Evolution And History

The Rise of the Primates 133

Primates

Anthropoid Prosimian

Relatively

short snout

Relatively

long snout

Orbit completely

enclosed in bone

Postorbital bar

No bony

plate behind

eye orbit

Figure 6.6 Ancestral features seen in Eocene and Oligocene

primates are still seen in prosimians today. Like modern

lemurs, these fossil prosimians have a postorbital bar, a bony

ring around the eye socket that is open in the back. Anthropoid

primates have orbits completely enclosed in bone. Note also

the difference in the relative size of the snout in these two

groups. Paleoanthropologists make these kinds of comparisons

as they reconstruct our evolutionary history.

wet conditions of the Eocene sustained extensive rainfor-

ests. Relative to ancestral primatelike mammals, these early

primate families had enlarged braincases, slightly reduced

snouts, and a somewhat forward position of the eye orbits,

which, though not completely walled in, are surrounded by

a complete bony ring called a postorbital bar (Figure 6.6).

During the Eocene, the first signs of anthropoid

primates also begin to appear in the fossil record. Until

falls that injured or killed the individuals poorly adapted to
arboreal life, may have been a part of initial forays into the
trees. Natural selection would favor those that judged depth
correctly and gripped the branches strongly. Early primates
that took to the trees were probably in some measure pre-
adapted by virtue of behavioral flexibility, better vision, and
more dexterous fingers than their contemporaries.
Primatologist Matt Cartmill further suggests that primate
visual and grasping abilities were also promoted through the
activity of hunting for insects by sight. His visual predation
hypothesis accounts for the observation that other tree-dwell-
ing species and hunting species do not necessarily possess the
same combination of visual and manual abilities possessed
by the primates. The relatively small size of the early primates
allowed them to make use of the smaller branches of trees;
larger, heavier competitors, and most predators, could not
follow. The move to the smaller branches also gave them ac-
cess to an abundant food supply; the primates were able to
gather insects, leaves, flowers, and fruits directly rather than
waiting for them to fall to the ground.
The strong selection in a new environment led to an
acceleration in the rate of change of primate characteris-
tics. Paradoxically, these changes eventually made possible
a return to the ground by some primates, including the
ancestors of the genus Homo.

True Primates

The first well-preserved “true” primates appeared by
about 55 mya at the start of the Eocene epoch. During this
time period, an abrupt warming trend began on earth,
causing many older forms of mammals to become ex-
tinct, to be replaced by recognizable forerunners of some
of today’s forms. Among the latter was an adaptive radia-
tion of pro simian primates, of which over fifty fossil genera
are known. Fossils of these creatures have been found in
Africa, North America, Europe, and Asia, where the warm,

visual predation hypothesis A theory for primate evolution

that proposes that hunting behavior in tree-dwelling primates was

responsible for their enhanced visual acuity and manual dexterity.

Millions of years ago

Epochs

Eras

70 60 50 40 30 20 10 0

PALEOCENE EOCENE OLIGOCENE MIOCENE PLIOCENE

MESOZOIC CENOZOIC

Mass extinction

of dinosaurs

Adaptive radiation

of mammals begins

Prosimian fossil

primates common

in Laurasia

Anthropoid fossil

primates become

common in the New

and Old World

Old World monkeys

and apes appear as

distinctive groups

Evolutionary lines

to humans, chimps,

and gorillas split

Figure 6.5 This timeline depicts some of the major events of primate evolution.