Pathways to cultural adaptation: the coevolution of cumulative culture and social networks

Humans have adapted to an immense array of ecologies by accumulating culturally transmitted knowledge and skills. Culture accumulates in at least two ways: via more distinct cultural traits, or via improvements of existing cultural trait. A trade-off is expected between these owing to the fact that social learning opportunities are finite and social learning often requires multiple exposures. Furthermore, what kind of culture accumulates depends on, and coevolves with, the social structure of societies. Here we show that the coevolution of social networks for learning and cumulative culture results in two distinct pathways to cultural adaptation: highly connected populations with high proficiency but low cultural trait diversity vs. sparsely connected populations with low proficiency but more cultural trait diversity. Importantly, we show there is a general conflict between group-level payoffs, which is maximised in highly connected groups that attain high proficiency, and individual level selection, which favours disconnection. This conflict emerges from the interaction of social learning with population structure and causes populations to cycle between the two cultural and network states. The same conflict creates a paradox where improving individual innovation rates lowers the payoffs of groups. Finally, we explore how populations navigate these two pathways in heterogeneous and changing environments, and show that high heterogeneity in payoffs and slow rate of environmental change favours high proficiency, while fast rate of environmental change favours more trait diversity. We also find that the proficiency pathway to cultural adaptation is favoured with increased population size, but only in slow changing environments. Our results uncover previously unrecognised trade-offs and tensions in the coevolutionary dynamics of cumulative culture and social structure, with broad implications for human social evolution.