History Stories

About 5,000 to 7,000 years ago the number of men having children fell dramatically—possibly by as much as 95 percent—throughout Asia, Africa and Europe, genetic research shows. Now, scientists think they might know why.

A new study argues that conflict between warring patrilineal clans may have wiped out large swathes of the male population, including entire male lineages. Over time, these devastating clan wars likely caused a collapse of diversity in the Y chromosome, or a genetic bottleneck.  

Back in 2015, researchers uncovered this collapse in the Y chromosome, which is passed from father to son, during the Neolithic era. Yet they noticed no corresponding pattern in mitochondrial DNA, passed from mother to child. This imbalance suggested a social cause for the drop in males reproducing, rather than an environmental one.  

For a possible explanation, the genetic researchers looked back some 12,000 years, when agriculture was introduced and hunter-gatherer groups began transitioning into larger, more settled clans. These clans tended to be patrilineal, where membership follows the male line. Women could marry in and out of different clans, but men remained where they were. That would already have limited genetic diversity within the clan, as many of its men would have descended from a common male ancestor. Then when these clans fought against each other, the researchers suggest, entire male lineages may have been wiped out, causing Y-chromosome diversity to collapse across the spectrum.

Y Chromosomes

(Credit: Polesnoy/Getty Images)

In the new study, published in Nature Communications, a team of researchers from Stanford University used mathematical modeling and computer simulations to test this theory. When they pitted patrilineal clans against each other in fierce battles over the resources necessary for their survival, they found that Y-chromosome diversity dropped. On the other hand, when they simulated battles between groups in which both men and women were allowed to move across clan lines, genetic diversity didn’t suffer as much.

Among the other possible explanations for the genetic bottleneck was male competition. When social hierarchies emerged in these early farming societies, powerful men within clans could have fathered hundreds of children, while other men had none. In the most famous example of this dynamic, millions of Asian men bear Y chromosomes similar to that of Mongol leader Genghis Khan and his descendants. Similarly, the social structures might have dictated that some men had multiple wives while others stayed single, also reducing Y-chromosome diversity.

But Mark Thomas, who co-authored the original 2015 genetic study, told Newsweek that the clan-war explanation may be the most probable, based on the “excellent” new research. “If it’s true, well, 5,000 to 7,000 years ago wasn’t the best time to be a male, unless you were a member of one of those lucky few clans we are all now descended from.”

The authors of the new study, led by Stanford sociologist Tian Chen Zeng, now plan to use their methodology—a potent combination of sociology, biology and mathematics—to study other cultures, in order to see how genetic variation among groups might correspond with political history. As they write,  “Cultural changes in political and social organization—phenomena that are unique to human beings—may extend their reach into patterns of genetic variation in ways yet to be discovered.”

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