From Stanford University at Eurekalert:
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.
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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. More.
Well, unlike many theses in human evolution, it’s plausible (unfortunately in this case). Early societies lived all too close to nature and the resulting desperation could certainly lead to wars of annihilation. Later in history, it usually made more sense to subjugate conquered peoples and make them pay tribute. But tribute only works if the average human can produce more than he himself needs to survive.
Some questions: Why 7000 years ago in particular? What was happening just then? Human beings cannot have just suddenly discovered violence. New weapons perhaps?
This thesis would be greatly strengthened if we found considerable evidence of violent deaths in male remains from that period.
See also: Researcher asks, if ecology caused the human brain to grow so large, what about the role of language?
Human evolution researchers: Social challenges decreased brain size
Early human religion: A 747 built in the basement with an X-Acto knife
and
Human origins: The war of trivial explanations