There was no vast migration of Eurasians into Africa

Last fall while at ASHG this paper came out, Ancient Ethiopian genome reveals extensive Eurasian admixture throughout the African continent:

Characterizing genetic diversity in Africa is a crucial step for most analyses reconstructing the evolutionary history of anatomically modern humans. However, historic migrations from Eurasia into Africa have affected many contemporary populations, confounding inferences. Here, we present a 12.5x coverage ancient genome of an Ethiopian male (‘Mota’) who lived approximately 4,500 years ago. We use this genome to demonstrate that the Eurasian backflow into Africa came from a population closely related to Early Neolithic farmers, who had colonized Europe 4,000 years earlier. The extent of this backflow was much greater than previously reported, reaching all the way to Central, West and Southern Africa, affecting even populations such as Yoruba and Mbuti, previously thought to be relatively unadmixed, who harbor 6-7% Eurasian ancestry.

Turns out that there was a bioinformatics error which negates the magnitude of these results. Erratum to Gallego Llorente et al. 2015:

The results presented in the Report “Ancient Ethiopian genome reveals extensive Eurasian admixture throughout the African continent“ were affected by a bioinformatics error. A script necessary to convert the input produced by samtools v0.1.19 to be compatible with PLINK was not run when merging the ancient genome, Mota, with the contemporary populations SNP panel, leading to homozygote positions to the human reference genome being dropped as missing data (the analysis of admixture with Neanderthals and Denisovans was not affected). When those positions were included, 255,922 SNP out of 256,540 from the contemporary reference panel could be called in Mota. The conclusion of a large migration into East Africa from Western Eurasia, and more precisely from a source genetically close to the early Neolithic farmers, is not affected. However, the geographic extent of the genetic impact of this migration was overestimated: the Western Eurasian backflow mostly affected East Africa and only a few Sub-Saharan populations; the Yoruba and Mbuti do not show higher levels of Western Eurasian ancestry compared to Mota.

We thank Pontus Skoglund and David Reich for letting us know about this problem.

First, scientists are humans and mistakes happen. So respect that the authors owned up to it. On the other hand, the conclusion never smelled right to many people. I was confused by it. I asked Iosif Lazaridis at ASHG. He was confused by it. I asked Pontus Skoglund. He was confused by it.

Unfortunately the result from the bioinformatics error was emphasized on the abstract, and in the press. In The New York Times:

“The most astonishing thing is there’s quite a lot of backflow in all modern African populations,” Dr. Pinhasi said. He and his colleagues estimate that 7 percent of the genomes of the Yoruba people of Nigeria are of Eurasian origin. In the genomes of Mbuti pygmies who live in the rain forest in the Democratic Republic of Congo, 6 percent of the DNA comes from Eurasians.

Ryan L. Raaum, an anthropological geneticist at Lehman College, part of the City University of New York, called the new study “fantastic” but questioned its conclusions. If people from the Near East moved into Africa, he argued, a drastic shift in the archaeology of the region would logically follow. But no such shift occurred. It is also possible that Eurasian DNA moved into Africa earlier than 3,000 years ago, Dr. Raaum argued. Mota might have simply lived in an isolated community that never encountered people with those genes.

The best way to test the conclusions of Dr. Pinhasi and his colleagues, Dr. Raaum said, would be to gather more DNA from African fossils of the same age. If the researchers are right, they would also lack Eurasian DNA. “Then the argument starts to seem a lot more plausible,” Dr. Raaum said.

A rule of thumb in science is when you get a shocking and astonishing result, check to make sure you didn’t make some error along the sequence of analysis. That clearly did not happen here. The blame has to be distributed. Authors work with mentors and collaborators, and peer reviewers check to make sure things make sense. The idea of massive admixture across the whole of Africa just did not make sense.

If something like this happened to me I’d probably literally throw up. This is horrible. But then again, this paper made it into Science, and Nature wrote articles like this: First ancient African genome reveals vast Eurasian migration. The error has to be corrected.

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