Biology Now, 2e

Neanderthal Sex ■ 311

two legs (Figure 17.8). Many skeletal changes

accompanied the switch to walking upright,

including the loss of opposable toes, as you will

notice if you try touching your little toe with the

big toe on the same foot.

The loss of opposable toes that accompanied

walking upright would have been a handicap in

trees, since opposable toes help grasp branches

during climbing. It is therefore likely that biped-

alism was an adaptation for living on the ground.

Walking on two feet freed the hands to carry

food, tools, and weapons, and it also elevated the

head, enabling the walker to see farther and over

more things.

The shift to life on the ground was probably

not sudden or complete. The skeletal structure of

some of the oldest fossil hominins (3–3.5 million

years old) indicates that they walked upright.

However, foot bones and fossilized footprints

show that the hominins living at that time still

had partially opposable big toes (Figure 17.9).

Perhaps they still occasionally climbed trees.

The earliest known hominin is Sahelanthro-

pus tchadensis, identified from a 6- to 7-million-

year-old skull discovered in 2002. Other early

hominins include Ardipithecus ramidus, who

lived 4.4 million years ago, and several Australo-

pithecus species that are 3–4.2 million years old,

including the first full-time walker with the first

modern foot: Australopithecus afarensis. All

of these hominins are thought to have walked

upright. Their brains were still relatively small

(less than 400 cubic centimeters in volume),

and their skulls and teeth were more similar

to those of other apes than to those of humans

(Figure 17.10). A typical modern human has a

brain volume of about 1,400 cubic centimeters,

about the same volume as a 1.5-liter soda bottle.

Within the hominin branch is the Homo genus.

The fossils identified at Riparo Mezzena were

believed to be Homo neanderthalensis bones, but

they had never been closely studied. So, Longo,

the curator from Verona, assembled a team of

Partially opposable

big toe

These bones

(shown in tan)

are based on

fossils of a

similar age.

These hominin

foot bones,

3–3.5 million

years old, were

discovered

in 1995.

These footprints of two early

hominins walking upright, side

by side, were found in Africa.

Figure 17.9

Early hominins had an upright stance and partially opposable

big toes

Fossilized foot bones show that some hominins living between 3.5 and

3 million years ago walked upright but had partially opposable big toes.

Q1: What other reason besides continuing to use trees might explain

why early hominins had partially opposable big toes?

Q2: In what way does the pattern of footprints in this figure suggest

that the print makers were walking upright?

Q3: Why do you think we no longer have partially opposable big toes?

Q1: Through natural selection, deleterious traits will tend to disappear from a population over time. Which traits might

have been deleterious for ground-dwelling early hominins?

Q2: Through natural selection, advantageous traits will tend to persist in a population over time. Which traits might have

been advantageous for ground-dwelling early hominins?

Q3: Adaptations to upright walking also mean that human females have more difficulty giving birth than do females of

other species. What adaptation would you predict has had the greatest impact on this result?

researchers to analyze them, including Condemi,

the anthropologist. Condemi had long been inter-

ested in the movement of Neanderthals across

Europe and how their populations overlapped

with modern Homo sapiens populations, and she

wanted to compare the Riparo Mezzena fossils to

fossils from Neanderthal groups that had been

dug up elsewhere around Europe, hoping to see

what the fossils might say about how Neander-

thals and modern humans interacted.