Chimpanzee, sometimes colloquially chimp, is the common name for the two extant hominid species of apes in the genus Pan. The Congo River forms the boundary between the native habitats of the two species: Common chimpanzee, Pan troglodytes (West and Central Africa) Bonobo, Pan paniscus (forests of the Democratic Republic of the Congo) Chimpanzees are members of the Hominidae family, along with gorillas, humans, and orangutans. Chimpanzees split from the human branch of the family about four to six million years ago. The two chimpanzee species are the closest living relatives to humans, all being members of the Hominini tribe (along with extinct species of Hominina subtribe). Chimpanzees are the only known members of the Panina subtribe. The two Pan species split only about one million years ago. The genus Pan is considered to be part of the subfamily Homininae to which humans also belong. These two species are the closest living evolutionary relatives to humans, sharing a common ancestor with humans about four to six million years ago. Research by Mary-Claire King in 1973 found 99% identical DNA between human beings and chimpanzees, although research since has modified that finding to about 94% commonality, with some of the difference occurring in noncoding DNA. It has been proposed, for instance by J. Diamond in his book where he refers to man as The Third Chimpanzee, that P. troglodytes and P. paniscus belong with H. sapiens in the genus Homo, rather than in Pan. One of the arguments for this is other species have been reclassified to belong to the same genus on the basis of less genetic similarity than that between humans and chimpanzees. The male common chimp is up to 1.7 m (5.6 ft) high when standing, and weighs as much as 70 kg (150 lb); the female is somewhat smaller. The common chimpís long arms, when extended, have a span one and a half times as long as the bodyís height and a chimpanzee's arms are longer than its legs. The bonobo is a little shorter a d thinner than the common chimpanzee, but has longer limbs. Both species use their long, powerful arms for climbing in trees. On the ground, chimpanzees usually walk on all fours using their knuckles for support with their hands clenched, a form of locomotion called knuckle-walking. Chimpanzee feet are better suited for walking than are those of the orangutan because the chimpís soles are broader and the toes shorter. Both the common chimpanzee and bonobo can walk upright on two legs when carrying objects with their hands and arms. The bonobo has proportionately longer upper limbs and tends to walk upright more often than the common chimpanzee. The coat is dark; the face, fingers, palms of the hands, and soles of the feet are hairless; and the chimp has no tail. The exposed skin of the face, hands and feet varies from pink to very dark in both species, but is generally lighter in younger individuals, darkening as maturity is reached. A University of Chicago Medical Centre study has found significant genetic differences between chimpanzee populations. A bony shelf over the eyes gives the forehead a receding appearance, and the nose is flat. Although the jaws protrude, the lips are thrust out only when a chimp pouts. The brain of a chimpanzee has been measured at ~337 cc, ~393 cc, with a general range of 282-500 cc. Human brains, in contrast, have been measured as being three times larger, variously reported volumes include ~1,299 cc, ~1,158 cc, and averages of ~1330 cc. Chimpanzee testicles are unusually large for their body size, with a combined weight of about 4 oz (110 g) compared to a gorilla's 1 oz (28 g) or a human's 1.5 ounces (43 g). This is generally attributed to sperm competition due to the polyandrous nature of chimpanzee mating behavior. Chimpanzees reach puberty at an age of between eight and 10 years, and rarely live past age 40 in the wild, but have been known to live more than 60 years in captivity.