A primate is any member of the biological orderPrimates, the group that contains all lemurs, monkeys, and apes, including humans. The English singular primate is a back-formation from the Latin name Primates, which itself was the plural of the Latin primas ("one of the first, excellent, noble").
All primates have five fingers (pentadactyly), a generalized dental pattern, and a primitive (unspecialized) body plan. Another distinguishing feature of primates is fingernails. Opposing thumbs are also a characteristic primate feature, but are not limited to this order; opossums, for example, also have opposing thumbs. In primates, the combination of opposing thumbs, short fingernails (rather than claws) and long, inward-closing fingers is a relic of the ancestral practice of brachiating through trees. Forward-facing color binocular vision was also useful for the brachiating ancestors of humans, particularly for finding and collecting food. All primates, even those that lack the features typical of other primates (like lorises), share eye orbit characteristics that distinguish them from other taxonomic orders.
As the table below illustrates, in many primate species, the males are larger than the females. However this picture is incomplete. All but one of these are Old World species, and in this group the mating system is usually polygynous; sexual dimorphism is expected with this kind of social structure. As the table shows, sexual dimorphism is much less in the marmosets (New World) than in the other species listed, and this is characteristic of New World monkeys in comparison with the Old World monkeys and apes. This is because the New World monkeys generally form pair bonds.
In more modern, cladistic reckonings, the Primate order is a true clade and can be further divided into three main clades. The most primitive clade is the suborderStrepsirhini, which contains all of the extant prosimians except for the tarsiers. The seven strepsirhine families are the four related lemur families and the three remaining families that include the lorises, the Aye-aye, the galagos, and the pottos. Some classification schemes wrap the Megaladapidae into the Lemuridae and the Galagonidae into the Loridae, yielding a three-two family split instead of the four-three split as presented here.
The suborder Haplorhini is composed of the remaining two sister clades: the tarsiers in family Tarsiidae (monotypic in its own superfamily) and the two superfamilies of New and Old World monkeys, including humans and the other apes.
The Tree shrews have sometimes been classified as primates, but are now usually placed in their own order, Scandentia.
