The answer just might have to do with how pathetic we are when we're born. Helpless infants need smart parents to take care of them, and intelligent parents need bigger brains. But giving birth to offspring that will develop big brains is a challenge, because the mechanics of getting a big head out of a mother's body are, well, difficult. That means babies need to be born at an earlier stage of development, before their heads get too big — when they're even more tiny and helpless.
In a study published Monday in the Proceedings of the National Academy of Sciences, Piantadosi and his colleague Celeste Kidd modeled how this evolutionary positive feedback system could have led to runaway natural selection that resulted in absurdly smart humans (but still not smart enough to behave well on the Internet). The effect was surprisingly strong: "The comparative helplessness of the species was a stronger predictor of intelligence than almost any other factor," Piantadosi said.
He and Kidd tested their theory on humans' primate relatives. Using a common measure of intelligence established by primatologists — "it's basically a primate IQ test," Kidd said — they cross referenced each species' smarts with its weaning time (how long infants rely on their mother's milk). The relationship was immediately obvious.
Marmosets, which are clever but not exactly brilliant, wean their young after just a few months. But young chimpanzees, which can learn language, use tools and perhaps even mourn their dead, depend on their mothers for nearly four years.
This relationship held even when the researchers controlled for other factors that could be used to explain it — brain size, a child's body size relative to an adult's, etc. Weaning time turned out to explain 70 percent of all variation in primate intelligence.
Humans weren't included in the model; our intelligence is so comparatively off the charts it would have essentially ruined the data. But we certainly validate the trend. Whereas even baby bonobos — with chimpanzees, our closest animal relatives — can grab onto their mothers as soon as they are born, human infants can't grasp, walk, or even lift their own heads.
"Human babies are exceptionally useless," Kidd said. With a laugh, she amended her adjective use: "I mean, helpless."
And though humans are weaned relatively young, our children still rely on their parents a lot longer than other creatures' babies. As Kidd pointed out, a 3- or 4-year-old would fare very poorly on its own in the wild — which bonobos do all the time.
This relationship makes sense if you think about the kinds of thinking that humans are good at, according to Piantadosi. We excel at social reasoning — interpreting others' motivations and intentions and figuring out how to act accordingly. Existing theories about human intelligence have attributed this to our communal lifestyles: the fact that we live and hunt in groups, our need to communicate through language.
"Social understanding is ... a really important thing for taking care of kids that are born helpless, because you have to be able to figure out what they need," Piantadosi said. "In that sense this fits nicely with those kinds of theories about what makes humans special."
Helplessness at birth might also go a long way toward explaining why intelligence came about in mammals, even though there are clades that have been around a lot longer and have dealt with similar circumstances. Many insects live in complex social groups, for instance, and they've had about 400 million years to cultivate their smarts. But they don't give birth to live young — their infants develop inside eggs. This means that insects didn't face the same selective pressure for super smart parents that mammals do, so they didn't get to tap into the positive feedback loop that may have driven the evolution of intelligence in humans.
"It doesn't quite explain, 'Why humans?' but it explains why you get human level intelligence in mammals," Piantadosi said.