Why did the male population decrease in the Neolithic? This is how science explains it

The notable decline in the male population recorded thousands of years ago throughout the world would be explained more by a social change that due to an unprecedented impulse of violence, according to a study published on Wednesday.

A French team from the National Center for Scientific Research (CNRS), the National Museum of Natural History (MNHN) and Paris Cité University believes that this decline resulted from a transition from a very diverse reproductive system to one based on a patrilineal line. That is to say, men align themselves according to their fathers’ clans.

Some of these clans had less capacity to reproduce, which ended up harming their entire lineage.

The episode occurred at the end of the Neolithic, between 3,000 and 5,000 years ago, and resulted in an abrupt drop in the diversity of the Y chromosome, responsible for male sexual characteristics.

That drop was identified only recently, through analysis of the Y chromosomes of current men.

A method that allows us to “go back in time,” explains Raphaëlle Chaix, a specialist in genetic anthropology at the CNRS and co-author of the study published in Nature Communications, to AFP.

This method allowed, in a study published in 2015, to identify a “very specific event regarding men: the collapse of their diversity about 5,000 years ago, when everything indicates that there was only one man for every 17 women participating in the reproduction in Europe”, continues this CNRS researcher.

The collapse, particularly severe in Europe, affected other regions, such as the Near East, Siberia or Africa, on a broader time scale.

A change “not necessarily violent”

The study confirmed by Léa Guyon, a doctoral student in genetic anthropology under the supervision of Raphaëlle Chaix and Evelyne Heyer, explains this event as a “change in social organization, not necessarily violent.”

This hypothesis contradicts a 2018 study according to which “clans kill each other, causing certain lines associated with a certain Y chromosome to disappear,” resulting in a loss of diversity on this chromosome.

That study counted a loss of 15% of men per generation.

The problem is that to date the archaeological record is too scarce and uncertain to establish that the Neolithic world experienced a universal and lasting episode of violence, the study recalls.

The model designed by Léa Guyon is based on a segmental patrilineal system.

Clans split when they become too large, forming subclans where “the most closely related males are grouped together, thus helping to select the Y chromosomes of the clans.”

Then some clans will disappear compared to other clans that will be more successful in reproducing, “because they have a higher social position, more power or resources,” adds this expert.

This model explains the sharp decline in genetic diversity after 2,000 to 3,000 years.

Regarding the causes of this social transition, the authors point to the emergence of agro-pastoralism, when hunter-gatherer populations were replaced by farmers and livestock breeders.

“When we compare current hunter-gatherer populations and agro-pastoral populations, the former are much less patrilocal and patrilineal than the latter,” observes Raphaëlle Chaix.

This study allowed the collection of more than a thousand genomes in patrilineal populations, which according to the authors demonstrate that segmental patrilineal systems experience a significant loss of genetic diversity of the Y chromosome.

The emergence of an agro-pastoral economy, which allows the accumulation of resources such as livestock, would have favored patrilocality – where the married couple settles in the husband’s community – and patrilineality, the study recalls.

The team would now like to study “these signs on each continent, trying to tell a story a little more specific for different regions of the world,” according to Raphaëlle Chaix.