Starting 7,000 years ago, and lasting until 5,000 years ago, there was a collapse in the genetic diversity of men. As recounted by a new Nature Communications study, the diversity of their Y chromosomes collapsed somewhat, to the point wherein it was as if there was only one man left for every 17 women in much of the Old World.
The question, of course, is what caused this event to take place? According to the researchers from Stanford, generations of warfare between patrilineal clans – those defined and controlled by men and their ancestors and descendants – are to blame.
The idea of this 7,000-year-old genetic collapse in men isn’t new: Analyzing the male-inherited Y chromosome present in contemporary humans gives researchers a good indication as to how many reproducing males there were back in days long lost – and it’s come up in plenty of studies. They’ve tended to come to different conclusions, though.
The 2015 paper describing the phenomenon for the first time, for example, noted that the global (but non-uniform) collapse occurred when there was a major shift from a hunter-gatherer lifestyle to sedentary agricultural lifestyle. One hypothesis was that, when agricultural societies permitted the emergences of social hierarchies, fewer men of higher status had control over the population’s reproductive success, which led to a collapse in genetic diversity.
This doesn't necessarily mean the total male population decreased, mind you; the number of men reproducing dropping would have the same effect on the genetic diversity of the Y chromosome.
Inspired by speculative blog posts, the Stanford team decided to re-open the cold case and try a different approach. Using mathematical models and computer simulations, they sent those hypothesized patrilineal clans to war over survival-dependent resources. The team tracked the state of the Y chromosome throughout the entire population.
These clans already had their own very low level of Y chromosome diversity by their nature, but as it turns out, wars between these clans took the Y chromosome diversity down considerably overall too. Conflicts between non-patrilineal clans, in which women and men could move to and fro whenever they wished, did not produce the same effect.
Bottlenecks can have a variety of initial triggers. The genetic variation of a population can drop due to a major environmental or zoological change, profuse inbreeding, an epidemic, and more. Drastic reductions in population sizes – of an entire species, or a group (male/female) within a species – also coincides with genetic bottlenecks.
This study’s hypothesis certainly seems to explain this particular bottleneck in human history. It is, however, only a model, which the authors themselves note has plenty of limitations. Saying that, conflict is hardly an unreasonable inference to make.
Warfare leads to the deaths of men, women, and children, no matter when it’s taken place. As men make up disproportionately more of the military forces during conflicts, however, they disproportionately make up most of the casualties. This leads to women outnumbering men in many situations.
Although the last few centuries featured societies that were fairly different from those of the Neolithic, one can see how wiping out male lineages through war – when taken to the extreme – might create such a bottleneck.
The key problem here is that it’s difficult to infer social patterns and changes by looking through the archeological record dating back that far. The Y chromosome data generally only tells you that something happened, not what caused it.
So with that in mind, as fascinating as this study is, it’s somewhat tentative, not conclusive, just like many previous studies before it. The timing of the bottleneck, and the fact that it only affected men, certainly suggests it was a social event that caused the change, but the real answer remains elusive.