At a psychology laboratory in Leipzig, Germany, two toddlers eye gummy bears that lie on a board beyond their reach. To get the treats, both tots must pull in tandem on either end of a rope. If only one child pulls, the rope detaches, and they wind up with nothing. A few miles away, in a plexiglass enclosure at Pongoland, the ape facility at the Leipzig Zoo, researchers repeat the identical experiment, but this time with two chimpanzees. If the primates pass the rope-and-board test, each one gets a fruit treat. By testing children and chimps in this way, investigators hope to solve a vexing puzzle: Why are humans so successful as a species? Homo sapiens and Pan troglodytes share almost 99 percent of their genetic material. Why, then, did humans come to populate virtually every corner of the planet—building the Eiffel Tower, Boeing 747s and H-bombs along the way? And why are chimps still foraging for their supper in the dense forests of equatorial Africa, just as their ancestors did seven or so million years ago, when archaic humans and the great apes separated into different species?