Biologists classify species into a nested series of groups or taxa (see TAXONOMY); the term primate labels one such taxon, at the classification level called an Order, a group within the more inclusive Class Mammalia. The primate order presents great variation within it, which some have treated as an evolutionary trend towards greater reliance on the VISUAL SYSTEM rather than OLFACTION, increased postnatal dependence (see MATERNAL BEHAVIOUR), and increased size and complexity of the CEREBRAL CORTEX. While retaining a kernel of truth, this gives an unfortunate impression of evolution directed at progressively closer approaches to human capability, which is of course nonsense. The simplest, and unique, defining feature of all primates is the possession of a fingernail on at least one digit. This ADAPTATION allows greater support for the pad of the digit, and so enables the extreme dexterity with which some primates can manipulate delicate objects, while retaining great power in the GRIP. (The five-fingered hand, another crucial element in primate dexterity, is a retained primitive feature, anatomically rather similar to that of many reptiles.)
Generalization about characteristics becomes a little easier for the main subgroups of the primates.
The main division is between strepsirhines (lemurs and lorises) and haplorhines (monkeys and apes). Strepsirhines retain the wet RHINARIUM (nose-tip) and long snouts of many other mammals, relying extensively on olfaction, and the reflective TAPETUM of the eye indicates their generally nocturnal adaptation. The haplorhines possess many derived characteristics, with (in general) enhanced COLOUR VISION and DEPTH PERCEPTION, far less reli ance on smell, and larger brains. Those species found in the New World form a separate CLADE (see EVOLUTION), and many of them have been far less studied than Old World forms. The latter are called catarrhines (the term refers to their close-set nostrils, with nasal septum), and include monkeys and apes. Most species are relatively large-brained and highly social, living typically in semi-permanent groups; unlike any New World monkeys, some are quite terrestrial and large-bodied. The apes differ strikingly from any other mammal in their LOCOMOTION, since their motile shoulder-blades permit suspension below a branch and clambering or swinging (BRACHIATION) through trees. This taxon also includes humans, separated from their nearest ape relatives by surprisingly short spans of independent evolution (very approximately: chimpanzee 6 million years, gorilla 8 million, orangutan 16 million, gibbon 18 million).
RICHARD W.BYRNE
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