Abstract
Modern human societies show hierarchical social modularity (HSM) in which lower-order social units like nuclear families are nested inside increasingly larger units. It has been argued that this HSM evolved independently and after the chimpanzee–human split due to greater recognition of, and bonding between, dispersed kin. We used network modularity analysis and hierarchical clustering to quantify community structure within two western lowland gorilla populations. In both communities, we detected two hierarchically nested tiers of social structure which have not been previously quantified. Both tiers map closely to human social tiers. Genetic data from one population suggested that, as in humans, social unit membership was kin structured. The sizes of gorilla social units also showed the kind of consistent scaling ratio between social tiers observed in humans, baboons, toothed whales, and elephants. These results indicate that the hierarchical social organization observed in humans may have evolved far earlier than previously asserted and may not be a product of the social brain evolution unique to the hominin lineage.
Highlights
How did human society transition from small, autonomous groups, to multi-tiered, hierarchically nested structures in which networks of association and cooperation coalesce into successively higher-level units? And when did this happen? According to the dominant narrative, the transition to a complex, multitiered society in humans was part of a broader trend in mammalian evolution in which brain size increase is associated with a suite of social cognition capacities referred to as the Social Brain [1]
Membership of pairs of groups or solitaries in the same first tier associations detected by clustering, strongly predicted their presence in the same second tier associations detected by modularity analysis (Lokouez 1⁄4 7.144 Pr(.jzj), 0.0001, Mbeli z 1⁄4 5.245 Pr(.jzj), 0.0001), demonstrating consistency between the two approaches and that the structure detected was hierarchically inclusive
Our results suggest a social structure in western gorillas with striking parallels to human society, from the kin bias of social modules, to the hierarchical scaling in their size, providing a potential link between human social structure and the modular societies observed in many other primate species [5]
Summary
How did human society transition from small, autonomous groups, to multi-tiered, hierarchically nested structures in which networks of association and cooperation coalesce into successively higher-level units? And when did this happen? According to the dominant narrative, the transition to a complex, multitiered society in humans was part of a broader trend in mammalian evolution in which brain size increase is associated with a suite of social cognition capacities referred to as the Social Brain [1]. The social brain enhancements that evolved in the context of collaborating with large coalitions of dispersed kin facilitated the development of HSM when the transition to single male social groups brought more structure to kin interactions In this narrative, the extension to even larger networks of reciprocity among non-kin is due to further expansion of social brain capacity [7]. HSM has not been previously studied in western gorillas in good part because their home ranges span large swaths of thick tropical forest, making observations of inter-group social interaction difficult. To circumvent this problem, we analyse observational data from two mineral-rich forest clearings in the Republic of Congo. We evaluate whether the sizes of gorilla social units show hierarchical scaling [4] and use genetic data to test whether these social units have kin structure similar to humans
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