Humans are generally highly cooperative and often impressively altruistic, quicker than any other animal species to help out strangers in need. A new study suggests that our lineage got that way by adopting so-called cooperative breeding: the caring for infants not just by the mother, but also by other members of the family and sometimes even unrelated adults. In addition to helping us get along with others, the advance led to the development of language and complex civilizations, the authors say.

Cooperative breeding is not unique to humans. Up to 10% of birds are cooperative breeders, as are meerkats and New World monkeys such as tamarins and marmosets. But our closest primate relatives, great apes such as chimpanzees, are not cooperative breeders. Because the human and chimpanzee lineages split between 5 million and 7 million years ago, and humans are the only apes that engage in cooperative breeding, researchers have puzzled over how this helping behavior might have evolved all over again on the human line.

In the late 1990s, Sarah Blaffer Hrdy, now an anthropologist emeritus at the University of California, Davis, proposed the cooperative breeding hypothesis. According to her model, early in their evolution humans added cooperative breeding behaviors to their already existing advanced ape cognition, leading to a powerful combination of smarts and sociality that fueled even bigger brains, the evolution of language, and unprecedented levels of cooperation. Soon after Hrdy’s proposal, anthropologists Carel van Schaik and Judith Burkart of the University of Zurich in Switzerland began to test some of these ideas, demonstrating that cooperatively breeding primates like marmosets engaged in seemingly altruistic behavior by helping other marmosets get food with no immediate reward to themselves.

Yet researchers debated how to interpret such experiments, and also their relevance to human evolution. One big issue was whether apes such as chimpanzees, which do help each other out in certain circumstances, do so out of selfless motives or need to be cajoled to lend a helping hand. If chimpanzees are sometimes altruistic even though they are not cooperative breeders, some scientists pointed out, that would weaken support for the cooperative breeding hypothesis. But experiments to determine whether chimps and other primates do engage in selfless behavior have been plagued by methodological problems and arguments about how they should be interpreted.

In the new study, published online today in Nature Communications, Burkart, van Schaik, and their colleagues took a different approach. The team designed a standardized apparatus that, with small modifications, could be used to test helping behavior in pretty much any primate species. The apparatus consists of a movable board that’s put outside of the animals’ cages. The researchers placed a favored food treat on one end of the board and trained the animals to operate a handle that pulls the food within reach of other individuals (see video, above). The animal operating the handle cannot get at the food himself, making it more likely that he is engaging in altruistic behavior toward his fellow primates.

The researchers used the apparatus to test helping behavior in 15 different primate species, including cooperative breeders like marmosets and tamarins, lemurs, spider monkeys, capuchin monkeys, macaques, chimps, and human children ranging from 5 to 7 years old. The basic design of the apparatus was the same for each species, although it was adjusted for the size of the animals and the shape of their hands. The food treats included items that the primates particularly like: dead crickets for the marmosets and tamarins, crackers for the macaques, and Smarties candies for the human children.

When the results of the trials were subjected to statistical analysis, the team found a close linear correlation between the degree to which a species engages in cooperative breeding and the likelihood that members of the group would help fellow animals get the food treat. Other possible factors that might explain or influence the altruistic behavior—such as higher cognition (measured by brain size), hunting in groups, or stronger social bonds between group members—showed either much weaker correlations or no correlation at all with helping behaviors.

The researchers suggest that cooperative breeding might have developed when our earliest ancestors, who evolved in Africa, first moved from life in the trees to a more precarious existence in savanna and woodland environments, several million years ago. “From other species, such as birds, we know that [cooperative breeding] is typically associated with adverse environmental conditions where it is difficult to survive,” Burkart says. “Once they moved into those savannah habitats, it may simply have become impossible for mothers to rear and provision their offspring alone.” Among the advantages of cooperative breeding, Burkart adds, is that mothers can give birth to new offspring while the previous ones are still dependent on adult care, thus increasing their reproductive success.

“The authors did an amazing job of developing a standardized approach” to test these ideas, says Katherine Cronin, a primate researcher at the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands. Cronin adds that the results carry special weight because the animals were tested while in their “natural group environment” rather than being removed from their cages and put into artificial situations, as has often been the case in past experiments.

Yet she cautions that cooperative breeding may be only one of a number of explanations for why humans evolved altruistic, highly cooperative behavior.

Andrew Whiten, a psychologist at the University of St. Andrews in the United Kingdom, agrees. “This is an impressively comprehensive study,” he says, and the authors have “applied the same relatively simple and elegant method to so many primate species.” Nevertheless, “it was not just one magic factor like cooperative breeding that made us what we are.”

Hrdy, for her part, calls the new results “a thing of beauty” because they show that cooperative breeding “does the best job of predicting” altruistic behavior. She agrees with Burkart and her co-workers that cooperative breeding “goes a long way towards explaining why only the apes in the line leading to Homo developed the necessary neural underpinnings” for highly cooperative behavior. “But we still have a long ways to go to explain why humans are so interested in the thoughts and feelings, intentions, and needs and desires of others”—qualities that are essential, she says, for uniquely human “hyper-cooperation.”

(Video credit: Judith Burkart)