Great apes are members of the family Hominidae, which includes humans, chimps, gorillas, and orangutans. There is one species of human (Homo sapiens), two species of chimpanzee (common chimpanzee and pygmy chimpanzee, also known as the bonobo), two species of gorilla (western gorilla and eastern gorilla), and three species of orangutan (Bornean orangutan, Sumatran orangutan, and Tapanuli orangutan), though throughout the last few million years there have been dozens of other great apes, some of which (like Homo neanderthalis) were arguably as intelligent as modern humans.
Great apes are large, tailless primates, distinguished from "lesser apes" (gibbons) by a larger size, greater brain-to-body ratio, and generally more human-like and less monkey-like anatomical features. Still, within this group there is considerable variation. These apes evolved from African lesser apes about 18 million years ago, orangutans split from the rest of the apes about 14 million years ago, gorillas split off about 7 million years ago. Until recently, it was thought that chimps and humans split between 3 and 5 million years ago, but more fossil finds (Sahelanthropus) suggest the split happened earlier, between 6 and 7 million years ago.
Great apes are all extremely intelligent. Each one is probably smarter than every other member of the animal kingdom. All the great apes engage in some degree of tool use. Their semi-precision grip allows them to use rocks or sticks for various purposes, including as weapons. Gorillas have been observed testing water depth with sticks. Chimpanzees are known to lure termites out of their nests using sticks. Though chimpanzee tool use in the wild was popularized by Jane Goodall in the 1960s, all the great apes have since been observed using tools. It is uncertain which ape aside from humans is the most intelligent, but recent research seems to point to the orangutan, which are intelligent enough to build leak-proof roofs over their nightly nests.
Of the great apes, only one, the orangutan, is primarily arboreal (tree-dwelling). The others dwell on the ground, with only one, the human, being able to swim. In central Africa, the Congo River divides the two primary chimp species: the common chimp and the bonobo. Their genetic separation occurred about 1.5 - 2 million years ago, when the Congo River first formed. The differences between the two are superficial in anatomy, but deep in diet and social structure. The bonobos eat only fruit, and have an egalitarian, nonviolent, matriarchal, highly sexual society, while common chimps are omnivorous and have a social structure based on an alpha male leading a troop of beta males.
Of course, the most unusual of the great apes is the human. Humans have numerous anatomical differences with the other apes, including large brains, bipedal locomotion, tall stature, hairless bodies, excess body fat, and more. Evolutionary biologists have yet to come up with decisive reasons to explain all of these differences, but various theories have been presented.