Thursday, April 05, 2018

Social Inequality Leaves An Outsized Genetic Mark |  In humans, the profound biological differences that exist between the sexes mean that a single male is physically capable of having far more children than is a single female. Women carry unborn children for nine months and often nurse them for several years prior to having additional children.1 Men, meanwhile, are able to procreate while investing far less time in the bearing and early rearing of each child. So it is that, as measured by the contribution to the next generation, powerful men have the potential to have a far greater impact than powerful women, and we can see this in genetic data.

The great variability among males in the number of offspring produced means that by searching for genomic signatures of past variability in the number of children men have had, we can obtain genetic insights into the degree of social inequality in society as a whole, and not just between males and females. An extraordinary example of this is provided by the inequality in the number of male offspring that seems to have characterized the empire established by Genghis Khan, who ruled lands stretching from China to the Caspian Sea. After his death in 1227, his successors, including several of his sons and grandsons, extended the Mongol Empire even farther—to Korea in the east, to central Europe in the west, and to Tibet in the south. The Mongols maintained rested horses at strategically spaced posts, allowing rapid communication across their more than 8,000-kilometer span of territory. The united Mongol Empire was short-lived—for example, the Yüan dynasty they established in China fell in 1368—but their rise to power nevertheless allowed them to leave an extraordinary genetic impact on Eurasia.2

A 2003 study led by Chris Tyler-Smith showed how a relatively small number of powerful males living during the Mongol period succeeded in having an outsize impact on the billions of people living in East Eurasia today.3 His study of Y chromosomes suggested that one single male who lived around the time of the Mongols left many tens of millions of direct male-line descendants across the territory that Mongols occupied. The evidence is that about 8 percent of the male population in the lands the Mongol Empire once occupied share a characteristic Y-chromosome sequence and a cluster of similar sequences differing by just a few mutations. Tyler-Smith and his colleagues called this a “Star Cluster” to reflect the idea of a single ancestor with many descendants, and estimated the date of the founder of this lineage to be 1,300 to 700 years ago based on the estimated rate of accumulation of mutations on the Y chromosome. The date coincides with that of Genghis Khan, suggesting that this single successful Y chromosome may have been his.

Star Clusters are not limited to Asia. Geneticist Daniel Bradley and his colleagues identified a Y-chromosome type that is present in 2 to 3 million people today and derives from an ancestor who lived around 1,500 years ago.4 It is especially common in people with the last name O’Donnell, who descend from one of the most powerful royal families of medieval Ireland, the “Descendants of Niall”—referring to Niall of the Nine Hostages, a legendary warlord from the earliest period of medieval Irish history. If Niall was real, he would have lived at about the right time to match the Y-chromosome ancestor.

Star Clusters capture the imagination because they can be tied, albeit speculatively, to historical figures. But the more important point is that Star Cluster analysis provides insights about shifts in social structure that occurred in the deep past that are difficult to get information about in other ways. This is therefore one area in which Y-chromosome and mitochondrial DNA analysis can be instructive, even without whole-genome data. For example, a perennial debate among historians is the extent to which the human past is shaped by single individuals whose actions leave a disproportionate impact on subsequent generations. Star Cluster analysis provides objective information about the importance of extreme inequalities in power at different points in the past.