Tuesday, June 30, 2009

On becoming cultural - models for evolution of human behavior

Ruth Mace describes articles by Bowles and by Powell et al. on the origins of altruism toward one's own social group and the emergence of cultural complexity. They invoke evolution and selection of behavioral traits at the level of groups, a topic I've mentioned in numerous previous posts (enter 'group selection' in the search box at left to review them). Clips from Mace's review:
...Genetic and cultural traits are both heritable and subject to evolutionary processes, but cultural traits are not transmitted in a Mendelian way; they can be inherited from almost anyone, including people who may not share your genetic interests...Perhaps the central difference between genetic and cultural transmission is that we can change our cultural phenotype during our lives—for example, to conform to group norms. Cultural differences between groups might be easier to maintain than are genetic ones, due to processes such as conformist social learning and punishment; several models show that if these processes occur, cultural group selection could explain the evolution of prosocial or altruistic behavior.
In what is essentially an extension of the Baldwin Hypothesis (again, check the blog's search box) Bowles makes the claim that:
...the demographic structure of hunter-gatherer populations allowed group-selected genetic traits to evolve in humans. He argues that lethal warfare was endemic and that altruistic, group-beneficial behaviors that hurt the survival chances of individuals but improved the likelihood for groups to win conflicts could emerge by group selection. This argument was originally espoused by Darwin, but few formal tests of it have been done.

In his model, Bowles identifies two key determinants of whether group selection can favor altruistic behavior: the individual and group costs and benefits of altruism in warfare, and the extent of genetic differentiation among groups. An array of ethnographic and archaeological evidence shows that hunter-gatherers certainly did kill each other. How much of this killing was within or between groups is tricky to ascertain (especially from archaeological data) and varies among sites, but on average, 14% of adult deaths appear to have been due to warfare...based on genetic studies of extant hunter-gatherers, the model shows that a realistic level of inbreeding within groups allows group benefits to offset fitness costs of roughly 3% associated with being an "altruistic warrior" relative to nonaltruists. Ironically, lethal hostility toward other groups could thus underpin cooperation and support within human communities.
Powell et al. argue:
...that changes in population size and structure can explain the patterns of acquisition (and loss) of culturally inherited skills (like the ~10,000-40,000 year old harpoon shown) ...[Their model]..examines a structured population, in which individuals live in groups (subpopulations) and inherit (learn) skills from others in the group or by contact due to migration between groups. The results show that the time since first occupation of a region is a far less reliable predictor of the accumulation of cultural skills than is the density of subpopulations and the degree of migration between them.

The two models paint rather different pictures of Pleistocene life. Were early modern humans in frequent contact with neighboring groups to exchange cultural innovations, or were they inward looking, unwilling to travel, and constantly engaging their neighbors in lethal conflict? Probably both, at different times and in different places (although it may be possible to steal someone's cultural innovations and kill them too).


  1. In the first paragraph of the quoted clip from Mace's review the combination of two words "cultural traits" is used consistently. Is it not the same thing as "memes" coined by Prof. Dawkins and expanded by Sue Blackmore and others?

  2. Can you talk of cultural complexity in the stone age? The only kind of culture in that time besides the making of stone tools is related to language - the first writing systems appeared much later about 3000 BC, and caused an explosion of cultural evolution. Here it makes sense to examine the emergence of cultural complexity. There is an interesting model from Robert Axelrod about the emergence of cultural complexity (named "The Dissemination of Culture: A Model with Local Convergence and Global Polarization").

    Can you talk of tolerance in the stone age? Hardly. Apparently tolerance towards strangers appeared together with language and culture. If this is true, then the question whether ancestral humans were largely "peaceful" or "warlike" is a moot question.