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The report ran in the major journal Current Biology, and people seemed to find it interesting. In the week that followed, Pruetz's findings were featured in more than 300 news and science outlets, including New Scientist, the New York Times, the Washington Post, and NPR's Science Friday. The Smithsonian Institution requested one of the spears. In short, it was the most widely talked about primatology news since the reports of infanticide and cannibalism at Jane Goodall's site at Gombe in the 1970s.

Pruetz and I watch the chimps climb from their nests. A large male hangs from a low branch by one arm, swinging gently, in no hurry. The silhouette is utterly erect, arrestingly humanoid. He lets go, drops to the ground, and moves off across the plateau. The symbolism is impossible to miss. Here is a chimpanzee, thought by many to be the closest thing we have to a living model of our early hominin ancestors, literally dropping from the trees and moving out into the open expanses of the savanna. It is as though we are watching time-lapse footage of human evolution, the dawn of man unfolding in our binoculars.

Chimps that live on the ground, rather than in the safety of treetops, tend to be wary of large strangers. Jill Pruetz spent four years getting the Fongoli chimpanzees accustomed to the presence of humans—what primatologists call habituating them—and the past three summers observing them. Six days a week, from dawn to dusk, she follows the chimps.

It is not glamorous work. It's hot and filthy and exhausting. Home is a mud-walled hut and a drop toilet shared with 30 Fongoli villagers. Dinner is peanut sauce over rice, except when it's peanut sauce over millet. If the chimps wander unusually far, Pruetz gets back to the village so late that her portion has long ago been fed to the dogs. Sometimes, rather than hike the five miles back to camp, she curls up and sleeps on the ground (or takes a nap in an abandoned chimp nest). She has gotten malaria seven times.

Yet you rarely meet people who love what they do as much as Pruetz does. Right now she is sitting on the ground, jotting notes with one hand and slapping sweat bees with the other. Blood from a blister has soaked through the heel of her sock. To listen to Pruetz, we might as well be in Paris. "Sometimes," she says, scratching a bite, "I think I'm going to wake up and it's all a dream." The payoffs have been dramatic. In addition to using tools to hunt, Fongoli chimps have been exhibiting some other novel behaviors: soaking in a water hole, passing the afternoon in caves.

At 24 square miles, Fongoli is the largest home range of any habituated chimpanzee group ever studied. (Jane Goodall's Gombe chimps, by comparison, roam over five square miles.) Craig Stanford likens foraging over a large range to knowing one's way around an enormous supermarket. Like Pruetz, he believes the chimpanzees are not foraging at random, but moving with foresight and intent. "You don't stroll down the aisles hoping to catch a glimpse of the broccoli. You know where each item is, and in which months seasonal foods are likely to be in stock." The same, he thinks, holds true for chimpanzees.

"Ecological intelligence" is the name of the theory that some primates, including those of our lineage, have evolved larger, more complex brains because it helped them adapt to the challenges of surviving in a less giving habitat. "The first push toward a larger brain," writes Stanford, "may have been the result of a patchily distributed, high-quality diet and the cognitive mapping capabilities that accompanied it."

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