• New World monkeys, such as the marmosets, spider monkeys, and howler monkeys

61
Figure 10.1 Primate family tree.
10
20
30
40
50
60
Lemur
Present
Tarsier
Spider
monkey
Colobus monkey
Baboon
Gibbon
Orangutan Gorilla
Human
Chimpanzee
Hominoids
Time (millions of y
ears ag
o)
O
ld
W
or
ld
m
on
ke
ys
N
ew
W
o
rl
d
m
o
n
ke
ys
P
ro
sim
ia
n
s
New World monkeys
Old World monkeys and apes
Lemurs
Figure 10.2 Geographical
distribution of living primates: Living
primates are principally creatures of the
tropics.

---

62
Part Two: Background to Human Evolution
Figure 10.3 Modes of primate
locomotion: The monkey (top right) walks
quadrupedally, while the gibbon (top left) is
an adept brachiator (it swings from branch
to branch like a pendulum). The orangutan
(mid-left) is also adept in the trees, but as a
four-handed climber. The gorilla (bottom
left), like the chimpanzee, is a knuckle-
walker (it supports its weight through the
forelimbs on the knuckles of the hand
rather than using a flat hand as the monkey
does). The tarsier ( foreground) moves by
vertical clinging and leaping. The hominin
(right) is a fully committed biped. Note the
grasping hands and forward-pointing eyes
characteristic of primates. (Courtesy of John
Gurche/Maitland Edey.)
lost its grasping function in favor of forming a “platform”
adapted to habitual upright walking. In modern primates,
fingers and toes have nails, not claws; and finger and toe pads
are broad and ridged, which aids in preventing slippage on
arboreal supports and in enhancing touch sensitivity. But
some primates have retained claws on certain digits.
Primate locomotion is hindlimb-dominated, whether it
consists of vertical clinging and leaping (various small spe-
cies), quadrupedal walking (monkeys and the African great
apes), brachiation (apes), or bipedalism (humans). In each
case, the center of gravity of the body is located near (or over)
the hindlimbs, which produces the typical diagonal gait
(forefoot preceding hindfoot on each side). (See figure 10.3.)
It also means that the body is frequently held in a relatively
vertical position, making the transition to habitual bipedal-
ism in humans a less dramatic anatomical shift than is often
imagined.
Vision is greatly emphasized in primates, while the olfact-
ory (smell) sense is diminished. In all primates, the two eyes
have come to the front of the head, producing stereoscopic
vision, to a greater extent than in other mammals. Although
some primates (the diurnal species) have color vision, this
character does not discriminate the order from many other
vertebrate groups. The shifting of the eyes from the side of
the head to the front, combined with the diminution of olfac-
tion, produces a shorter snout; this character is accompanied
by a reduction in the number of incisor and premolar teeth
from the ancestral condition of three incisors, one canine,
four premolars, and three molars (denoted 3.1.4.3) to a max-
imum of 2.1.3.3. (Prosimians and New World monkeys
demonstrate this latter pattern, whereas Old World monkeys
and hominoids have one fewer premolar.) (See figure 10.4.)
Hylobates dental
formula
2.1.2.3.
2.1.2.3.
M
3
3
M
2
2
M
1
1
P
3
3
P
4
4
C
1
1
I
2
2
I
1
1