An international team of anthropologists and economists have developed a new theory of human male parental investment.

How did human fatherhood come about? Paternal care (i.e. investment in offspring provided by a biological father) is rare among mammals but widespread across modern human subsistence societies. Much of men’s parental investment consists of provisioning relatively helpless children with food for prolonged periods of time (up to two decades in modern hunter-gatherers). Paternal provisioning is absent among other great apes, whose observed mating systems wouldn't have encouraged paternal provisioning. That paternal provisioning arose in humans seems remarkable and puzzling. With promiscuous mating, a would-be “Dad” who provides food for a mate and their joint offspring without seeking other mates risks being outcompeted in terms of biological fitness by a “Cad”, who focuses only on promiscuous mating instead of investing in offspring. Such a competitive disadvantage creates a formidable barrier for Dads to emerge when Cads abound.

An oft-invoked explanation for the evolution of paternal provisioning in humans is that ancestral females started mating preferentially with males who provided them with food, in exchange for female sexual fidelity. In their new study, which appears in PNAS, Ingela Alger, Jonathan Stieglitz and co-authors Donald Cox (Boston College), Paul Hooper (University of New Mexico), and Hillard Kaplan (Chapman University) argue that this explanation is insufficient for several reasons. Instead they posit that ecological change would have sufficed to trigger the spread of Dads, even in the face of female sexual infidelity.

The key force in the theory of paternal provisioning is complementarities between females and males, and also between males. In this context complementarities are synergistic effects that increase per-capita benefits, which may arise from dividing labor and/or pooling resources. The path to complementarities began roughly 5-8 million years ago, with a gradual drying in Africa, and a progressively greater need to rely on nutritious, diverse, spatially dispersed and relatively hard-to-obtain foods including animal products. In response to ecological change, ancestral hominins adapted in various ways including efficient bipedal locomotion, dietary flexibility, and an ability to thrive in diverse environments, facilitated by tool use. Complementarities between males and females would have resulted from the nutrients that each sex specialized in acquiring: protein and fat acquired by males pair well with carbohydrates acquired by females. Complementarities between males would have resulted from higher returns from hunting in groups instead of in isolation, and from food sharing to lower starvation risk. Dietary reliance on animal products is thus a key feature underlying these complementarities between and within sexes.

These complementarities would have led to a substantial increase in the impact of food provided by a Dad on the survival of his mate’s offspring. Using evolutionary game theory the researchers show that this impact can lead Dads to gain a fitness advantage over Cads, although Cads may still co-exist with Dads under certain conditions. If sons inherit their biological father’s traits, then over time Dads will increase in number in a population. Theoretically connecting the evolution of paternal provisioning to ecological change allows the authors to make novel predictions about the paleontological and archeological record.

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