Dual-inheritance theory: The evolution of human cultural capacities and cultural evolution

In the early 1930's Wintrop and Luella Kellogg (1933) began co-rearing their 10.5-month-old son, Donald, with a 7.5-month-old female chimpanzee named Gua. The Kelloggs expected that Gua, with the chimpanzee's popular reputa­ tion for aping, would acquire numerous behaviors and practices via imitation from both Donald and themselves. Unexpectedly, however, while Gua did finally acquire a few human patterns (e.g. combing his hair), Donald was the one who began to imitate the chimpanzee in some dramatic ways. Following Gua, Donald acquired the habits of knuckle walking (which he contin­ ued well after achieving full bipedality), chewing on shoes, scraping his teeth against interior walls, and hard biting. Donald even adopted some stereotypical chimpanzee food grunts, barks and hoots, using a particular bark as the word for orange. Thus, it was the human who did most of the aping. People in many small-scale societies believe that a human fetus is formed by many repeated ejaculations of sperm into the womb. This belief means that a child can have multiple fathers, who share paternity according to the number of times they had sex with the mother prior to birth (in anthropological parlance, 'partible paternity'). In response to this cultural belief, women in many of these societies actively seek out extra­ marital copulations, often to provide their child with extra fathers. And, while male jealously from the husband is sometimes a problem, it is regarded as socially inappropriate and thus suppressed. Detailed statistical analyses from two such societies, the Bad of Venezuela (Beckerman et aI., 2002) and Ache of Paraguay (Hill and Hurtado. 1996), show that the optimal number of fathers for a child's survival is more than one. These 'other fathers' (non-husbands of mom) provide resources, in the form of fish and meat, to their offspring and the mothers, both during pregnancy and while the child is growing up. Interestingly, since much of the sex associated with 'extra fathers' occurs after conception, many of these social fathers cannot be the genetic fathers. Culturally transmitted beliefs in partible paternity have been recorded in various linguistically unrelated societies across lowland