Primates constitute the class of mammals that includes lemurs, monkeys, and apes. Humans are apes, therefore primates. The term primate, used by scientists for several centuries, suggests that this group, which includes humans, is the most important among mammalian classes. While scientists no longer rank classes by their importance relative to humanity, the term primate continues in use.

Biologists classify primates into approximately 230 living species of prosimians, tarsiers, and anthropoids. Primates, in general, share these characteristics:

- They have the largest brains among mammals, after adjustment for body size.

- They often live in large, complex social groups. Large brains aid survival but are considered to be most important for social interactions.

- They take a longer time to reach maturity than most other mammals, a characteristic related to the evolution of intelligence.

The first primates lived in trees, and most still do, except for a few groups such as baboons and humans that have (independently from one another) evolved the ability to live on the ground. This adaptation is complete enough that in humans, uniquely among primates, the lower limbs (legs) are relatively long. Some, like galagos, have remarkable leaping abilities: Though less than a foot and a half (50 cm) in length, galagos can leap over six feet (2 m). Leaping from branch to branch in trees has encouraged the evolution of important characteristics that primates possess more than most other mammals:

- The ability to grasp. This results primarily from an opposable thumb, rare in other mammals. Primates not only have the ability to grasp branches in trees, but the juvenile primates can efficiently cling to the fur of their mothers. Most primates also have opposable big toes, although primates that have evolved a ground-based existence have lost this characteristic. Nonhuman apes have opposable big toes; humans do not; the australopithecine ancestors of humans had big toes that were intermediate in their adaptation for grasping branches.

- Excellent vision. Nocturnal primates have big eyes. For example, each of the tarsier's eyes is larger than its brain. Diurnal primates have inherited big eyes from nocturnal ancestors. Visual acuity is important not only for night vision but for jumping around in trees. For prosimians, smell remains an important sense, while most anthropoids rely primarily on vision. . .

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