64 CHAPTER 3 | Living Primates
and transmit the appropriate response back down to the
motor nerves. The enlarged, responsive cerebral cortex
provides the biological basis for flexible behavior patterns
found in all primates, including humans.
There are many reasons for the increased learning ca-
pacity of the primate brain, but it likely started as the ear-
liest primates, along with many other mammals, began to
carry out their activities in the daylight hours. Prior to
65 million years ago, mammals seem to have been noc-
turnal in their habits. The extinction of the dinosaurs
and climate change at that time opened new ecological
niches—a species’ way of life considered in the full con-
text of its environment, including other species, geology,
climate, and so on. With the change to a diurnal life, the
sense of vision took on greater importance, and so visual
acuity was favored by natural selection. Unlike reptile vi-
sion, where the information-processing neurons are in
the retina, mammalian vision is processed in the brain,
permitting integration with information received through
other senses such as sound, touch, taste, and smell.
If the evolution of visual acuity led to larger brains, it
is likely that the primates’ insect predation in an arboreal
setting also played a role in enlargement of the brain. This
would have required great agility and muscular coordina-
tion, favoring development of the brain centers. Thus it is
of interest that much of the higher mental faculties are ap-
parently developed in an area alongside the motor centers
of the brain.^3
Another related hypothesis that may help account for
primate brain enlargement involves the use of hands as
tactile instruments to replace the teeth and jaws or snout.
The hands assumed some of the grasping, tearing, and di-
viding functions of the jaws, again requiring development
of the brain centers for more complete coordination.The Primate Skeleton
The skeleton gives animals with internal backbones, or
vertebrates, their basic shape or silhouette, supports the
soft tissues, and helps protect vital internal organs (Fig-
ure 3.6). In primates, for example, the skull protects the
brain and the eyes. A number of factors are responsible for
the shape of the primate skull as compared with those of
most other mammals: changes in dentition, changes in the
sensory organs of sight and smell, and increased brain size.
The primate braincase, or cranium, tends to be high
and vaulted. A solid partition exists in anthropoid pri-
mates between the eye and the temple, affording maxi-
mum protection to the eyes from the contraction of the
chewing muscles positioned directly next to the eyes.for acutely clear perception without sacrificing visual con-
tact with the object’s surroundings.
The primates’ emphasis on visual acuity came at the
expense of their sense of smell. Smells are processed in the
forebrain, a part of the brain that projects into the snout of
animals depending upon smells. A large protruding snout,
however, may interfere with stereoscopic vision. But smell
is an expendable sense to diurnal tree-dwelling animals
in search of insects; they no longer needed to live a “nose
to the ground” existence, sniffing the ground in search of
food. The anthropoids especially have the least-developed
sense of smell of all land animals. Though our sense of
smell allows humans to distinguish perfumes, and even
to distinguish family members from strangers, our brains
have come to emphasize vision rather than smell. Prosim-
ians, by contrast, still rely more on smell than on vision,
possessing numerous scent glands for marking objects in
their territories.
Arboreal primates also possess an acute sense of
touch. An effective feeling and grasping mechanism helps
prevent them from falling and tumbling while speeding
through the trees. The early mammals from which pri-
mates evolved possessed tiny touch-sensitive hairs at the
tips of their hands and feet. In primates, sensitive pads
backed up by nails on the tips of the animals’ fingers and
toes replaced these hairs.
The Primate Brain
These changes in sensory organs have corresponding
changes to the primate brain. In addition, an increase in
brain size, particularly in the cerebral hemispheres—the
areas supporting conscious thought—occurred in the
course of primate evolution. In monkeys, apes, and hu-
mans, the cerebral hemispheres completely cover the cer-
ebellum, the part of the brain that coordinates the muscles
and maintains body balance.
One of the most significant outcomes of this develop-
ment is the flexibility seen in primate behavior. Rather
than relying on reflexes controlled by the cerebellum, pri-
mates constantly react to a variety of features in the envi-
ronment. Messages from the hands and feet, eyes and ears,
as well as from the sensors of balance, movement, heat,
touch, and pain, are simultaneously relayed to the cerebral
cortex. Obviously the cortex had to evolve considerably in
order to receive, analyze, and coordinate these impressions
ecological niche A species’ way of life considered in the full
context of its environment, including factors such as diet, activ-
ity, terrain, vegetation, predators, prey, and climate.
vertebrate An animal with a backbone, including fish, am-
phibians, reptiles, birds, and mammals.
cranium The braincase of the skull.(^3) Romer, A. S. (1945). Vertebrate paleontology (p. 103). Chicago: University
of Chicago Press.