Evolution And History

Primate Taxonomy 57

The ancestral primates possessed biological character-

istics that allowed them to adapt to life in the forests. Their

relatively small size enabled them to use tree branches not

accessible to larger competitors and predators. Arboreal

life opened up an abundant new food supply. The primates

were able to gather leaves, flowers, fruits, insects, bird eggs,

and even nesting birds, rather than having to wait for them

to fall to the ground. Natural selection favored those who

judged depth correctly and gripped the branches tightly.

Those individuals who survived life in the trees passed on

their genes to the succeeding generations.

Although the earliest primates were nocturnal, today

most primate species are diurnal (active in the day). The

transition to diurnal life in the trees involved important

biological adjustments that helped shape the biology and

behavior of humans today.

Primate Taxonomy

Taxonomies are ways of organizing the natural world.

Because taxonomies reflect scientists’ understanding of

the evolutionary relationships among living things, these

classificatory systems are continually under construction.

With new scientific discoveries, taxonomic categories have

replaced by the permanent or adult teeth. The specializa-

tions of mammalian teeth allow species and evolutionary

relationships to be identified through dental comparisons.

Evidence from ancient skeletons indicates the first

mammals appeared over 200 million years ago as small

nocturnal (active at night) creatures. The earliest primate-

like creatures came into being about 65 million years ago

when a new mild climate favored the spread of dense trop-

ical and subtropical forests over much of the earth. The

change in climate and habitat, combined with the sudden

extinction of dinosaurs, favored mammal diversification,

including the evolutionary development of arboreal (tree-

living) mammals from which primates evolved.

CROCODILE

JAW

CHIMPANZEE

JAW

3 molars 2 premolars

2 incisors

1 canine

Identical teeth

Nursing their young is an important part of the general mammalian

tendency to invest high amounts of energy into rearing relatively few

young at a time. The reptile pattern is to lay many eggs, with the

young fending for themselves. Interestingly, ape mothers, such as

this one, tend to nurse their young for four or five years. The practice

of bottle-feeding infants in the United States and Europe is a mas-

sive departure from the ape pattern. Although the health benefits for

mothers (such as lowered breast cancer rates) and children (strength-

ened immune systems) are clearly documented, cultural norms have

presented obstacles to breastfeeding. Across the globe, however,

women nurse their children on average for about three years.

© Peter Arnold, Inc.

Figure 3.2 The crocodile jaw, like the jaw of all reptiles,

contains a series of identical teeth. If a tooth breaks or falls

out, a new tooth will emerge in its place. Mammals, by

contrast, possess precise numbers of specialized teeth, each

with a particular shape characteristic of the group, as indicated

on the chimpanzee jaw: Incisors in front are shown in blue,

canines behind in red, followed by two premolars and three

molars in yellow (the last being the wisdom teeth in humans).

nocturnal Active at night and at rest during the day.

arboreal Living in the trees.

diurnal Active during the day and at rest at night.