Patrilineal segmentary systems provide a peaceful explanation for the post-Neolithic Y-chromosome bottleneck

Abstract

Studies have found a pronounced decline in male effective population sizes worldwide around 3000–5000 years ago. This bottleneck was not observed for female effective population sizes, which continued to increase over time. Until now, this remarkable genetic pattern was interpreted as the result of an ancient structuring of human populations into patrilineal groups (gathering closely related males) violently competing with each other. In this scenario, violence is responsible for the repeated extinctions of patrilineal groups, leading to a significant reduction in male effective population size. Here, we propose an alternative hypothesis by modelling a segmentary patrilineal system based on anthropological literature. We show that variance in reproductive success between patrilineal groups, combined with lineal fission (i.e., the splitting of a group into two new groups of patrilineally related individuals), can lead to a substantial reduction in the male effective population size without resorting to the violence hypothesis. Thus, a peaceful explanation involving ancient changes in social structures, linked to global changes in subsistence systems, may be sufficient to explain the reported decline in Y-chromosome diversity.

Measuring the Efficiency of Purging by non-random Mating in Human Populations

“…In addition, Y chromosome data suggest that the patrilineal descent of the TUR may date from 2,600 years ago ( Marchi et al 2017 ). At the worldwide level, Y chromosome data suggest that patrilineal systems may be even older, dating from up to or exceeding 5,000 years ( Zeng et al 2018 ; Guyon et al 2024 ). TUR individuals are not allowed to marry within their own patrilineage but may still marry a cousin on the maternal side from a different lineage.…”