Haplorhini in the context of "Reginald Innes Pocock"

Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians (monkeys and apes). Primates arose 74–63 million years ago first from small terrestrial mammals, which adapted for life in tropical forests: many primate characteristics represent adaptations to the challenging environment among tree tops, including large brain sizes, binocular vision, color vision, vocalizations, shoulder girdles allowing a large degree of movement in the upper limbs, and opposable thumbs (in most but not all) that enable better grasping and dexterity. Primates range in size from Madame Berthe's mouse lemur, which weighs 30 g (1 oz), to the eastern gorilla, weighing over 200 kg (440 lb). There are 376–524 species of living primates, depending on which classification is used. New primate species continue to be discovered: over 25 species were described in the 2000s, 36 in the 2010s, and six in the 2020s.

Primates have large brains (relative to body size) compared to other mammals, as well as an increased reliance on visual acuity at the expense of the sense of smell, which is the dominant sensory system in most mammals. These features are more developed in monkeys and apes, and noticeably less so in lorises and lemurs. Some primates, including gorillas, humans and baboons, are primarily ground-dwelling rather than arboreal, but all species have adaptations for climbing trees. Arboreal locomotion techniques used include leaping from tree to tree and swinging between branches of trees (brachiation); terrestrial locomotion techniques include walking on two hindlimbs (bipedalism) and modified walking on four limbs (quadrupedalism) via knuckle-walking.

The simians, anthropoids, or higher primates are primates of the infraorder Simiiformes (/ˈsɪmi.ɪfɔːrmiːz/) containing all animals traditionally called monkeys and apes. More precisely, they consist of the parvorders Platyrrhini (New World monkeys) and Catarrhini, the latter of which consists of the family Cercopithecidae (Old World monkeys in the stricter sense) and the superfamily Hominoidea (apes – including humans).

The simians are sister group to the tarsiers (Tarsiiformes), together forming the haplorhines. The radiation occurred about 60 million years ago (during the Cenozoic era); 40 million years ago, simians colonized South America, giving rise to the New World monkeys. The remaining simians (catarrhines) split about 25 million years ago into Cercopithecidae and apes (including humans).

Adapiformes is a group of early primates. Adapiforms radiated throughout much of the northern continental mass (now Europe, Asia and North America), reaching as far south as northern Africa and tropical Asia. They existed from the Eocene to the Miocene epoch. Some adapiforms resembled living lemurs.

Adapiforms are known from the fossil record only, and it is unclear whether they form a monophyletic or paraphyletic group. When assumed to be a clade, they are usually grouped under the "wet-nosed" taxon Strepsirrhini, which would make them more closely related to the lemurs and less so to the "dry-nosed" Haplorhini taxon that includes monkeys and apes.

Tarsiers (/ˈtɑːrsiərz/ TAR-see-ərz) are haplorhine primates of the family Tarsiidae, which is the lone extant family within the infraorder Tarsiiformes. Although the group was prehistorically more globally widespread, all of the existing species are restricted to Maritime Southeast Asia, predominantly in Brunei, Indonesia, Malaysia and the Philippines.

They are found primarily in forested habitats, especially forests that have liana, since the vine gives tarsiers vertical support when climbing trees.

In his Systema Naturae of 1758, Carl Linnaeus divided the Order Primates within Mammalia into four genera: Homo, Simia, Lemur, and Vespertilio. His Vespertilio included all bats, and has since been moved from Primates to Chiroptera. Homo contained humans, Lemur contained four lemurs and a colugo, and Simia contained the other Primates. Linnaeus did not think that Homo should form a distinct group from Simia, classifying them separately mainly to avoid conflict with religious authorities. If this is taken into account, Simia (including Homo) would be roughly equivalent to the Suborder Haplorhini of the Primates (while Lemur would be roughly equivalent to the Suborder Strepsirrhini).

Homo, Lemur, and Vespertilio have survived as generic names, but Simia has not. All the species have since been moved to other genera, and in 1929, the International Commission on Zoological Nomenclature ruled in its Opinion 114 that Simia be suppressed. The genus Simias is distinct and remains valid, containing a single species, the pig-tailed langur (Simias concolor).

The tapetum lucidum (Latin for 'bright tapestry, coverlet'; /təˈpiːtəm ˈluːsɪdəm/ tə-PEE-təm LOO-sih-dəm; pl.: tapeta lucida) is a layer of tissue in the eye of many vertebrates and some other animals. Lying immediately behind the retina, it is a retroreflector. It reflects visible light back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image).

The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep-sea animals. Similar adaptations occur in some species of spiders. Haplorhine primates, including humans, are diurnal and lack a tapetum lucidum.