Simian in the context of Archicebus

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.

Haplorhini (/hæpləˈraɪnaɪ/), the haplorhines (Greek for "simple-nosed") or the "dry-nosed" primates is a suborder of primates containing the tarsiers and the simians (Simiiformes or anthropoids), as sister of the Strepsirrhini ("moist-nosed"). The name is sometimes spelled Haplorrhini. The simians include catarrhines (Old World monkeys and apes, including humans), and the platyrrhines (New World monkeys).

Haplorhini was proposed by Pocock in 1918 when he realized the tarsiers were actually sister to the monkeys rather than the lemurs, also following findings of Hugh Cuming 80 years earlier and Linnaeus 160 years earlier. For Linnaeus, this ensemble of primates constituted a genus "Simia". For religious reasons, Homo constituted its own genus (which has remained).

Apes, collectively Hominoidea (/ˌhɒmɪˈnɔɪdi.ə/; hominoids), are a superfamily of Old World simians native to sub-Saharan Africa and Southeast Asia. They were more widespread in Africa, Asia, and Europe in prehistory, and, including humans, are found globally. Apes are more closely related to Old World monkeys (family Cercopithecidae) than to the New World monkeys (Platyrrhini), with both Old World monkeys and apes placed in the clade Catarrhini. Apes do not have tails due to a mutation of the TBXT gene. In traditional and non-scientific use, the term ape can include tailless primates taxonomically considered Cercopithecidae (such as the Barbary ape and black ape), and is thus not equivalent to the scientific taxon Hominoidea. There are two extant branches of the superfamily Hominoidea: the gibbons, or lesser apes; and the hominids, or great apes.

• The family Hylobatidae, the lesser apes, include four genera and a total of 20 species of gibbon, including the lar gibbon and the siamang, all native to Asia. They are highly arboreal and bipedal on the ground. They have lighter bodies and smaller social groups than great apes.
• The family Hominidae (hominids), the great apes, includes four genera comprising three extant species of orangutans, two extant species of gorillas, two extant species of chimpanzees, and humans.

Except for gorillas and humans, hominoids are agile climbers of trees. Apes eat a variety of plants and animals. Plants, which can include fruits, leaves, stalks, roots, nuts, and seeds, make up the bulk of their diet. Human diets are sometimes substantially different from that of other hominoids, due in part to the development of technology and a wide range of habitation.

A roar /rɔːr/ is a type of animal vocalization that is loud, deep and resonating. Many mammals have evolved to produce roars and other roar-like vocals for purposes such as long-distance communication and intimidation. These include various species of big cats, bears, pinnipeds, deer, bovids, elephants, and simians.

The anatomical basis for the ability to roar often involves modifications to the larynx and hyoid bone and enlarged internal air spaces for low-frequency vocal resonation. While roaring, animals may stretch out their necks and elevate their heads to increase the space for resonance.

In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:

1. the grouping contains its own most recent common ancestor (or more precisely an ancestral population), i.e. excludes non-descendants of that common ancestor
2. the grouping contains all the descendants of that common ancestor, without exception

Monophyly is contrasted with paraphyly and polyphyly as shown in the second diagram. A paraphyletic grouping meets 1. but not 2., thus consisting of the descendants of a common ancestor, excepting one or more monophyletic subgroups. A polyphyletic grouping meets neither criterion, and instead serves to characterize convergent relationships of biological features rather than genetic relationships – for example, night-active primates, fruit trees, or aquatic insects. As such, these characteristic features of a polyphyletic grouping are not inherited from a common ancestor, but evolved independently.

The parvorder Catarrhini /kætəˈraɪnaɪ/ (known commonly as catarrhine monkeys, Old World anthropoids, or Old World monkeys) consists of the Cercopithecoidea (Old World monkeys) and Hominoidea (apes). In 1812, Geoffroy grouped those two groups together and established the name Catarrhini, "Old World monkeys", ("singes de l'Ancien Monde" in French). Its sister in the infraorder Simiiformes (simians) is the parvorder Platyrrhini (New World monkeys). There has been some resistance to directly designate apes (and thus humans) as monkeys despite the scientific evidence, so "Old World monkey" may be taken to mean the Cercopithecoidea or the Catarrhini. That apes are monkeys was already realized by Georges-Louis Leclerc, Comte de Buffon in the 18th century. Linnaeus placed this group in 1758 together with what we now recognise as the tarsiers and the New World monkeys, in a single genus "Simia" (sans Homo). The Catarrhini are all native to Africa and Asia. Members of this parvorder are called catarrhines.

The Catarrhini are the sister group to the New World monkeys, the Platyrrhini. Some six million years before the ape - Cercopithecoidea bifurcation, the Platyrrhini emerged within "monkeys" by migration to South America from Afro-Arabia (the Old World), likely by ocean.

New World monkeys are the five families of primates that are found in the tropical regions of Mexico, Central and South America: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. The five families are ranked together as the Ceboidea (/səˈbɔɪdi.ə/), the only extant superfamily in the parvorder Platyrrhini (/plætɪˈraɪnaɪ/).

Platyrrhini is derived from the Greek for "broad nosed", and their noses are flatter than those of other simians, with sideways-facing nostrils. Monkeys in the family Atelidae, such as the spider monkey, are the only primates to have prehensile tails. New World monkeys' closest relatives are the other simians, the Catarrhini ("down-nosed"), comprising Old World monkeys and apes. New World monkeys descend from African simians that colonized South America, a line that split off about 40 million years ago.

Afrotarsius is a primate found in the Paleogene of Africa.

The first species to be named, Afrotarsius chatrathi, was named in 1985 on the basis of a single lower jaw from the Oligocene of Fayum, Egypt, and tentatively referred to the tarsier family (Tarsiidae). However, this relationship immediately proved controversial, and in 1987 the animal was placed in a separate family Afrotarsiidae related to simians. A tarsier-like tibiofibula was allocated to Afrotarsius in 1998, but the identity of this bone is controversial. In 2010, a second species of the genus, Afrotarsius libycus, was named from the Eocene of Dur At-Talah, Libya, on the basis of isolated upper and lower teeth. Features of these teeth were interpreted as additional evidence for a relationship between Afrotarsius and anthropoids. A second afrotarsiid genus, Afrasia, was named in 2012 from the Eocene Pondaung Formation of Myanmar. In the same paper, Afrotarsiidae was placed together with the Asian Eosimiidae in an infraorder Eosimiiformes, in the simians. However, some studies indicate that it should be placed in Tarsiiformes.

Eosimiidae is the possible family of extinct primates believed to be the earliest simians.

The evolutionary history of the primates can be traced back 57-90 million years. One of the oldest known primate-like mammal species, Plesiadapis, came from North America; another, Archicebus, came from China. Other such early primates include Altiatlasius and Algeripithecus, which were found in Northern Africa. Other similar basal primates were widespread in Eurasia and Africa during the tropical conditions of the Paleocene and Eocene.Purgatorius is the genus of the four extinct species believed to be among the earliest example of a primate or a proto-primate, a primatomorph precursor to the Plesiadapiformes, dating to as old as 66 million years ago.

The surviving tropical population of primates, which is seen most completely in the upper Eocene and lowermost Oligocene fossil beds of the Faiyum depression southwest of Cairo, gave rise to all living species—lemurs of Madagascar, lorises of Southeast Asia, galagos or "bush babies" of Africa, and the anthropoids: platyrrhine or New World monkeys, catarrhines or Old World monkeys, and the apes, including Homo sapiens.

Prosimians are a group of primates that includes all living and extinct strepsirrhines (lemurs, lorisoids, and adapiforms), as well as the haplorhine tarsiers and their extinct relatives, the omomyiforms, i.e. all primates excluding the simians. They are considered to have characteristics that are more "primitive" (ancestral or plesiomorphic) than those of simians (monkeys, apes, and humans).

Simians emerged within the Prosimians as sister group of the haplorhine tarsiers, and therefore cladistically belong to this group. Simians are thus distinctly closer related to tarsiers than lemurs are. Strepsirrhines bifurcated some 20 million years earlier than the tarsier–simian bifurcation. However, simians are traditionally excluded, rendering prosimians paraphyletic. Consequently, the term "prosimian" is no longer widely used in a taxonomic sense, but is still used to illustrate the behavioral ecology of tarsiers relative to the other primates.

Menstruation is the shedding of the uterine lining (endometrium) in some mammals. It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.

Although there is some disagreement in definitions between sources, menstruation is generally considered to be limited to primates. It is common in simians (Old World monkeys, New World monkeys, and apes), but completely lacking in strepsirrhine primates and possibly weakly present in tarsiers. Beyond primates, it is known only in bats, the elephant shrew, and the spiny mouse species Acomys cahirinus. Overt menstruation (where there is bleeding from the uterus through the vagina) is found primarily in humans and close relatives such as chimpanzees.