Other species are capable of displaying dazzling feats of intelligence. Crows can solve multistep problems. Apes display numerical skills and empathy. Yet, neither species has the capacity to conduct scientific investigations into other species' cognitive abilities. This type of behavior provides solid evidence that humans are by far the smartest species on the planet.

Besides just elevated IQs, however, humans set themselves apart in another way: Their offspring are among the most helpless of any species. A new study, published recently in Proceedings of the National Academy of Sciences (PNAS), draws a link between human smarts and an infant’s dependency, suggesting one thing led to the other in a spiraling evolutionary feedback loop. The study, from psychologists Celeste Kidd and Steven Piantadosi at the University of Rochester, represents a new theory about how humans came to possess such extraordinary smarts.

Like a lot of evolutionary theories, this one can be couched in the form of a story—and like a lot of evolutionary stories, this one is contested by some scientists. Kidd and Piantadosi note that, according to a previous theory, early humans faced selection pressures for both large brains and the capacity to walk upright as they moved from forest to grassland. Larger brains require a wider pelvis to give birth whereas being bipedal limits the size of the pelvis. These opposing pressures—biological anthropologists call them the “obstetric dilemma”—could have led to giving birth earlier when infants’ skulls were still small.

Thus, newborns arrive more immature and helpless than those of most other species. Kidd and Piantadosi propose that, as a consequence, the cognitive demands of child care increased and created evolutionary pressure to develop higher intelligence. This in turn led to larger brains, requiring earlier birth—and that produced offspring who were more helpless, requiring even higher parental intelligence, a circle that led to “runaway selection” and superintelligent modern humans. “We were playing with my niece, thinking, ‘It takes a lot of human abilities to take care of this kid,’” Piantadosi says. “You have to figure out what they need, and when.” You also have to understand goals, Kidd adds: “If the goal is to grab at something in a dangerous location, being able to read that quickly is very useful.” Humans are particularly good at this kind of social reasoning and various theories have been suggested to explain this, usually involving living in social groups. “Our theory is an alternative, which says that social reasoning evolved to take care of kids.” Piantadosi says.

To test this the pair developed a mathematical model that first links the likelihood of surviving birth to a baby’s head size, and second the likelihood of surviving childhood with parental intelligence. The model shows that, given the right starting points (big brains), the pressures for increasing brain size and lowering birth age can become self-reinforcing, leading to runaway evolution. “When you look at how unusual humans are, it must be true that some form of runaway selection has been active,” says anthropologist Owen Lovejoy of Kent State University in Ohio, who edited the paper for PNAS. An implication of the theory is that some human abilities are a by-product of this selection for skills relevant to child care. “We have too many odd behaviors and capabilities that don't relate to reproductive fitness,” Lovejoy says. “They have to be collateral effects that came along for the ride.”

The researchers then tested the theory's most fundamental prediction: Higher intelligence should be associated with more helpless newborns in other primates. They used weaning time as an indicator of helplessness and showed that this predicted a measure of intelligence derived from a previous analysis of many studies of primate cognition. Helplessness was an even better predictor of intelligence than brain size, which may seem puzzling—but the relationship between brain size and intelligence is complex. Brain organization matters more than volume. Having a bigger brain does not necessarily make a species smarter—but if that species is to evolve higher intelligence, its brain has to get bigger. “Intelligence is associated with brain size but it's more strongly determined by environmental pressures,” Piantadosi explains. “Maximum intelligence might be bounded by brain size but the level you actually get may be determined by other pressures, which in our case are about child care.”

The authors also say the theory explains why superintelligence evolved relatively late, specifically in primates. “Other accounts don't explain why primates developed superintelligence but other species didn't,” Kidd says. “If it was about dealing with environmental fluctuations and hardships, why didn't insects or reptiles, who had much longer than primates, develop human-level intelligence?” The model requires both big brains and live births because there is no link between the maturity of newborns and head size or intelligence in egg-laying species. “The key is live birth,” Kidd adds.

Some researchers who were not part of the study are not buying Kidd’s theory, however. Evolutionary psychologist Robin Dunbar of the University of Oxford contends that the study misconstrues evolutionary theory. Dunbar argues that intelligence evolves in reaction to the need to cope with environmental upheavals, which in turn can lead to pressures that affect child-rearing strategies and subsequently brain size. "What they have hit on is the consequence of evolution, not the cause," Dunbar says.

Another critic took a different tack. Anthropologist Dean Falk of The Florida State University says the study ignores extensive fossil and primate data suggesting that bipedalism came with altered brain motor systems that caused increased helplessness, millions of years before brain size began to increase. “The evidence suggests that hominin infants became helpless long before the obstetric dilemma kicked in, which would not have occurred until after brain size began to increase,” Falk says. This incapacity would then have sparked changes in child care, providing a major impetus for later brain evolution and the eventual emergence of language and other cognitive abilities unique to humans, she explains.

Chet Sherwood, an anthropologist at The George Washington University, says the study merely represents an extension of old ideas. “The model formalizes a notion that has been discussed by anthropologists for a long time, that the evolution of human cognition was shaped by a shift in brain development, which necessitated more intense caregiving from mothers and other helpers,” he says.

Anthropologist Wenda Trevathan of New Mexico State University argues the authors have largely oversimplified the complex relationship between parental intelligence and infant survival. Kidd and Piantadosi acknowledge this and say their model is not meant as a full account but as “a piece of a much more complex evolutionary and reproductive history in which multiple traits are interrelated.”

Trevathan, however, agrees with the general thrust of the researchers’ arguments. “The helpless infant and requisite intense and long-term parenting had a huge impact on human evolution and help account for who we are today,” she says. An understanding of where evolution has brought us may even help new parents adjust to life with baby. “I hope it makes new parents feel better when they're wondering why is this infant so much work?” Kidd says. “Having useless babies is what makes us special.”

Dedicated to the memory of Jenny O'Sullivan, whose extraordinary intelligence and humanity will be sorely missed.