News feature: The search for what sets humans apart.

In a quiet room, away from the bustle of their daycare playmates, two 3 year olds pull a rope that’s wrapped around a board, which sits inside a transparent box. A single pile of riches, stickers and gummy bears, can be theirs but only if they work together. To bring the board to the front of the box and ferry the reward to within reach, each must pull one end of the rope simultaneously because the ends of the rope are too far apart for one person to manage. First, each child works with an adult to accomplish the feat. Then the children work together, bringing the spoils close enough to grab. At that point, each must make a decision: should they share? Usually, they do (1). Chimpanzees, it turns out, do not share, unless the food reward is split equally into two piles (See Fig. 2). Otherwise, the dominant chimpanzee takes it all (2). This is one example of differing social-cognitive skills—cooperative skills in this case—between humans and other animals. “And you can see that humans are adapted for that in a way other apes are not,” says Michael Tomasello, codirector of the Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany, who oversaw both experiments. Whether or not the experiment reveals an adaptation that improves reproductive success, the Tomasello work asserts that there’s something unique about the way humans cooperate, distinguishing us from other animals. Such contentions about human uniqueness have driven research in animal behavior, cognitive development, neuroscience, linguistics, genetics, and other fields. Although these claims address fundamental questions about our place in nature, many scientists think they fail to reflect the subtleties of sound scientific inquiry. That’s because researchers are constantly learning more, both about aspects of cognitive abilities and about how animals behave—and such findings always have the potential to move the human-uniqueness goalpost. Indeed, thus far, every claim that humans are qualitatively different from other animals has fallen short. Distinctions seem to be a matter of degree. “This has been a theme in comparative cognition,” says Laurie Santos, director of Yale University’s Comparative Cognition Laboratory in New Haven, CT. “And perhaps an embarrassing one for researchers: oftentimes they stake this claim—‘Aha, this is the cognitive capacity that makes us special’—but then when you look carefully at other animals you see things in them that look pretty special, too,” she says. Frans deWaal, director of the Center for the Advanced Study ofApe andHumanEvolution at Emory University in Atlanta, GA, agrees. “We’re better at a lot of things, but it’s never a qualitative difference.” de Waal predicts a similar outcome for this apparent cooperation distinction: “It is going to fall apart, too.”