Humans are primates

Humans are members of a particular sub-group of mammals known as the primates (Order Primates).

Primate diversity

The first primates appeared more than 60 million years ago. Many different types of primates have evolved over this vast period of time and many of these no longer exist. Others have survived and there are now more than 350 different species of living primates. Almost all of today’s primates live in tropical and subtropical areas of Africa, Madagascar, Asia, Central America and South America. They include lemurs, lorises, tarsiers, monkeys and apes are all primates.

Primate features

Primates (including humans) are different to all other animals because they are the only mammals that have the following combination of features:

• relatively large, complex brain
• forward-facing eyes with overlapping fields of view that allow depth perception
• eye sockets with a ring or cup of bone surrounding and supporting the eyes
• grasping hands with long fingers to curl around objects
• opposable thumbs and/or big toes (able to touch the other digits on the same hand or foot)
• flat nails (rather than claws) on some fingers and toes
• sensitive pads under the tips of the fingers and toes containing special touch receptors called Meissner’s Corpuscles
• a well-developed collarbone (clavicle)
• two nipples (but sometimes more) on the chest (in females, these supply milk to the young)
• penis and testes that permanently hang down from the body (in males)
• long childhood that extends well beyond weaning

Primitive primates – the strepsirrhines

The strepsirrhines are, in many ways, the most primitive primates. This group contains about 90 species, most of which are nocturnal (active at night). They include lemurs, lorises, bushbabies and the Aye-aye. Sometimes these primates are grouped in with the tarsiers and are referred to as ‘prosimians’ but it is now known that tarsiers are more advanced than lemurs and lorises and belong in a different group.Most strepsirrhines live in forests in Madagascar but some live in Africa or southern Asia.

Strepsirrhines have the following features:

• large ears and a keen sense of hearing
• eye sockets that have a narrow ring of bone around each eye
• eyes with a special light reflecting layer that aids night vision
• long snouts with moist noses for a highly developed sense of smell
• long, projecting lower front teeth (incisors) usually modified into a fur grooming comb
• a claw rather than a nail on the second toe of each foot
• most have more than two nipples as several offspring are usually born at a time

Advanced primates – the haplorrhines

The more advanced primates are placed together into a group known as the haplorrhines. The primates included in this group are the:

• tarsiers
• ‘New World’ monkeys
• ‘Old World’ monkeys
• apes

Tarsiers

The tiny tarsiers share many features with monkeys and apes but also retain some primitive strepsirrhine-like features. There are more than 10 species and all are nocturnal (active at night). Tarsiers live in forests on parts of Indonesia, Malaysia and the Philippines. They were once grouped with the strepsirrhines as ‘prosimians’ because they share the following primitive features:

• lower jaw in two pieces (the left and right sides are separated by a joint at the chin)
• claws rather than nails on some toes
• two or three pairs of nipples on the chest and abdomen

Tarsiers are now considered to be close relatives of monkeys and apes because, like monkeys and apes, they have following features which evolved more recently:

• eye sockets with walls of bone that form a cup-like cavity for each eye
• eyes which do not have a light reflecting layer
• small, dry noses and a reduced sense of smell
• lower front teeth (incisors) which are not modified into a grooming comb.

New World monkeys

There are two major groups of monkeys. One group with about 120 species is known as the New World monkeys and includes the marmosets, tamarins, howler monkeys and spider monkeys. These monkeys are mostly tree-dwellers. New World monkeys are found in Central and South America.

New World monkeys differ from other monkeys and apes because they have:

• no ear canal (instead, the eardrum is located near the skull’s surface)
• widely spaced nostrils that open to the sides
• six premolar teeth and either four or six molars in each jaw
• claws on some fingers and toes (in the marmosets and tamarins)
• a prehensile (grasping) tail (in the larger species)

Old World monkeys

There are about 130 species of Old World monkeys including baboons, macaques, rhesus monkeys and colobus monkeys. Most live in trees although some live fully or partly on the ground. Old World monkeys are found in Africa, Asia and in southern Europe on the Rock of Gibraltar.

Old World monkeys are more closely related to apes than to New World monkeys because, like apes, they have:

• ear canal (a bony tube that connects the external ear to the eardrum)
• closely spaced nostrils that open forward or downward
• four premolar teeth and six molars in each jaw
• flat nails on all fingers and toes (no claws)

Old World monkeys differ from apes because they have:

• molar teeth that tend to have four cusps (raised bumps) arranged in pairs. Each pair of cusps is joined to form a cutting ridge resulting in two ridges on the surface of each molar tooth
• a shoulder structure that does not allow them to fully rotate their arms
• a chest cavity that is narrow and deep
• an external tail

Apes

There are about 20 species of apes and most of these are gibbons. Orang-utans, gorillas, chimpanzees and humans make up the remaining types of apes. Most ape species (other than humans) live in South-east Asia but some species are only found in central Africa. Humans are also classified as apes but unlike other apes, we have an almost world-wide distribution.

Apes differ from all other primates because they have:

• molar teeth with five distinct cusps (raised bumps) in the lower jaw and four cusps in the upper jaw (these cusps are separate rather than joined in ridges)
• a shoulder structure that allows them to fully rotate their arms
• a wide, shallow chest cavity
• no external tail