Monkey is a common term for many groups or species of simian mammals belonging to the infraorder Simiiformes. The word is used to describe groupings of primates, such as New World monkey families and Old World monkey families. Many monkey species are arboreal (live in trees), while some, such as baboons, live mostly on the ground. The majority of animals are most active during the day (diurnal). Monkeys are often seen to be clever, especially Catarrhini’s Old World monkeys.

Simians and tarsiers first appeared 60 million years ago among haplorrhines. New World monkeys and catarrhine monkeys appeared 35 million years ago within the simians. Some 25 million years ago, Old World monkeys and Hominoidea originated from the catarrhine monkeys. Primatologists regard extinct basic simians such as Aegyptopithecus or Parapithecus, eosimiidea, and even the Catarrhini group to be monkeys.

Strepsirrhine primates, such as lemurs, lorises, and galagos, are not monkeys. Tarsiers are haplorhine primates, like monkeys, yet they are not monkeys.

Apes evolved from “monkeys” as sisters of the Cercopithecidae in the Catarrhini, making them cladistically monkeys. Because it has been difficult to label apes (and consequently humans) as monkeys, “Old World monkey” may refer to the Cercopithecoidea or Catarrhini. In the 18th century, Georges-Louis Leclerc, Comte de Buffon recognized that apes are monkeys.

The word “monkey” may have originated in a German version of the Reynard the Fox tale published about 1580, according to the Online Etymology Dictionary. A figure named Moneke is the offspring of Martin the Ape in this version of the story. As there was no clear distinction between “ape” and “monkey” in English at the time, the 1911 Encyclopaedia Britannica page for “ape” states that it is either a synonym for “monkey” or a tailless humanlike primate. The names “monkey” and “ape” are frequently interchanged in everyday speech. A few monkey species, such as the Barbary ape, include the word “ape” in their common name.

The concept that there were tendencies in primate evolution arose later in the first half of the twentieth century, and that the surviving members of the order could be placed in a sequence, leading through “monkeys” and “apes” to humans. Monkeys were therefore separated from “apes” as a “grade” on the evolutionary road to humans.

Monophyletic groupings, or groups made up of all the offspring of a common ancestor, are increasingly more commonly used in scientific classifications. Although the New World and Old World monkeys are both monophyletic groupings, their merger was not because it excluded hominoids (apes and humans). As a result, the name “monkey” no longer referred to a scientifically recognized taxon.

The infraorder Simiiformes, or simians, is the smallest recognized taxon that includes all monkeys. However, this includes the hominoids, making monkeys non-hominoid simians in terms of currently accepted taxa. The term monkey is ambiguous in colloquial and popular usage, and it sometimes refers to non-human hominoids. Furthermore, many arguments for a monophyletic use of the term “monkey” are presented from the standpoint that language should reflect cladistics.

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