Evolution And History

Primate Characteristics 65

Femur Humerus

Radio-ulna

Carpals

Metacarpals

Fibula

Femur

Humerus

Radius

Ulna

Carpals

Tibia Metacarpals

Tarsals

Metatarsals

Phalanges

(2)

Phalanges (5)

Phalanges (5)

Tibia

Tarsals

Metatarsals

Phalanges

(2)

Figure 3.6 All primates possess the same ancestral vertebrate limb pattern seen in reptiles and
amphibians, consisting of a single upper long bone, two lower long bones, and five radiating digits
(fingers and toes), as seen in this gorilla (right) skeleton. Other mammals such as bison (left) have a
modified version of this pattern. In the course of evolution, bison have lost all but two of their digits,
which form their hooves. The second long bone in the lower part of the limb is reduced. Note also
the joining of the skull and vertebral column in these skeletons. In bison (as in most mammals) the
skull projects forward from the vertebral column, but in semi-erect gorillas, the vertebral column is
further beneath the skull.

The foramen magnum (the large opening at the base
of the skull through which the spinal cord passes and con-
nects to the brain) is an important clue to evolutionary
relationships. In most mammals, as in dogs and horses,
this opening faces directly backward, with the skull pro-
jecting forward from the vertebral column. In humans, by
contrast, the vertebral column joins the skull toward the
center of its base, thereby placing the skull in a balanced
position as required for habitual upright posture. Other
primates, though they frequently cling, sit, or hang with
their bodies upright, are not as fully committed to up-
right posture as humans, and so their foramen magnum
is not as far forward.
In anthropoid primates, the snout or muzzle portion
of the skull reduced as the acuity of the sense of smell de-
clined. The smaller snout offers less interference with ste-
reoscopic vision; it also enables the eyes to take a frontal
position. As a result, primates have flatter faces than some
other mammals.
Below the primate skull and the neck is the clavicle,
or collarbone, a bone found in ancestral mammals though
lost in mammals such as cats. The size of the clavicle is
reduced in quadrupedal primates like monkeys that pos-
sess a narrow sturdy body plan. In the apes, by contrast,
it is broad, orienting the arms at the side rather than at
the front of the body and forming part of the suspensory
hanging apparatus of this group (see Table 3.2). The

clavicle also supports the scapula (shoulder blade) and

allows for the muscle development that is required for

flexible yet powerful arm movement—permitting large-

bodied apes to hang suspended below tree branches and

to brachiate, or swing from tree to tree.

The limbs of the primate skeleton follow the same ba-

sic ancestral plan seen in the earliest vertebrates. Other

animals possess limbs specialized to optimize a particular

behavior, such as speed. In each primate arm or leg, the

upper portion of the limb has a single long bone, the lower

portion two long bones, and then hands or feet with five

radiating digits (phalanges). Their grasping feet and hands

have sensitive pads at the tips of their digits, backed up (ex-

cept in some prosimians) by flattened nails. This unique

foramen magnum A large opening in the skull through

which the spinal cord passes and connects to the brain.

clavicle The collarbone connecting the sternum (breastbone)

with the scapula (shoulder blade).

suspensory hanging apparatus The broad powerful shoul-

der joints and muscles found in all the hominoids, allowing

these large-bodied primates to hang suspended below the tree

branches.

scapula The shoulder blade.

brachiation Using the arms to move from branch to branch,

with the body hanging suspended beneath the arms.