americans having 17% to 18% European admixture. I wondered: What`s the latest on that number? The technology has certainly improved over the years.

Last May 22, Sarah A. Tishkoff of Penn published a paper in Science called "The Genetic Structure and History of Africans and African Americans." It featured a huge number of samples from within Africa, and a small number of African-Americans.

Anthro blogger Dienekes said:

The importance of this new paper from the Tishkoff Lab cannot be emphasized enough. It is probably the most comprehensive study of African genetic variation to date. The supplementary material (pdf) is itself 102 pages long and should keep you busy reading for a while (free for non-subscribers [the Science paper is not free, however]).

What this study has found in a nutshell is that "black" Africans belong to 14 distinct clusters. Black Americans belong overwhelmingly to the Niger-Kordofanian cluster [beginning mostly in Cameroon and Nigeria, and spreading broadly from their], consistent with their origin largely from Western Africa. ...

As I have stated many times before, Bantu speakers have recently expanded from their cradle and contributed genetically to almost all other Africans, while remaining relatively pure in their own homeland. [See p. 12 of Tishkoff`s supplementary material.]

You hear a lot of stuff about how "Africans are the most genetically diverse population on earth, therefore, they have the most geniuses, etc." Malcolm Gladwell was trumpeting that argument way back in 1997 with his New Yorker article about "Why blacks are like boys and whites are like girls."

Unfortunately, this whole line of thought is based on a misunderstanding of what kind of genetic diversity population geneticists are interested in. You, me, and Malcolm Gladwell are interested in genes that affect IQ, sprinting skills, and the like. But population geneticists don`t like to look at genes that do important things because those get altered over time by selection precisely because they are important. They like to look at genes that don`t do much of anything, because they only change by random mutation, so they are the most useful for genealogical purposes.

The press release for Tishkoff`s paper says:

A median proportion of European ancestry in African-Americans of 18.5 percent, with large variation among individuals.
Which is very similar to Shriver`s work. Shriver had more crude technology but a larger sample of African-Americans from a couple of dozen areas, while Tishkoff has 365 drawn from Chicago, Pittsburgh, Baltimore, and North Carolina.

Yet, here`s Tishkoff`s abstract, which says Europeans make up about 13% of African-American genetic ancestry.

Africa is the source of all modern humans, but characterization of genetic variation and of relationships among populations across the continent has been enigmatic. We studied 121 African populations, four African American populations, and 60 non-African populations for patterns of variation at 1327 nuclear microsatellite and insertion/deletion markers. We identified 14 ancestral population clusters in Africa that correlate with self-described ethnicity and shared cultural and/or linguistic properties. We observed high levels of mixed ancestry in most populations, reflecting historical migration events across the continent. Our data also provide evidence for shared ancestry among geographically diverse hunter-gatherer populations (Khoesan speakers and Pygmies). The ancestry of African Americans is predominantly from Niger-Kordofanian (~71%), European (~13%), and other African (~8%) populations, although admixture levels varied considerably among individuals. This study helps tease apart the complex evolutionary history of Africans and African Americans, aiding both anthropological and genetic epidemiologic studies.
If you look at Table S6 on page 89 of 102 in her supplementary materials, you can see that her 13% figure apparently comes from a subsample of 98 African-Americans in four locations.

Interestingly, they come up with less than 1% of the genetic ancestry of African-Americans are American Indians but 5% are Asian Indian! Dienekes suggests that may be related to backflow from Out of Africa populations that went back In to Africa. Tishkoff says they are probably getting Asian Indians and Europeans confused in their analyses (they`re basically all Caucasians), so the actual European admixture figure is likely higher and the actual Asian Indian figure lower:

Low levels of ancestry from several additional populations were also detected (Table S6): Fulani (means 0.0 - 0.03, individual range 0.00-0.14), Cushitic East African (means 0.02, individual range 0.05 - 0.10), Sandawe East African (means 0.01- 0.03, individual range 0.00 - 0.12), East Asian (means 0.01 – 0.02, individual range 0.0 - 0.08), and Indian (means 0.04 – 0.06, individual range 0.01 -0.17). The Fulani are present across West Africa and, therefore, would be expected to have contributed to the slave trade, and the Cushitic and Sandawe ancestry could represent slave trade originating from the east coast of Africa (S126). It should be noted that the levels of Indian ancestry in African Americans may be slightly overestimated, and the levels of European ancestry slightly underestimated, due to moderate levels of the Indian AAC in European/Middle Eastern individuals (Figs. 3 and 4). We did not observe significant levels of Native American ancestry. However, other regions of the US, may reveal Native American Ancestry, as previously reported (S125). Finally, European and African ancestry levels varied considerably among individuals (Fig. 6).
Also, her Other African origins include some Saharan and Ethiopian groups that are somewhat Caucasian.

So, the 18.5% figure in the press release jibes fairly well with the supplementary materials if you add in the Asian Indians and some of the Northern Africans.

So, seven years later, Shriver`s work is reasonably well confirmed.

By the way, Figure S28 shows Tishkoff`s best guess for the origin of modern humans (the Atlantic coast of Namibia) and the Out of Africa exit point (half way up the Red Sea — that may just be because there were two exit points, one at the Sinai and the other at Djibouti-Yemen, and their statistics are just splitting the difference).

Lots of interesting stuff in Tishkoff`s paper on Pygmies, Bushmen, and others within Africa, but the African-American stuff is basically what I`ve been telling you all decade.