Evolution And History

Primate Characteristics 61

teeth (a 2-1-2-3 dental formula shared with the Old World

monkeys and apes) are fewer than the teeth of some pri-

mates and more generalized than those of most primates.

However, this trend does not indicate that species with

more teeth are less evolved; it only shows that their evolu-

tionary history followed different trends.

The canines of most primates develop into long, dag-

gerlike teeth that enable them to rip open tough husks

of fruit and other foods. In many species, males possess

larger canine teeth compared to females. This sex differ-

ence is an example of sexual dimorphism—differences

between the sexes in the shape or size of a feature. These

large canines are used frequently for social communica-

tion. All an adult male gorilla, baboon, or mandrill needs

to do to get a youngster to be submissive is to raise his up-

per lip to display his large, sharp canines.

Primate Sensory Organs

The primates’ adaptation to arboreal life involved changes in

the form and function of their sensory organs. The sense of

smell was vital for the earliest ground-dwelling, night-active

The evolutionary trend for primate dentition has been
toward a reduction in the number and size of the teeth. The
ancestral dental formula, or pattern of tooth type and num-
ber in mammals, consisted of three incisors, one canine,
five premolars, and three molars (expressed as 3-1-5-3) on
each side of the jaw, top and bottom, for a total of forty-eight
teeth. In the early stages of primate evolution, one incisor
and one premolar were lost on each side of each jaw, result-
ing in a dental pattern of 2-1-4-3 in the early fossil primates.
This change differentiated primates from other mammals.
Over the millennia, as the first and second premolars
became smaller and eventually disappeared altogether, the
third and fourth premolars grew larger and added a sec-
ond pointed projection, or cusp, thus becoming “bicuspid.”
In humans, all eight premolars are bicuspid, but in other
Old World anthropoids, the lower first premolar is not
bicuspid. Instead, it is a specialized, single-cusped tooth
with a sharp edge to act with the upper canine as a shear-
ing mechanism. The molars, meanwhile, evolved from a
three-cusp pattern to one with four and even five cusps.
The five-cusp pattern is characteristic of the lower molars
of living and extinct hominoids (see Figure 3.4). Because
the grooves separating the five cusps of a hominoid lower
molar looks like the letter Y, hominoid lower molars are
said to have a Y5 pattern. In humans there has been some
departure from the Y5 pattern associated with the reduc-
tion in tooth and jaw size such that the second and third
molars generally have only four cusps. Four- and five-cusp
molars economically combined the functions of grasping,
cutting, and grinding in one tooth.
The evolutionary trend for human dentition has gen-
erally been toward economy, with fewer, smaller, more ef-
ficient teeth doing more work. Thus our own thirty-two

Primate Group Skull and Face

Dental Formula and

Specializations

Locomotor Pattern and

Morphology

Tail and Other Skeletal

Specializations

Earliest fossil

primates

Eye not fully surrounded by bone 2-1-4-3

Prosimians Complete ring of bone surrounding eye

Upper lip bound down to the gum

Long snout

2-1-3-3

Dental comb for

grooming

Hind leg dominance

for vertical clinging

and leaping

Tail present

Anthropoids Forward-facing eyes fully enclosed in bone

Free upper lip

Shorter snout

New World

monkeys

2-1-3-3 Quadrupedal Prehensile

(grasping) tail

in some

Old World

monkeys

2-1-2-3

Four-cusped molars

Quadrupedal Tail present

Apes 2-1-2-3

Y5 molars on lower jaw

Suspensory hanging

apparatus

No tail

dental formula The number of each tooth type (incisors,

canines, premolars, and molars) on one half of each jaw. Unlike

other mammals, primates possess equal numbers on their upper

and lower jaws so the dental formula for the species is a single

series of numbers.

sexual dimorphism Within a single species, differences

between males and females in the shape or size of a feature not

directly related to reproduction, such as body size or canine

tooth shape and size.

Table 3.2 Primate Anatomical Variation and Specialization