Wednesday, October 14, 2015

Ancient DNA and a History of Human Migration

Fig.1. Members of the Ari tribe in modern southwest Ethiopia are probably direct
 descendants of the Mota man (Credit: Carsten ten Brink via Flickr)

Around 60,000 to 100,000 years ago, modern humans migrated out of Africa and proceeded to populate every corner of the world. Although ancient DNA analyses have added complexity to the routes of human migration, from Neolithic farmers to early hunter-gatherers in Europe, the birthplace of humanity has remained elusive. Among other challenges, regional hot temperatures that quickly degrade genetic material barred ancient African remains from such studies. However, in research published last week in the journal Science, a skeleton found in a remote cave in Ethiopia produced Africa’s first ancient human genome.

Ron Pinhasi, an archaeologist at the University College Dublin, worked with an international team of scientists to extract and sequence DNA from a 4,500-year-old skeleton from Mota cave in southwest Ethiopia. They believe the cool, dry conditions preserved enough high-quality DNA to compare with modern populations in Africa, Europe, and Asia. The comparisons showed a surprising find: the Mota skeleton lacked around 4 to 7 percent of DNA found in modern African individuals.

The researchers traced this genetic discrepancy to an influx of Eurasians who returned to Africa around 3,000 years ago. Although this migration back into Africa was already known through previous studies of ancient grains on the continent, the size of the migration had been vastly underestimated. Using the Mota skeleton as a baseline for Africa before the influx, Pinhasi et al. examined modern genomes around the continent for evidence of Eurasian DNA. Even relatively isolated groups, such as the Mbuti in the heart of the Congo, show a small percentage of Eurasian genes. Further comparisons determined that the Ari people of Ethiopia were the closest descendants to the Mota skeleton.

If the backflow of humans into Africa was much greater than previously thought, what caused the move? The researchers could not find any obvious climatic reasons, but the arrival of crops, such as wheat and barley, around the same time might explain the extent to which Eurasian genes spread through the continent. The success of such crops would have caused populations to increase across Africa.

Until now, most studies of historic migrations relating to Africa rely on a thorough analysis of living populations. The most recent hypothesis of how humans left the continent analyzed genome sequences from 225 people in Egypt and Ethiopia to understand which region acted as a gateway to the rest of the world. Indeed, understanding genetic diversity in Africa is undoubtedly crucial for mapping human evolution and migration. Research into ancient DNA, however, unlocks secrets hidden in bones that may already be sitting in our museums. As seen with the Mota skeleton and even with a recent find in South Africa, fossils reveal a much more complex tale of how our ancestors and their close relatives lived or traveled millennia ago.


References:
Callaway, E. (2015, October 8). First ancient African genome reveals vast Eurasian migration. Nature. Retrieved from http://www.nature.com/news/first-ancient-african-genome-reveals-vast-eurasian-migration-1.18531

Llorente, M. G., et al. (2015, October 8). Ancient Ethiopian genome reveals extensive Eurasian admixture throughout the African continent. Science. doi: 10.1126/science.aad2879

Pagani, L., et al. (2015, June 4). Tracing the route of modern humans out of Africa by using 225 human genome sequences from Ethiopians and Egyptians. AJHG, vol. 96 (6): 986-991. doi: 10.1016/j.ajhg.2015.03.019

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