When Australopithecus split off from other primates and moved out of the rainforests into drier and “predator-rich” habitats, they needed large groups to survive, says Peter Richerson, a biologist and professor emeritus at the University of California, Davis.

To survive, Australopithecus learned to work with people who weren’t biologically related to them. These social networks enabled strategy, weapon development and the formation of large “mobs that could hunt really tough predators,” Richerson says.

As australopithecines evolved to be bipedal, childbirth also became more laborious and dangerous. This change probably encouraged australopithecine mothers to help each other give birth, says Lesley Newson, an evolutionary biologist and co-author of A Story of Us: A New Look at Human Evolution. For Australopithecus mothers, “it was really beneficial if they cooperated and said, ‘I’ll pull your baby out if you pull my baby out,'” says Newson.

Sarah Hrdy, evolutionary anthropologist and author of Father of Time: A Natural History of Men and Babies, says that cooperation in child-rearing evolved. Hrdy found that early humans relied on family members other than the mother to raise and care for the baby, a concept called “cooperative reproduction” that is not seen in other primate species that are genetically similar to us.

Group members did this “in exchange for being accepted within the group,” says Hrdy. The baby, aware of its various caregivers, learned to observe, associate with, and ingratiate itself with non-family members. This “sets the stage for cooperation,” says Hrdy.