Chimpanzees are the closest living relatives, sharing more than 98 percent of their genetic blueprint. Humans and chimpanzees are also believed to have a common ancestor that lived four to eight million years ago. Although they typically walk on all fours (knuckle-walking), chimpanzees can stand and walk upright. By swinging from branch to branch, they can move quite efficiently in the trees, where they obtain most of their food. Chimpanzees also usually sleep in trees, building nests out of leaves.