Abstract
Our species exhibits spectacular success due to cumulative culture. While cognitive evolution of social learning mechanisms may be partially responsible for adaptive human culture, features of early human social structure may also play a role by increasing the number potential models from which to learn innovations. We present interview data on interactions between same-sex adult dyads of Ache and Hadza hunter-gatherers living in multiple distinct residential bands (20 Ache bands; 42 Hadza bands; 1201 dyads) throughout a tribal home range. Results show high probabilities (5%–29% per year) of cultural and cooperative interactions between randomly chosen adults. Multiple regression suggests that ritual relationships increase interaction rates more than kinship, and that affinal kin interact more often than dyads with no relationship. These may be important features of human sociality. Finally, yearly interaction rates along with survival data allow us to estimate expected lifetime partners for a variety of social activities, and compare those to chimpanzees. Hadza and Ache men are estimated to observe over 300 men making tools in a lifetime, whereas male chimpanzees interact with only about 20 other males in a lifetime. High intergroup interaction rates in ancestral humans may have promoted the evolution of cumulative culture.
Highlights
It has been hypothesized that cumulative culture and extensive non-kin cooperation allowed Homo sapiens to replace other hominin species in the Pleistocene and facilitated the biological dominance of our species in the Holocene [1]
Within-band interactions such as non-kin food sharing [2], cooperative food acquisition, and provisioning of multiple goods and services [3] are well documented for recent hunter-gatherers [4,5] and part of a cooperative breeding life history that may be critical for explaining human success
While Ache and Hadza ritual relationships are different in character, both types of ritual relationships define and reinforce patterns of behavior between individuals, and may influence patterns of inter-band interaction
Summary
It has been hypothesized that cumulative culture and extensive non-kin cooperation allowed Homo sapiens to replace other hominin species in the Pleistocene and facilitated the biological dominance of our species in the Holocene [1]. In order to understand the emergence of these features we must examine aspects of social behavior in our ancestors that may have favored their evolution. Observations of modern hunter-gatherers offer the opportunity to examine features of that lifestyle that may be associated with important evolved human traits. Within-band interactions such as non-kin food sharing [2], cooperative food acquisition, and provisioning of multiple goods and services [3] are well documented for recent hunter-gatherers [4,5] and part of a cooperative breeding life history that may be critical for explaining human success. Inter-band social networks are hypothesized to explain evolved brain expansion [6,7], extensive non-kin cooperation [8,9,10] and the emergence of cumulative culture [11,12,13]
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