Evolution, 4th Edition

550 CHAPTER 21

efficiency compared with other apes, which sway from side to side when they

occasionally walk. The curvature of our spine, especially in the lumbar region,

also improves stability (FIGURE 21.4). The foramen magnum, the hole at the

base of the skull through which the spinal cord exits from the brain, is shifted

forward so that we more easily face straight ahead when standing. Our feet are

highly modified for running: the big toe is not opposable, but instead is enlarged

and directed forward. Together with the rigid, curved arch of the foot and the

toe joints that flex upward, this helps push us forward and upward at the end of

each stride.

By the time bipedality evolved, the African climate had become drier, and hom-

inins inhabited open woodland instead of wet forests. Natural selection favored

walking rather than climbing in this new environment. An erect posture may also

have aided in picking fruit on low trees and running while hunting prey. When

hominins became runners, sweating was important for evaporative cooling. This

probably selected for reduced body hair in the species that an anthropologist

dubbed the “naked ape” [47].

There are many important differences between the hands of humans and other

apes. We have shorter fingers with straighter phalanges, and longer, more oppos-

able thumbs. Strong muscles provide our hands with both strength and preci-

sion—no other ape has the dexterity to play a guitar. Other distinctive features are

our small teeth (especially the canines) and our flat, nonprojecting face.

Futuyma Kirkpatrick Evolution, 4e

Sinauer Associates

Troutt Visual Services

Evolution4e_21.04.ai Date 01-18-2017

Pelvis

Face

Pelvis Face

Lumbar

region

Cranium

Human

Foramen

magnum

Chimpanzee

Lumbar

region

Cranium

Leg

FIGURE 21.4 Comparison of chimpanzee and human

skeletons. Human bipedality is reflected in many features,

including the very different pelvis, S-shaped lumbar re-

gion of the vertebral column, foot structure, and anterior

position of the foramen magnum of the skull.

21_EVOL4E_CH21.indd 550 3/22/17 1:51 PM