60 CHAPTER 3 | Living Primates
(humans included), on each side of each jaw, in front, are
two straight-edged, chisel-like broad teeth called inci-
sors (Figure 3.4). Behind each incisor is a canine tooth,
which in many mammals is large, flaring, and fanglike.
The canines are used for defense as well as for tearing and
shredding food.
In humans, canine tooth size is relatively small, al-
though it has an oversized root, suggestive of larger ca-
nines some time back in our ancestry. Behind the canines
are the premolars and molars (the “cheek teeth”) for grind-
ing and chewing food. Molars erupt through the gums
while a young primate is maturing (6-year molars, 12-year
molars, and wisdom teeth in humans). Thus the functions
of grasping, cutting, and grinding were served by different
kinds of teeth. The exact number of premolars and molars
and the shape of individual teeth differ among primate
groups (Table 3.2).name games: They reflect theoretical relationships among
closely related species.
Though the DNA sequences of humans and African
apes are 98 percent identical, the organization of DNA into
chromosomes differs between humans and the other great
apes. Bonobos and chimps, like gorillas and orangutans,
have an extra pair of chromosomes compared to humans, in
which two medium-sized chromosomes have fused together
to form chromosome 2. (Chromosomes are numbered ac-
cording to their size as they are viewed microscopically,
so that chromosome 2 is the second largest of the human
chromosomes. Recall Figure 2.4.) Of the other pairs, eigh-
teen are virtually identical between humans and the African
apes, whereas the remaining ones have been reshuffled.
Overall, the differences between humans and other
African apes are not as great as the differences between gib-
bons (with twenty-two pairs of chromosomes) and siamangs
(twenty-five pairs of chromosomes)—closely related spe-
cies that, in captivity, have produced live hybrid offspring.
Although some studies suggest a closer relationship between
the two species in the genus Pan (chimps and bonobos) and
humans than either has to gorillas, others disagree; the saf-
est course at the moment is to regard all three genera—Pan,
humans, and gorillas—as having an equal degree of rela-
tionship. (Chimps and bonobos are, of course, more closely
related to each other than either is to gorillas or humans.)^2
Primate Characteristics
While the living primates are a varied group of animals,
they do share a number of features. We humans, for exam-
ple, can grasp, throw, and see in three dimensions because
of shared primate characteristics. Compared to other mam-
mals, primates possess a relatively unspecialized anatomy
while their behavioral patterns are diverse and flexible.
Many primate characteristics are useful to arboreal an-
imals, although (as any squirrel knows) they are not essen-
tial to life in the trees. For animals preying upon the many
insects living on the fruit and flowers of trees and shrubs,
however, primate characteristics such as dexterous hands
and keen vision would have been enormously adaptive.
Life in the trees, along with the visual predation of insects,
played a role in the evolution of primate biology.
Primate Teeth
The varied diet available to arboreal primates—shoots,
leaves, insects, and fruits—did not require the specializa-
tion of teeth seen in other mammals. In most primates
(^2) Rogers, J. (1994). Levels of the genealogical hierarchy and the problem of
hominid phylogeny. American Journal of Physical Anthropology 94, 81.
GORILLA
JAW
PROSIMIAN
JAW
2 incisors
6 front teeth
project to form
dental comb
1 canine
3 premolars
2 premolars
3 molars
3 molars
Tongue
12
543
Molar
2 incisors
1 canine
Figure 3.4 Because the exact number and shape of the
teeth differ among primate groups, teeth are frequently used
to identify evolutionary relationships and group membership.
Prosimians (top), with a dental formula of 2-1-3-3, possess
two incisors, one canine, three premolars, and three molars on
each side of their upper and lower jaws. Also, lower canines
and incisors project forward, forming a “dental comb,” which
is used for grooming. A dental formula of 2-1-2-3, typical of
Old World monkeys and apes, can be seen in the gorilla jaw
(bottom). Note the large projecting canines. On one of the
molars, the cusps are numbered to illustrate the Y5 pattern
found in hominoids.