In the view of the primatologist Sarah Blaffer Hrdy, the extraordinary social skills of an infant are at the heart of what makes us human. Through its ability to solicit and secure the attentive care not just of its mother but of many others in its sensory purview, a baby promotes many of the behaviors and emotions that we prize in ourselves and that often distinguish us from other animals, including a willingness to share, to cooperate with strangers, to relax one’s guard, uncurl one’s lip and widen one’s pronoun circle beyond the stifling confines of me, myself and mine.

As Dr. Hrdy argues in her latest book, “Mothers and Others: The Evolutionary Origins of Mutual Understanding,” which will be published by Harvard University Press in April, human babies are so outrageously dependent on their elders for such a long time that humanity would never have made it without a break from the great ape model of child-rearing. Chimpanzee and gorilla mothers are capable of rearing their offspring pretty much through their own powers, but human mothers are not.

Human beings evolved as cooperative breeders, says Dr. Hrdy, a reproductive strategy in which mothers are assisted by as-if mothers, or “allomothers,” individuals of either sex who help care for and feed the young. Most biologists would concur that humans have evolved the need for shared child care, but Dr. Hrdy takes it a step further, arguing that our status as cooperative breeders, rather than our exceptionally complex brains, helps explain many aspects of our temperament. Our relative pacifism, for example, or the expectation that we can fly from New York to Los Angeles without fear of personal dismemberment. Chimpanzees are pretty smart, but were you to board an airplane filled with chimpanzees, you “would be lucky to disembark with all 10 fingers and toes still attached,” Dr. Hrdy writes.