Chimpanzees are our closest living relatives, sharing more than 98 percent of our genetic blueprint. Humans and chimps are also thought to share a common ancestor who lived some four to eight million years ago. Although they normally walk on all fours (knuckle-walking), chimpanzees can stand and walk upright. By swinging from branch to branch they can also move quite efficiently in the trees, where they do most of their eating. Chimpanzees usually sleep in the trees as well, employing nests of leaves.