Summary:

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Significance

The authors explain the study's significance—to uncover migrations between Africa and Europe that gave rise to the particularly diverse genomes of modern African populations.

 

Abstract

The abstract clearly states the variables in the study: relevant migrations, allele frequencies, and  languages.

 

Results

It was determined that people from western and southern Africa have admixture from Eurasians (specifically western Europe and the Middle East). This admixture occurred around 43 generations ago, under the assumption that a new generation occurs every 30 years. By comparing Khoisan, French, and Yoruba participants, the researchers discovered not one but two sources of non-Khoisan ancestry that exist in the Khoisan populations. They developed another method to determine when these two admixture events occurred, which yielded an event around 39 generations ago and a more recent event 4 generations ago. They further tested this by comparing admixed African populations and non-admixed populations and found the Nama to have very recent admixture. Additional comparisons showed the Nama share 14% of their genes with western Eurasians, the most among all the groups. There are also linguistic similarities between the Nama and ancient Eurasians that support this resemblance.

 

The researchers considered that these “Eurasian admixers" could in fact be western African populations that had already been admixed and carried the Eurasian genes into southern African populations. After comparing another set of populations, the researchers discovered that an additional two admixture events may have occurred 30 and 109 generations ago, although large confidence intervals suggest these dates are very flexible. These additional admixture events suggest mixing occurred between African populations from North to South. Nonetheless, where did the Eurasians ancestry originate? The team examined data collected from ancient western Eurasians, southern Africans, and western Africans. They found lots of mixture between populations in eastern Africa and admixture of western Eurasians 80–110 generations ago, having occurred before the introduction of Eurasian genes into South Africa. This provides evidence that Eurasian genes traveled South through eastern Africa.

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Discussion

Multiple clues as to who conducted the migrations, where they came from, and where they ended up were answered by many sources, including genetic evidence, architecture, language, and archeological artifacts. The influence of Arabian architecture in modern Ethiopia combined with linguistic clues suggest a back-to-Africa migration occurred. Western ancestry was found in the genes of southern Africans indicating Eurasians migrated there through eastern Africa. A lactase gene consistent with this migration also provides evidence of Eurasian ancestry in southern Africa.

 

Conclusions

The study developed a model that with additional archaeological samples could uncover more of the ancestral history of Eurasians in East Africans.

 

Materials and Methods

The authors list the programs and sources used with links for readers to access.
 

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Questions:

Who sponsored the study?

The study was sponsored by NIH, the Max Planck Society, and awarded a grant from the Deutsche Forschungsgemeinschaft and multiple smaller NIH grants.

 

How are outliers handled in the data?

What seemed to be genetic outliers in the collection of information on Khoisan populations were removed.

 

Are both P values and confidence intervals reported?

Yes, the researchers were very transparent with their results. They even mentioned that their confidence interval was particularly large, and explained that their dates for the two older admixture events were only approximates.

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What was the aim of the study? What hypothesis did the researchers test? Are the conclusions reached (assuming they are valid) important to you and others (explain)?

The hypothesis is that there was a back-to-Africa event shown by admixture of Eurasians into African populations. The aim of the study was to explore these migrations and admixture events in combination with archaeological and linguistic evidence. Similarly, the findings of this study provide evidence for other disciplines who need genetic information to conduct their own research. To me, this study illustrates the power of interdisciplinary science and what it can accomplish. The social implications of migrations should be considered. People of various ethnicities may seem so different, but in fact their genes say otherwise.

 

Were enough data obtained to reach valid conclusions?

Pickrell et al. were very resourceful and gathered sufficient amounts of data. After defining their regions, populations of focus, and excluding outliers, they had identified 565,259 SNPs and genotyped 1,040 people from 75 populations all using the same method (Affymetrix Human Origins array). This is a large sample size, and 565,259 SNPs is a very substantial discovery.

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Was the statistical analysis (if used) appropriate for the study?

Reference curves allowed Pickrell et al. to determine relatedness of selected reference populations. The precision of this relatedness was represented by how well the points fit the exponential curve.

 

A similar curve was used to determine the date of the admixture events. A Z-test yielded significant results (3.2), concluding their model produced results that were very likely true.

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Have the authors discussed possible limitations of the study?

They examined specific migrations, but did not explore the “diffusion” of genes between regions. The second model developed to date the two admixture events was limited in terms of identifying whether the “admixed” ancestry developed due to admixture from Eurasians or gene flow between the Yoruba populations, who were already admixed. The authors also mention their graphs are limited in that they do not represent the complexity of gene flow between African populations. In a perfect world, scientists could genotype everyone and learn a lot of interesting things, but as seen in most studies, time and money are major limitations.

 

Are the results plausible?

Considering the fact that the study’s strong genetic evidence align with linguistic, archaeological and historical evidence is a sign Pickrell et al. have found very strong results.