Genetic differences between Africans and non-Africans

Howard Hughes Medical Institute:

In each gene, the researchers found distinctive sets of polymorphisms, which are sequence differences between different individuals. Blocks of linked polymorphisms are called haplotypes, whereby each haplotype is, in essence, a distinct genetic variant of the gene. They found that they could further break the haplotypes down into related variants called haplogroups. Their analysis indicated that for each of the two genes, one haplogroup occurs at a frequency far higher than that expected by chance, indicating that natural selection has driven up the frequency of the haplogroup. They referred to the high-frequency haplogroup as haplogroup D.

When the researchers compared the ethnic groups in their sample for haplogroup D of ASPM, they found that it occurs more frequently in European and related populations, including Iberians, Basques, Russians, North Africans, Middle Easterners and South Asians. That haplogroup was found at a lower incidence in East Asians, sub-Saharan Africans and New World Indians. For Microcephalin, the researchers found that haplogroup D is more abundant in populations outside of Africa than in populations from sub-Saharan Africa.

To produce more informative statistical data on the frequency of haplotype D among population groups, the researchers applied their methods to a larger population sample of more than one thousand people. That analysis also showed the same distribution of haplogroups.

Their statistical analysis indicated that the Microcephalin haplogroup D appeared about 37,000 years ago, and the ASPM haplogroup D appeared about 5,800 years ago - both well after the emergence of modern humans about 200,000 years ago. “In the case of Microcephalin, the origin of the new variant coincides with the emergence of culturally modern humans,” said Lahn. “And the ASPM new variant originated at a time that coincides with the spread of agriculture, settled cities, and the first record of written language. So, a major question is whether the coincidence between the genetic evolution that we see and the cultural evolution of humans was causative, or did they synergize with each other?”

Lahn said that the geographic origin and circumstances surrounding the spread of the haplogroups can only be surmised at this point. “One can make guesses, but our study doesn't reveal how these positively selected variants arrived," he said. "They may have arisen in Europe or the Middle East and spread more readily east and west due to human migrations, as opposed to south to Africa because of geographic barriers. Or, they could have arisen in Africa, and increased in frequency once early humans migrated out of Africa.”

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