Social change would explain the decline of the male population in the Neolithic

The dramatic decline in the male population recorded several thousand years ago around the world could be explained more by social changes than by a historical increase in violence between human groups, according to a study published on Wednesday.

The eco-anthropology team from the CNRS, the National Museum of Natural History (MNHN) and the University of Paris Cité argues that this decline is the result of a transition to a patrilineal system, in which men are linked to their fathers’ clans. A system that, through the fragmentation of groups and the disparity in their reproductive capacities, would lead to a dramatic decline in the number of men.

An episode that occurred in the world’s population at the end of the Neolithic period, between 3,000 and 5,000 years ago, resulted in a sharp decline in the diversity of the Y chromosome, responsible for male sexual characteristics.

It was only recently identified by analyzing the Y chromosome of modern men. A method that allows us to “go back in time”, Raphaëlle Chaix, a specialist in genetic anthropology at the CNRS and co-author of the study published in Nature Communications, explains to AFP.

This method made it possible, in a study published in 2015, to identify an event “very specific to men, the collapse of their diversity around 5,000 years ago, as if at that time only one man participated for every 17 women. reproduction in Europe”, continues this CNRS researcher.
The collapse, particularly serious in Europe, also affected other regions, such as the Middle East, Siberia or Africa, on a wider temporal scale.

The study, co-authored by Lea Guyon, a PhD student in genetic anthropology under the supervision of Raphaëlle Chaix and Evelyne Heyer, explains this event by “a change in social organization, not necessarily violent.”

This hypothesis contradicts a 2018 study in which “clans kill each other, causing a particular lineage linked to a particular Y chromosome to die out,” which ultimately causes a loss of diversity on this chromosome. Scenario based on a loss of 15% of males per generation.

The problem is that, to date, the archaeological record is too thin and uncertain to establish that the Neolithic world experienced an episode of universal and sustained violence, the study reminds.

The model designed by Léa Guyon is based on the so-called segmental patrilineal system. With clans undergoing fission when they become too large, forming sub-clans in which “most closely related people will group together, thus helping to sort the Y chromosomes of the clans”.

Then certain clans will disappear, facing other clans that will have more reproductive success, “because they have a higher social position, more power or resources,” she continues.
This model manages to explain the sharp decline in genetic diversity after 2,000 to 3,000 years.

As for the causes of social transition in action, the authors point to the emergence of agro-pastoralism, in which hunter-gatherer populations were replaced by farmers and breeders.

“When we compare the current hunter-gatherer population and the farmer-herder population, the former are much less patrilocal and patrilineal than the latter,” notes Raphaëlle Chaix. An observation that fueled the team’s working hypothesis, as well as the latter’s field observations.

This work made it possible to assemble more than a thousand genomes in patrilineal populations and to show that segmental patrilineal systems suffer a significant loss of Y chromosome genetic diversity.

The emergence of an agro-pastoral economy, allowing the accumulation of resources such as livestock, would favor patrilocality, – which sees a married couple settling in the husband’s community – and patrilineality, the study recalls.

The team would now like to study “these signals on each continent, to try to tell a story that is a little more specific to different regions of the world,” says Raphaëlle Chaix.