But the Neolithic Y-chromosome bottleneck, as it is known, has been something of a puzzle since its
discovery in 2015. This is because it was only observed on the genes on the Y chromosome that get passed down from father to son - which means it only affected men.
This points to a social, rather than an environmental, cause, and given the social restructures between 12,000 and 8,000 years ago as humans shifted to more agrarian cultures with patrilineal structures, this may have had something to do with it.
In fact, a drop in genetic diversity doesn't mean that there was necessarily a drop in population. The number of men could very well have stayed the same, while the pool of men who produced offspring declined.