The suborder Anthropoidea represents the branch of Primates that includes all monkeys, apes, and humans. These creatures are informally known as anthropoids, or sometimes simians, and they share a set of distinct physical and behavioral traits. The term Anthropoidea literally translates from Greek roots to mean “human-like,” reflecting the close relationship of this grouping to our own species.
Defining the Suborder Anthropoidea
Anthropoids were historically classified as a suborder of Primates, differentiating monkeys, apes, and humans from prosimians like lemurs. Modern taxonomy places anthropoids within the infraorder Simiiformes, which is nested within the suborder Haplorhini, alongside the tarsiers. This shift reflects that anthropoids and tarsiers share more recent genetic commonalities than either group does with the Strepsirrhini (lemurs and lorises).
Anthropoids are defined by a suite of evolutionary novelties that emerged approximately 40 million years ago, distinguishing them from their earlier primate ancestors. The primary difference is the general trend toward increased body and brain size, which supports more complex behaviors and social structures. Their evolution involved a transition to a predominantly diurnal, or daytime, activity pattern, leading to significant changes in sensory perception.
Key Anatomical and Behavioral Characteristics
A defining anatomical feature of anthropoids is the complete enclosure of the eye socket by a bony plate, known as post-orbital closure. This bony partition fully separates the eye from the chewing muscles in the temporal fossa. This adaptation supports a strong reliance on vision, including highly developed stereoscopic vision and color perception, which is advantageous for diurnal foraging.
The increased focus on sight is paired with a corresponding reduction in the sense of smell, evidenced by the loss of the rhinarium (wet nose) found in strepsirrhine primates. Anthropoids possess a dry nose and a significant reduction in the internal nasal turbinals related to olfaction. Furthermore, the vomeronasal organ, which detects pheromones, is non-functional or absent, reflecting the reduced importance of chemical communication.
Anthropoid teeth are characterized by a generalized dental formula. While the ancestral formula of 2.1.3.3 is retained by New World monkeys, Catarrhines (Old World monkeys, apes, and humans) later evolved a reduced formula of 2.1.2.3. Anthropoids also exhibit complex social behaviors, long periods of infant dependency, and a trend toward neocortex expansion, which supports higher cognitive functions.
The Two Major Anthropoid Infraorders
Anthropoids are classified into two primary infraorders, defined by geographic isolation and nasal morphology: Platyrrhini and Catarrhini. Platyrrhines, meaning “flat-nosed,” are the New World monkeys found exclusively in Central and South America. Their nostrils are wide-set and face outward.
Many Platyrrhines, such as spider monkeys and howler monkeys, possess prehensile tails used as a fifth limb for arboreal locomotion. This infraorder includes diverse groups like marmosets and tamarins, which retain the ancestral dental formula of 2.1.3.3. Their evolutionary path diverged from the Old World anthropoids approximately 35 to 40 million years ago.
The second infraorder, Catarrhini, or “downward-nosed,” includes all Old World monkeys, apes, and humans, native to Africa and Asia. Their nostrils are narrow and face downward, and none possess prehensile tails. Catarrhines share a derived dental formula of 2.1.2.3, having lost one premolar in each quadrant compared to their New World relatives.
This infraorder is further subdivided into two superfamilies: Cercopithecoidea (Old World Monkeys) and Hominoidea (Apes and Humans). A distinction lies in their molars: Cercopithecoids, like baboons and macaques, possess bilophodont molars characterized by four cusps arranged in two parallel ridges. Conversely, Hominoidea (gibbons, gorillas, chimpanzees, and humans) is defined by the presence of Y-5 molars, where five cusps are separated by a distinctive Y-shaped groove.