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Prehistoric Man Hunting Bears by Emmanuel Benner the Younger.

Neolithic Male Genetic Diversity Plummeted – Here’s Why

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Starting about 7,000 years ago, something weird seems to have happened to men: Over the next two millennia, recent studies suggest, their genetic diversity - specifically, the diversity of their Y chromosomes - collapsed. So extreme was that collapse that it was as if there was only one man left to mate for every 17 women.

Anthropologists and biologists were perplexed, but Stanford researchers now believe they've found a simple - if revealing - explanation. The collapse, they argue, was the result of generations of war between patrilineal clans, whose membership is determined by male ancestors.

The outlines of that idea came to Tian Chen Zeng, a Stanford undergraduate in sociology, after spending hours reading blog posts that speculated - unconvincingly, Zeng thought - on the origins of the "Neolithic Y-chromosome bottleneck," as the event is known. He soon shared his ideas with his high school classmate Alan Aw, also a Stanford undergraduate in mathematical and computational science.

"He was really waxing lyrical about it," Aw said, so the pair took their idea to Marcus Feldman, a professor of biology in Stanford's School of Humanities and Sciences. Zeng, Aw and Feldman published their results May 25 in Nature Communications .

"Woman Triumphant" by Rudolf Cronau. (1919). (Public Domain)

"Woman Triumphant" by Rudolf Cronau. (1919). ( Public Domain )

A Cultural Culprit

It's not unprecedented for human genetic diversity to take a nosedive once in a while, but the Y-chromosome bottleneck, which was inferred from genetic patterns in modern humans, was an odd one. First, it was observed only in men - more precisely, it was detected only through genes on the Y chromosome, which fathers pass to their sons. Second, the bottleneck is much more recent than other biologically similar events, hinting that its origins might have something to do with changing social structures.

Certainly, the researchers point out, social structures were changing. After the onset of farming and herding around 12,000 years ago, societies grew increasingly organized around extended kinship groups, many of them patrilineal clans - a cultural fact with potentially significant biological consequences. The key is how clan members are related to each other. While women may have married into a clan, men in such clans are all related through male ancestors and therefore tend to have the same Y chromosomes. From the point of view of those chromosomes at least, it's almost as if everyone in a clan has the same father.

That only applies within one clan, however, and there could still be considerable variation between clans. To explain why even between-clan variation might have declined during the bottleneck, the researchers hypothesized that wars, if they repeatedly wiped out entire clans over time, would also wipe out a good many male lineages and their unique Y chromosomes in the process.

Cave art in Magura cave from between 10000-8000 years ago. (Public Domain)

Cave art in Magura cave from between 10000-8000 years ago. ( Public Domain )

Computing Clans

To test their ideas, the researchers turned to mathematical models and computer simulations in which men fought - and died - for the resources their clans needed to survive. As the team expected, wars between patrilineal clans drastically reduced Y chromosome diversity over time, while conflict between non-patrilineal clans - groups where both men and women could move between clans - did not.

Zeng, Aw and Feldman's model also accounted for the observation that among the male lineages that survived the Y-chromosome bottleneck, a few lineages underwent dramatic expansions, consistent with the patrilineal clan model, but not others.

Now the researchers are looking at applying the framework in other areas - anywhere "historical and geographical patterns of cultural interactions could explain the patterns you see in genetics," said Feldman, who is also the Burnet C. and Mildred Finley Wohlford Professor.

Feldman said the work was an unusual example of undergraduates driving research that was broad both in terms of the academic disciplines spanned - in this case, sociology, mathematics and biology - and in terms of its potential implications for understanding the role of culture in shaping human evolution. And, he said, "Working with these talented guys is a lot of fun."

Top image: Prehistoric Man Hunting Bears by Emmanuel Benner the Younger. Source: Public Domain

The article, originally titled ‘Wars and clan structure may explain a strange biological event 7,000 years ago,’ was first published on Science Daily.

Stanford University. "Wars and clan structure may explain a strange biological event 7,000 years ago." ScienceDaily. ScienceDaily, 29 May 2018. www.sciencedaily.com/releases/2018/05/180529185356.htm

References

Tian Chen Zeng, Alan J. Aw, Marcus W. Feldman. Cultural hitchhiking and competition between patrilineal kin groups explain the post-Neolithic Y-chromosome bottleneck . Nature Communications , 2018; 9 (1) DOI: 10.1038/s41467-018-04375-6

Comments

There some good thinking here, I wish more would adopt a cross-disciplinary approach, it may unlock what was really going on rather than opinions which are often prejudiced. This may suggest a huge war between all the clans. I wonder if it was somehow recorded when the writing first started to appear, e.g. the clash of the titans and similar may be a memory of this. Something similar seemed to happen after thera errupted, with the lack of tin coming into the med, the city states still had a need for the bronze and seemed to raid all the ports and other cities to try to acquire stockpiles - it brought in a dark age. It would not have been tin around 5000 BCE, but I wonder what it was they were fighting for? Copper maybe, or farms. This is just before the founding of Minoan and Egyptian civilisation. It's interesting that they both had good defences to stifle invasion or raiding. The Minoans favoured islands (a big body of water), the Egyptians (the med on one side, a dessert on the other), it would be interesting to see if this is a critical factor to allow civilisation to develop, I suspect it might have been.

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