Primate Characteristics Suggest Arboreal Ancestry

Fossil evidence indicates that the primates arose very early in the Cenozoic from a stock of small, tree-dwelling insectivores not unlike the living tree shrews (order Scandentia: family Tupaiida). The group soon split into several evolutionary lineages that have likely had independent histories since.

A tree shrew, thought to resemble the common ancestor of all primates. They are not closely related to squirrels, which they resemble superficially.

Although living primates are a diverse group, most share the following characteristics:

1. enlargement of the clavicle (reduced or absent in many other mammals)
2. a highly mobile shoulder joint and an elbow joint allowing some degree of rotation
3. retention of five functional digits on each limb
4. enhanced individual mobility of the digits, especially the thumb and big toe, which are often opposable
5. modification of claws into flattened nails
6. development of sensitive tactile pads on the digits
7. shortening of the snout
8. three-dimensional (stereoscopic) vision
9. enhanced visual acuity and color vision
10. enlargement of the brain, particularly the cerebral cortex
11. usually only two mammary glands and one young per pregnancy

Many of these traits are adaptations for an arboreal lifestyle. Among ground-dwelling mammals the limbs have tended to evolve enhanced stability and reduced range of motion. In contrast, tree-dwelling mammals, especially those that move via vertical clinging and leaping or brachiation (swinging by the forelimbs), need very mobile limbs and often position limbs at the sides of the body (rather than underneath it) to maximize their reach. Grasping hands and feet and dextrous, sensitive digits are also adaptive for an arboreal existence.

The position of the eyes also differs between ground-dwelling and arboreal mammals. Many terrestrial quadrupeds have eyes positioned at the sides of the head, giving them a very wide visual field, but having very little overlap in the fields of the two eyes. This arrangement allows a terrestrial animal to see predators (for example) even if they are almost directly behind the animal, but gives the animal relatively poor depth perception. Primates tend to have their eyes positioned relatively close together on the front of the head, reducing their visual field, but giving them excellent stereoscopic vision and depth perception. This is, of course, important in localizing objects and an animal jumping from limb to limb obviously must be able to detect the position of the next limb with considerable accuracy.

The Prosimians were the First Primates

The living primates are usually classified into suborders: The Prosimii (prosimians - including lemurs and lorises) and the Anthropidea (anthropoids - monkeys, apes, and humans). The name prosimian means "pre-monkey" and this group of primates are thought to have been the first primates to evolve from the groups insectivore ancestors. In general, prosimians have a mix of "typical" primate characteristics and traits more similar to those of ancestral insectivores. For example, prosimians have opposable first digits, but have longer snouts and long bushy tails - traits not seen in anthropoid primates.

Many Anatomical Differences Separate Apes from Humans

Some of the many anatomical changes that occurred in the course of evolution from an ape-like ancestor to modern humans are:

1. the jaw became shorter and more arc-shaped (rather than U-shaped as in apes) (see A below), and the teeth became smaller.
2. the point of attachment of the skull to the vertebral column shifted from the rear of the braincase to a position under the braincase (see C below), which balances the skull more on top of the vertebral column; permitting a more upright, vertical posture.
3. the braincase became much larger relative to body size and, as it did, a prominent vertical forehead developed.
4. crests and ridges on the skull, such as eyebrow ridges, were reduced as the muscles that attached to them became smaller (see B below).
5. the nose became more prominent, with a distinct bridge and tip.
6. the arms (though probably never as long as in modern apes) became shorter, and the legs became longer relative to the arms.
7. the posture became uproght and bipedal, which required a change in the curvature of the vertebral column (becoming more S-shaped), as well as modifications in the shape of the pelvis and positioning of the legs.
8. the feet became flattened for bipedal walking and then an arch developed.
9. the big toes moved back into line with the other toes and ceased to be opposable (see D below).

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