Modern humans (Homo sapiens) are grouped with the mammals (class Mammalia) in the subphylum Vertebrata. Within the mammals, humans are assigned by taxonomists to the primates (order Primates), along with lemurs, monkeys, and apes. Humans are grouped by most taxonomists together with the surviving species of Hominoidea, or great apes (the others include two species of gorilla, two species of chimpanzee, orangutans, and simangs). However, all members of the human family can stand upright with no difficulty and are naturally bipedal, whereas apes are naturally quadrapedal, only standing upright when necessary. Humans are also the only surviving members of the family Hominidae and genus Homo. Extinct members of the genus include Homo habilis and Homo erectus. The Homo sp. probably evolved from one of several species of australopithecines: Australopithecus africanus, A. robustus, A. boisei, and the recently discovered A. garhi. "Lucy" (Australopithecus afarensis) lived around 3.5 million...