Being a “dad” is uncommon in the primate world. Most male primates have little to do with their offspring, especially apes. Other primate males are invested in mating. They typically don’t take the time to care for their young. They donate their DNA, and then they are on their way to mate with as many females as possible. So why are human males different?

This has been a puzzle to scientists and evolutionary biologists. It was previously believed that our ancestors, around five to eight million years ago, struck up a balance between fidelity and food. Females would remain monogamous to one male, while males cared for their mate and offspring, providing things to eat.

A new study in the Proceedings of the National Academy of Sciences suggests that it is more complex than this – and fatherhood originated from changing environmental conditions.

“Paternal provisioning among humans is puzzling because it is rare among primates and absent in nonhuman apes,” say the authors.

The study divided pre-human males into two types. “Dads” were the males that stuck around and provided for their children. “Cads”, on the other hand, were those males that contributed DNA and little else, and spent most of their energy mating with as many females as possible.

Ordinarily, it seems that “Cads” would win out over “Dads”. They would mate more often, have more children, and hence make up a larger portion of the gene pool. So what triggered the emergence of a society with mostly “Dads”?

The authors say this happened about five to eight million years ago. During this time, Africa underwent a drying spell. Food was harder to find. Survival depended on coming up with new ways to obtain food and eating a more diverse diet.

Ripe fruit was less available, so our ancestors turned to hunting and scavenging. They ate meat, tubers, and nuts. This led to walking on two feet, which encouraged greater mobility. Brain size increased. Children needed a longer period to stay with their mothers.

And most importantly, survival depended on cooperation.

Societies where males teamed together to hunt obtained more meat per capita. Groups where males and females cooperated were able to share a wider variety of foods – meats, rich in proteins and fats, obtained by the males, and carbohydrates found by the females. The groups that worked together had a more balanced diet – and were healthier.

Using game theory, the researchers showed that eventually, the “Dad” mentality would take over. The offspring of the invested males were healthier. Gradually, there was a shift where males started to care for their young.

So why don’t we see this in other primates?

Perhaps, say the researchers, the answer lies in the fact that our ancestors began to walk on two feet. When walking on two feet, they had their hands free to use tools. Game was sparse and often large – using tools and working in groups made it easier to take down large game. Again, this fostered cooperation and a different social structure we see in other primates.