A perplexing biological mystery from thousands of years ago might be explained by a series of major wars between ancient human clans.
If you trace back through human history, there have been a number of occasions where population diversity has taken a massive nosedive. One example is the ‘genetic bottleneck’ that occurred around 70,000 years ago when a number of possible environmental disasters reduced the world’s human population to somewhere between 3,000 and 10,000 people.
A more recent event that occurred 7,000 years ago – which has remained a mystery for decades – saw that over a period of 2,000 years, the diversity in Y chromosomes just collapsed.
So extreme was this collapse, research has shown, that it resembled a plot from post-apocalyptic fiction, with only one man left to mate for every 17 women.
Generations of war
Now, however, a team from Stanford University has published a theory in Nature Communications that seems to suggest something relatively straightforward: all the men killed each other.
The team’s argument is that the collapse was due to generations of war between extended kinship groups known as patrilineal clans, whose membership was dominated by male ancestors.
First put forward by sociology undergraduate students Tian Chen Zeng and Alan Aw, the pair later collaborated with Prof Marcus Feldman to pick apart the peculiar bottleneck.
What made it so strange was, not only was it not observed in women, but it is much more recent than other biologically similar events, hinting that its origins might have something to do with changing social structures.
Changing social structures
At that time, human social structures were changing due to the onset of farming around 12,000 years ago and the emergence of patrilineal clans, which could have had significant biological consequences.
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, making it seem that everyone in a clan had the same father.
To explain how even between-clan variation might have declined during the bottleneck, the research team hypothesised that repeated wars between clans would also wipe out a good many male lineages and their unique Y chromosomes in the process.
This theory appeared sound after running a series of computer simulations, which showed a reduction in Y chromosome diversity after a series of clan battles.
Now, the researchers hope to apply the same framework in other areas where historical and geographical patterns of cultural interactions could explain the patterns you see in genetics.