Primate Social Organization Evolved from a Flexible Pair-Living Ancestor

Abstract Explaining the evolution of primate social organization has been fundamental to understand human sociality and social evolution more broadly. It has often been suggested that the ancestor of all primates was solitary and that other forms of social organization evolved later. However, previous research included the assumption that many understudied primate species were solitary, then finding transitions to more complex social systems being driven by various life history traits and ecological factors. Here we show that when intra-specific variation is accounted for, the ancestral social organization of primates was variable, with the most common social organization being pair-living but with approximatively 15-20% of social units of the ancestral population deviating from this pattern by being solitary living. We built a detailed database from primary field studies quantifying the number of individuals (social units) expressing different social organizations in each population. We used Bayesian phylogenetic models to infer the probability of each social organization, conditional on several socio-ecological predictors, in ancestral populations. Body size and activity patterns had large effects on transitions between types of social organizations. Our results challenge the assumption that ancestral primates were solitary and that pair-living evolved afterwards. Moreover, our results emphasize the importance of focusing on field data and accounting for intra-specific variation. Pair-living is evolutionary ancient, likely caused by reproductive benefits such as access to partners and reduced intra-sexual competition, with more complex social structure (pair-bonding) and care systems (biparental and allo-parental care) evolving later. Significance Statement Was the ancestor of all primates a solitary-living species? Did more social forms of primate societies evolve from this basic and simple society? The dogma has been that the answer is yes. We used a modern statistical analysis, including variations within species, to show that the ancestral primate social organization was most likely variable. Most lived in pairs, and only 15-20% of individuals were solitary. Living in pairs was likely ancient and caused by reproductive benefits, like access to partners and reduced competition with the sexes. More complex social elaborations like pair-bonds, and biparental and allo-parental care, probably evolved later.