The Role of Cooperative Breeding in Modern Human Evolution

Journal of Human Evolution 63 (2012): 52-63,

Allomaternal care, life history and brain size evolution in mammals

Isler, Karin, and Carel P. van Schaik.

Humans stand out among the apes by having both an extremely large brain and a relatively high reproductive output, which has been proposed to be a consequence of cooperative breeding. Here, we test for general correlates of allomaternal care in a broad sample of 445 mammal species, by examining life history traits, brain size, and different helping behaviors, such as provisioning, carrying, huddling or protecting the offspring and the mother. As predicted from an energetic-cost perspective, a positive correlation between brain size and the amount of help by non-mothers is found among mammalian clades as a whole and within most groups, especially carnivores, with the notable exception of primates. In the latter group, the presence of energy subsidies during breeding instead resulted in increased fertility, up to the extreme of twinning in callitrichids, as well as a more altricial state at birth. In conclusion, humans exhibit a combination of the pattern found in provisioning carnivores, and the enhanced fertility shown by cooperatively breeding primates. Our comparative results provide support for the notion that cooperative breeding allowed early humans to sidestep the generally existing trade- off between brain size and reproductive output, and suggest an alternative explanation to the contro- versial ‘obstetrical dilemma’-argument for the relatively altricial state of human neonates at birth.


Journal of Human Evolution 63 (2012): 180-190,

Social organization and the evolution of cumulative technology in apes and hominins

Pradhan, Gauri R., Claudio Tennie, and Carel P. van Schaik.

Culturally supported accumulation (or ratcheting) of technological complexity is widely seen as characterizing hominin technology relative to that of the extant great apes, and thus as representing a threshold in cultural evolution. To explain this divide, we modeled the process of cultural accumulation of technology, which we defined as adding new actions to existing ones to create new functional combinations, based on a model for great ape tool use. The model shows that intraspecific and inter-specific variation in the presence of simple and cumulative technology among extant orangutans and chimpanzees is largely due to variation in sociability, and hence opportunities for social learning. The model also suggests that the adoption of extensive allomaternal care (cooperative breeding) in early Pleistocene Homo, which led to an increase in sociability and to teaching, and hence increased efficiency of social learning, was enough to facilitate technological ratcheting. Hence, socioecological changes, rather than advances in cognitive abilities, can account for the cumulative cultural changes seen until the origin of the Acheulean. The consequent increase in the reliance on technology could have served as the pacemaker for increased cognitive abilities. Our results also suggest that a more important watershed in cultural evolution was the rise of donated culture (technology or concepts), in which technology or concepts was transferred to naïve individuals, allowing them to skip many learning steps, and specialization arose, which allowed individuals to learn only a subset of the population’s skills.


Journal of Human Evolution has two papers dealing with the phenomenon of cooperative breeding in humans, primates and mammals. Cooperative breeding (a.k.a. allomaternal care) describes such a social or kinship system in which nonmaternal helpers support offspring who are not their own. There’s a growing awareness among evolutionary biologists that cooperative breeding played a key role in anthropogenesis. Cooperative breeding is something big apes don’t have. Humans, on the other hand, are thoroughly allomaternal care-oriented. As Isler and Schaik (2012) describe,

“Group members of both sexes and including both reproductive men and non-reproductive group members, such as post-reproductive kinswomen (cf. Hawkes et al., 1998) and older siblings, help mothers in the form of food provisioning, carrying, protecting and babysitting the infants (reviewed in Hrdy, 2009). Indeed, among foragers mothers are provided with more food (about 3500 kJ per day, Kaplan et al., 2000) during both gestation and lactation than they actually need to cover the maximum additional costs of gestation or lactation (Butte and King, 2005; Sellen, 2007). Provisioning the children with mashed or cooked food that is both easy to chew and digest begins even before weaning and provisioning with solid food continues for years afterward (Bogin, 1998).”

Cooperative breeding evolved in humans presumably by the time of Homo erectus. As Pradhan et al. argue,

“Extensive allomaternal care in the form of systematic food sharing or even provisioning, similar to what is seen among cooperatively breeding animals, began after the routine deployment of cooperative hunting subsequent to 2.5 Ma, and was firmly established by the appearance of Homo erectus around 1.7 Ma (Hrdy, 2009; van Schaik and Burkart, 2010; Isler and van Schaik, in press).”

Technological evolution could have been accomplished with just the ape-size brains. It’s the social aspect of learning and sharing, which came through allomaternal care, that decisively differentiated humans from big apes.

“Because considerable cumulative cultural evolution is possible with great ape sized brains, as implied by results form the wild and experiments in captivity, the ratcheted technology of hominins should no longer be considered qualitatively unique, although they subsequently pushed it to much higher levels than the extant great apes. We speculate that a truly qualitative change in technological evolution came much later, in the form of donated technology, when individuals could use the products of others’ efforts as their starting point, allowing them to skip many steps in the learning process, and individuals could also specialize in acquiring particular subsets of the skills present in the population as a whole” (Pradhan et al. 2012, 189).

It’s noteworthy that, outside of humans, allomaternal care is very prominent among New World (not as much Old World, the sole exception being Colobinae) monkeys.

“Currently the only primates counted among full-fledged cooperative breeders – where allomothers provision as well as carry their charges – are at the extreme end of primate continuum of shared care, among marmosets and tamarins.” (Hrdy, Sarah B. “Comes the Child Before Man: How Cooperative Breeding and Prolonged Postweaning Dependence Shaped the Human Potential,” in Hunter-Gatherer Childhoods, edited by Barry S. Hewlett and Michael E. Lamb. New Brunswick: Transaction Publishers, 2005, p. 74).







The following diagram shows the intensity and the extent of prosocial behavior in four primate genera (from Burkart, J.M., S. B. Hardy, and C. P. van Schaik. “Cooperative Breeding and Human Cognitive Evolution,” Evolutionary Anthropology 18 (2009): 178). Humans and chimps are the most unlike in on these measures, with humans showing a very similar profile to Callitrichids.

Similarities between cooperative breeding among humans and among Platyrrhines are strikingly concrete and specific. For example, cottontop tamarins and humans have similarly high rates of abandonment of infants by their mothers in the situation when outside help is not available (Hrdy 2005, 74). Tamarins and humans are also equally astute at assessing the “character” of the individual with whom they share food. In experimental conditions, tamarins were more likely to assist an unrelated individual who had a “reputation” of sharing food and reciprocating in the past, rather than the one who was known not to reciprocate. Cooperative breeding seems to be a plausible springboard for the evolution of language capacity, too, and, again, New World monkeys furnish some of the best examples of parallel evolution of speech among primates. Babbling, reports Sarah Hrdy, is the “strangest of all these convergences”: long assumed to be uniquely human, it’s found in Callitrichids. Pygmy marmosets are highly vocal as infants producing over a dozen call types at high rates. The more vocal the child, the more attention and food he gets from adults. Speech, therefore, may have evolved out of the need to attract the attention of caregivers. Young chimpanzees, on the other hand, stay continuously with their mothers until the age of 10, hence all social interactions can be resolved with the help of simple vocal and gestural exchanges (Zuberbuhler, Klaus. “Cooperative Breeding and the Evolution of Vocal Flexibility,” in The Oxford Handbook of Language Evolution, edited by Maggie Tallerman and Kathleen R. Gibson.Oxford, 2011).

Evolutionary biologists use cooperative breeding to explain some of the important demographic, behavioral and cognitive differences between humans and great apes. For instance, as Isler & vam Schaik (2012, 60) found, the connection between cooperative breeding and fertility is very strong in Callitrichids: Callitrichid monkeys regularly produce twins, and the father and other group members carry them shortly after birth for most of the time. The obvious demographic success of modern humans over its hominin antecedents calls for an evaluation of the possibility that cooperative breeding may have played a role in the evolution of Homo sapiens sapiens in the Late Pleistocene. The difficulties with defining a modern human behavioral package in archaeological terms, the lack of material evidence for revolutionary cognitive/technological abilities around 50,000 YA in Africa and the increasing evidence for “symbolic” behaviors among Mid-Pleistocene African hominins (Blombos Cave, etc.) and European Neandertals (see more here) raise the possibility that something other than a “cognitive revolution” generated the sapient phenomenon. As Burkart et al (2009, 182) write,

“Our hypothesis is that while chimpanzees and, perhaps, all great apes, may have many of the relevant cognitive preconditions for uniquely human cognition to evolve, they lack the motivational preconditions. In humans alone, these two components have come together, the cognitive component due to common descent and the motivational component due to convergent evolution resulting from the selection pressures associated with cooperative breeding.”

Profound changes in the nature of cooperative breeding may have been responsible for the emergence of that distinct ancestral population that grew in numbers to finally replace all other hominin species and colonize the globe. As research among South American hunter-gatherers suggests, this change in the nature of allomaternal care may have involved the broadening of the social circle of helpers to include individuals other than post-reproductive females. The broadening of the range of allomaternal help in turn may have further spurred prosocial and cognitive abilities.

If cooperative breeding, and not individual cognition, is at the heart of behavioral modernity, then we can expect to see in the Pleistocene archaeological record multiple examples of pseudo-human “symbolic” and “technologically advanced” artifacts manufactured by the previous generation of hominins. For example, the finding of a particularly early (over 60,000 YA) bow-and-arrow technology in the African Middle Stone Age (Sibudu Cave, South Africa) cannot be used as evidence for the emergence of modern human behavior in Africa, as it could have been manufactured by hominin species predating Homo sapiens sapiens in Africa.