There is something disconcerting about watching chimpanzees in the wild. Whereas other African mammals simply seem like part of the natural landscape, chimpanzees can seem a little too big and resemble humans a little too much to be in the trees. You get the feeling while observing them that their primary food, ripe fruit, is inadequate to sustain them, perhaps because you cannot imagine eating so much fruit yourself, or perhaps because they seem too intelligent to sit around gorging on figs for hours on end. Or maybe it is their obvious and familiar social awareness and behavioral individuality that engender the impression that they are just slightly disconnected from the seamlessness of the environment around them. Or the fleeting but human-like social gestures that are invariably captivating – a light touch after a fight, juveniles chasing each other in play, a mother grooming her infant. Whatever the reasons, wild chimpanzees can seem to occupy an ambiguous place in the rain forest, necessarily part of their ecological communities yet somehow apart.

The traces of familiarity that ignite our visceral fascination with chimpanzees are also the foundation of our intense scientific interest in them. Chimpanzees and their sister species the bonobos are our closest genetic relatives, with whom we shared a common ancestor some 6 to 9 million years ago. Chimpanzees, in particular, exhibit what appear to be precursors of many of the behaviors long thought to make humans special among the primates: complex tool fabrication and use, reliance on hunting and meat eating, cooperative lethal aggression, language, and cultural behavior. Consequently, human evolutionary biologists draw on the behavior of chimpanzees to develop hypotheses about the evolution of these traits in our ancestors. Chimpanzees offer a jumping-off point for thinking about the kinds of changes that would have been necessary to propel our ancestors on the trajectory toward Homo sapiens. As Jane Goodall has remarked,

But in studying these apes in their natural habitats, we are necessarily confronted with the plights of their communities, and of the human communities that surround them.