Evolution And History

146 CHAPTER 7 | The First Bipeds

Though genetic evidence established that the human line
diverged from those leading to chimpanzees and gorillas
between 5 and 8 mya, for a long time the fossil evidence of
the early stages of human evolution was both sparse and
tenuous. Today, however, several interesting specimens
from Africa fill in this important period. Inclusion of any
fossil specimen in the human evolutionary line depends
upon evidence for bipedalism (also called bipedality),
the defining characteristic of the human line. The pos-
sible human ancestors from the Miocene recently found
in Chad (Sahelanthropus tchadensis) and Kenya (Orrorin
tugenensis), dated 6 to 7 mya, were described in the last
chapter. In this chapter, we will pick up our story with
a diverse array of fossil bipeds from the Pliocene—the
geological epoch that began around 5 mya beginning with
Ardipithecus from the chapter opener.^1
Before focusing on the fossils, however, let’s look at the
anatomy of bipedalism—the shared derived characteristic
distinguishing humans and their ancestors from the other
African apes.

The Anatomy of Bipedalism

For a hominoid fossil to be definitively classified as part of
the human evolutionary line, certain evidence of bipedal-
ism is required. Bipedalism is associated with anatomical
changes literally from head to toe.
Bipedalism can even be preserved in the skull ( Figure 7.1)
because balancing the head in an upright posture requires a
skull position relatively centered above the spinal column.
The spinal cord leaves the skull at its base through an open-
ing called the foramen magnum (Latin for “big opening”).
In a knuckle-walker like a chimp, the foramen magnum is
placed more toward the back of the skull while in a biped it
is in a more forward position.
Extending down from the skull of a biped, the spi-
nal column makes a series of convex and concave curves
that together maintain the body in an upright posture
by positioning the body’s center of gravity above the legs
rather than forward. The curves correspond to the neck
(cervical), chest (thoracic), lower back (lumbar), and pel-
vic (sacral) regions of the spine, respectively. In a chimp,
the shape of the spine follows a single arching curve

Human

Foramen magnum

Chimpanzee

Figure 7.1 Bipedalism can be inferred from the position of

the foramen magnum, the large opening at the base of the

skull. Note its relatively forward position on the human skull

(left) compared to the chimp skull.

bipedalism The mode of locomotion in which an organism

walks upright on its two hind legs, characteristic of humans and

their ancestors; also called bipedality.

Figure 7.2 Differences between skeletons of chimps and

humans reflect their mode of locomotion.

A

A

B

B

C

C

D

D E

E

F

F

G

G

H

H

Cervical vertebra

Thoracic vertebra

Lumbar vertebra

Sacrum

Ilium

Ischium

Pubis

Femur

Tibia

Pelvis

A B C D E F G H I I

I

(^1) White, T. D., et al. (2009, October). Ardipithecus ramidus and the paleo-
biology of early hominids. Science 326 (5949), 64, 75–86; see also the entire
October 2, 2009, issue of Science. (Figure 7.2). Interestingly, at birth the spines of human
babies have a single arching curve as seen in adult apes.
As humans mature the curves characteristic of bipedalism
appear, the cervical curve at about 3 months on average
and the lumbar curve at around 12 months—a time when
many babies begin to walk.