Newborns may seem tiny to their adoring moms and dads, but most other primate parents would disagree. According to Jeremy DeSilva, an anthropologist at Boston University, human babies weigh about 6% of what their mothers weigh, while other primate babies barely tip the scales at about 3% of their mothers’ weights. But according to a new study published by DeSilva in PNAS, our robust infants may have some something to do with how we came together as a society.
Not only are our babies bigger, they also have larger heads and fewer motor skills. On the whole, they are harder to carry around and more in need of being carried. (More on Time.com: Is There Any Biology Behind Our Political Affiliations?)
“The whole expression that it takes a village is in part rooted in the fact that we have really big infants that are pretty helpless,” DeSilva told NPR. “If we wanted to get anything done, we have to hand them off.”
DeSilva wanted to see if human ancestors also gave birth to large offspring, so he calculated the size range of infants born to Australopithecus — a species that predates modern humans by millions of years. One problem: there weren’t any fossils of newborn Australopithecus around to study. So, as NPR reported, DeSilva had to devise a way to calculate their size:
DeSilva came up with a clever way around that problem. It’s a two-step process: First, you use adult skulls to estimate the newborn’s skull size. That can be done very accurately for all primates, and DeSilva was able to analyze a dozen Australopithecus skulls.
“So once you have the size of the head, there is what researchers have called ‘the 12 percent rule,’” he says. The 12 percent rule says that the brain represents 12 percent of the total body weight. “It’s not exactly 12 percent; in fact in the apes it tends to be more like 10 percent.”
Even with that margin of error, DeSilva says, it was clear that the birth weight of Australopithecus infants was much closer to the 6 percent of modern humans than the 3 percent of apes.
He found that these ancient precursors to our own hefty, gurgling young were also closer to 6% of their mothers’ size than they were to the 3% seen in other primate babies. (More on Time.com: The Rich Are Different: More Money, Less Empathy)
While other primate species certainly engage in communal rearing of offspring, DeSilva theorizes that the stable, fixed nature of human communities — even nomadic communities camp for periods of time — allowed one group to take care of the not-exactly-portable infants while another hunted, gathered or cultivated the earth. In this way, our big babies may have helped to anchor our ancestors’ nomadic spirits and created the communities we now see as central to our humanity. It’s an interesting thought, even if it’s countered somewhat by the boom in baby strollers.