Evolution And History

62 CHAPTER 3 | Living Primates

objects hanging in space, such as vines or branches. Mon-

keys, apes, and humans achieved this through binocular

stereoscopic color vision (Figure 3.5), the ability to see the

world in the three dimensions of height, width, and depth.

Binocular vision (in which two eyes sit next to each other

on the same plane so that their visual fields overlap) and

nerve connections that run from each eye to both sides of

the brain confer complete depth perception characteristic

of three-dimensional or stereoscopic vision. This arrange-

ment allows nerve cells to integrate the images derived

from each eye. Increased brain size in the visual area in

primates, and a greater complexity at nerve connections,

also contribute to stereoscopic color vision.

Visual acuity, however, varies throughout the primate

order in terms of both color and spatial perception. Pro-

simians, most of whom are nocturnal, lack color vision.

The eyes of lemurs and lorises (but not tarsiers) are ca-

pable of reflecting light off the retina, the surface where

nerve fibers gather images in the back of the eye to in-

tensify the limited light available in the forest at night. In

addition, prosimian vision is binocular without the ben-

efits of stereoscopy. Their eyes look out from either side

of their muzzle or snout. Though there is some overlap of

visual fields, their nerve fibers do not cross from each eye

to both halves of the brain.

mammals. It enabled them to operate in the dark, to sniff

out their food, and to detect hidden predators. However,

for active tree life during daylight, good vision is a better

guide than smell in judging the location of the next branch

or tasty morsel. Accordingly, the sense of smell declined in

primates, while vision became highly developed.

Travel through the trees demands judgments concern-

ing depth, direction, distance, and the relationships of

Though the massive canine teeth of some male primates are serious

weapons, they are more often used to communicate rather than to

draw blood. Raising his lip to flash his canines, this mandrill will get

the young members of his group in line right away. Over the course of

human evolution, overall canine size decreased, as did differences in

canine size between males and females.

© Tom Brakefield/Corbis

Primary receiving area

for visual information

binocular vision Vision with increased depth perception

from two eyes set next to each other, allowing their visual fields

to overlap.

stereoscopic vision Complete three-dimensional vision (or

depth perception) from binocular vision and nerve connections

that run from each eye to both sides of the brain, allowing nerve

cells to integrate the images derived from each eye.

Figure 3.5 Monkeys, apes, and humans possess binocular

stereoscopic vision. Binocular vision refers to overlapping

visual fields due to forward-facing eyes. Three-dimensional

or stereoscopic vision comes from binocular vision and the

transmission of information from each eye to both sides of

the brain.