But as Tomasello argues in his book, this “social intelligence hypothesis” is something of an understatement. A social nature isn’t enough to fully distinguish between humans and chimpanzees—male chimpanzees can form political alliances, for example, and sometimes work together to hunt, both of which require advanced social skills. Humans are not just socially intelligent, then; as Tomasello and others have put it, we’re “ultra-social” in ways that the great apes are not, with an enhanced capacity for cooperation that arose somewhere along our species’ evolutionary path.

Tomasello has conducted dozens of studies to support this idea. In one study published in 2007, he and his colleagues gave 105 human toddlers, 106 chimpanzees, and 32 orangutans a battery of tests assessing their cognitive abilities in two domains: physical and social. The researchers found that the children and the apes performed identically on the physical tasks, like using a stick to retrieve food that was out of reach or recalling which cup had food in it. But with the social tests—like learning how to solve a problem by imitating another person, or following an experimenter’s gaze to find a treat—the toddlers performed about twice as well as the apes.

Related to this enhanced social ability is a greater tendency to work together, even on tasks where collaboration isn’t necessary. In a 2011 study by Tomasello and his Planck Institute colleagues, 3-year-old children and chimpanzees were given an opportunity to obtain a reward either on their own or by collaborating with another member of their species. The experiment was set up so that the children and the apes knew a) that they would get the reward regardless of whether they worked with a partner, and b) that working with a partner would mean both of them got the same reward. Children, the researchers found, were much more likely to collaborate than chimpanzees.

There are many theories for why humans became ultra-social. Tomasello subscribes to the idea that it’s at least partly a consequence of the way early humans fed themselves. After humans and chimpanzees diverged from their common ancestor around 6 million years ago, the two species adopted very different strategies for obtaining food: Chimpanzees, who eat mostly fruit, gather and eat the majority of their food alone; humans, by contrast, became collaborative foragers. The fossil record shows that as early as 400,000 years ago, they were working together to hunt large game, a practice that some researchers believe may have arisen out of necessity—when fruits and vegetables were scarce, early humans could continue the difficult work of foraging and hunting small game on their own, or they could band together to take home the higher reward of an animal with more meat.

Chimps show no signs of this ability. “It is inconceivable,” Tomasello has said, “that you would ever see two chimpanzees carrying a log together.” In one of the earliest studies of chimpanzee cooperation, published in 1937, chimpanzees only worked together to pull in a board with food on it after they’d been extensively trained by an experimenter—they showed no natural ability to do it on their own. (Even when chimpanzees do collaborate, there’s been no evidence to date that they have the ability to adopt complementary roles in group efforts or establish a complex division of labor.)