HUMAN BIOLOGY

prInCIpLes OF eVOLutIOn 451

What trends marked the evolution of

humans?

• Trends associated with the evolution of humans include refined
  hand movements, improved vision, dentition for an omnivorous
  diet, the interconnected development of a more complex brain
  and cultural behavior, and upright walking.

Take-Home message

bipedalism A two-legged

gait.

In time, hands also began to be freed from load-bearing
functions—they were not needed to support the body.
Much later, refinements in hand movements led to the pre-
cision grip and the power grip:

These hand positions would enable early humans to
make and use tools. They helped form the foundation for
the development of early technologies and culture.

Improved daytime vision Early primates had an eye
on each side of the head. Later ones had forward-directed
eyes, which is better for detecting shapes and movements
in three dimensions. Over time, modifications allowed the
eyes to respond to variations in color and light intensity
(dim to bright)—another advantage for life in the trees.

Changes in dentition Changes in dentition—the
teeth and jaws—of early primates accompanied a shift
from eating insects to fruits and
leaves, and on to an omnivorous
(mixed) diet. Rectangular jaws
and long canine teeth came to be
further defining features of mon-
keys and apes. On the road lead-
ing to humans, a bow-shaped jaw
and teeth that were smaller and
all about the same length evolved.

Changes in the brain and behavior Living on tree
branches also favored shifts in reproductive and social
behavior. Imagine the advantages of single births over
litters, for example, or of clinging longer to the mother.
In many primate lineages, parents started to invest more
effort in fewer offspring. They formed strong bonds with
their young, maternal care became more intense, and the
learning period grew longer.
Brain regions, such as the cerebral cortex, that are
involved in information processing began to expand, and
the brain case enlarged. New behavior promoted more
brain development—which in turn stimulated more new
behavior. In other words, brain modifications and behav-
ioral complexity became closely linked. We see the links
clearly in the parallel evolution of the human brain and
culture. Culture is the sum total of behavior patterns of
a social group, passed between generations by learning
and symbolic behavior—especially language. The capac-
ity for language arose among ancestral humans through
changes in the skull bones and expansion of parts of
the brain.

Figure 23.16 Animated! Compare the skeletal organization of two

primates. Pictured are A a gorilla and B a human.

Upright walking Of all primates, only humans can

stride freely on two legs for long periods of time. This two-

legged gait, called bipedalism, emerged as elements of the

ancestral primate skeleton were reorganized. As shown in

Figure 23.16, humans have a shorter,

S-shaped backbone as compared

with apes. In addition, in apes (and

monkeys) the foramen magnum, the

opening at the base of the skull

where the spinal cord can connect with the brain, is at the

back of the skull. In humans, it is close to the center of

the base of the skull. These and other features, such as the

position and shape of the knee and ankle joints and pelvic

girdle, make bipedalism possible. By current thinking, the

evolution of bipedalism was the key change in the origin

of human and humanlike species, both past and present.

© Cengage Learning

Jaw shape and teeth
of an early primate

© Cengage Learning

location of

foramen

magnum

A

foramen

magnum

B A: © Potapov Alexander/Shutterstock.com; B: © Cengage Learning/Gary Head

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