By the logic of family trees, apes and humans are monkeys too
Apes sit deep inside the monkey family tree: any ape is more closely related to baboons and macaques than South American monkeys are. So if a group should include all descendants of a common ancestor, humans are monkeys. Many people resist that conclusion, and the word has quietly stopped naming a formal scientific group.
Monkey is a common name for most members of the simians, the infraorder Simiiformes. Traditionally every simian except the apes counts as a monkey, which makes the group paraphyletic, meaning it leaves out some descendants of its common ancestor. Include the apes and monkey simply becomes another word for simian. Lemurs, lorises and galagos are not monkeys, and neither are tarsiers, the simians' closest relatives.
The timeline runs deep. Simians and tarsiers split around 70 million years ago. New World and Old World lineages separated roughly 35 million years ago, the New World group apparently reaching South America across the ocean, and apes branched from Old World monkeys about 25 million years ago. Fossil simians such as Aegyptopithecus, from 35 to 32 million years ago, also count as monkeys. In 1812 Étienne Geoffroy coined Catarrhini for apes plus Old World monkeys; the 18th-century naturalist Buffon had already classed apes as monkeys, and Linnaeus in 1758 lumped them with tarsiers in a single genus.
Early twentieth-century thinking pictured evolution as a ladder from monkeys through apes to people, with monkeys as one rung. Modern classification prefers monophyletic groups instead, and the combination of New World and Old World monkeys fails that test because it excludes apes. Some scientists now argue that monkey should be used to include apes, to match the tree.
The word itself may come from Moneke, son of Martin the Ape in a German retelling of Reynard the Fox printed around 1580. English long blurred ape and monkey, and the Barbary ape is actually a monkey. Most monkeys live in trees and are active by day, though baboons spend most of their time on the ground, and Old World species are especially noted for intelligence. All simians share two chest nipples and a lack of sensory whiskers.
Source: Monkey
