[3] Some nine million years before the divergence between the Cercopithecidae and the apes,[6] the Platyrrhini emerged within "monkeys" by migration to South America from Afro-Arabia (the Old World),[citation needed][7][8] likely by ocean.

Within suborder Haplorhini, the simians are a sister group to the tarsiers – the two members diverged some 70 million years ago.

Extinct basal simians such as Aegyptopithecus or Parapithecus (35–32 million years ago) are also considered monkeys by primatologists.

[26] Linnaeus placed this group in 1758 together with the tarsiers, in a single genus "Simia" (sans Homo), an ensemble now recognised as the Haplorhini.

[27] Monkeys, including apes, can be distinguished from other primates by having only two pectoral nipples, a pendulous penis, and a lack of sensory whiskers.

[28][better source needed] According to the Online Etymology Dictionary, the word "monkey" may originate in a German version of the Reynard the Fox fable, published c. 1580.

Later in the first half of the 20th century, the idea developed that there were trends in primate evolution and that the living members of the order could be arranged in a series, leading through "monkeys" and "apes" to humans.

[21][35][36][37][38] Several science-fiction and fantasy stories have depicted non-human (fantastical or alien) antagonistic characters refer to humans as monkeys, usually in a derogatory manner, as a form of metacommentary.

Apes (hominoids)—consisting of gibbons, orangutans, gorillas, chimpanzees and bonobos, and humans—are also catarrhines but were classically distinguished from monkeys.

Williams et al. outlined evolutionary features, including in stem groupings, contrasted against the other primates such as the tarsiers and the lemuriformes.

In some areas, some species of monkey are considered agricultural pests, and can cause extensive damage to commercial and subsistence crops.

[61] Some organizations train capuchin monkeys as service animals to assist quadriplegics and other people with severe spinal cord injuries or mobility impairments.

After being socialized in a human home as infants, the monkeys undergo extensive training before being placed with disabled people.

Around the house, the monkeys assist with daily tasks such as feeding, fetching, manipulating objects, and personal care.

[66][67] They are used primarily because of their relative ease of handling, their fast reproductive cycle (compared to apes) and their psychological and physical similarity to humans.

[68] A number of countries have used monkeys as part of their space exploration programmes, including the United States and France.

The television series Monkey and the literary characters Monsieur Eek and Curious George are all examples.

Author Terry Pratchett alludes to this difference in usage in his Discworld novels, in which the Librarian of the Unseen University is an orangutan who gets very violent if referred to as a monkey.

[72][73] Hanuman, a prominent deity in Hinduism, is a human-like monkey god who is believed to bestow courage, strength and longevity to the person who thinks about him or Rama.