HUMAN BIOLOGY

prinCiples of evolution 447

hoW does the study of patterns of body

form help our understanding of evolution?

• Comparing the patterns of body form in different groups of
  organisms provides clues about how body parts may have
  been modified in the course of evolution.

taKe-Home message

How did the adults of different vertebrate groups come

to be so different? At least some differences resulted from

mutations that altered the onset, rate, or time of completion

of certain developmental steps. For instance, Figure  23.9

depicts how change in the growth rate at a key point in

development may have produced differences in the pro-

portions of chimpanzee and human skull bones, which

are alike at birth. Later on, they change dramatically for

chimps but only slightly for humans. Chimps and humans

have almost identical genes. Yet on the separate evolution-

ary road leading to humans, some regulatory genes prob-

ably mutated in ways that proved adaptive. From then

on, instead of promoting the rapid growth required for

dramatic changes in skull bones, the mutated genes have

blocked it.

As Figure 23.10 indicates, the bodies of humans, pythons,

and other organisms can have what seem like useless

vestigial structures. For example, consider your own ear-

wiggling muscles—which four-footed mammals (such as

dogs) use to orient their ears. In humans, such body parts

are left over from a time when more functional versions

were important for an ancestor.

Figure 23.10 Animated! Vestigial body parts are “left over”

from ancestral species. A Some python bones correspond to the

pelvic girdle of other vertebrates, including humans. A snake has

small vestigial hind limbs that are remnants from a limbed ancestor.

B The human coccyx is a similar vestige from an ancestral species

that had a bony tail, although muscles still attach to it.

Figure 23.9 Animated! The skulls of a chimpanzee and
a human start out with similar proportions but diverge as
growth continues. A shows a chimp’s skull, B a human skull.
Imagine that the skulls represented here are paintings on a
blue rubber sheet divided into a grid. Stretching the sheet
deforms the grid’s squares. For the adult skulls, differences
in size and shape in corresponding grid sections reflect
differences in growth patterns. (© Cengage Learning)

infant

A Chimpanzee skull

B Human skull

adult

infant adult

backbone

(vertebral

column)

pelvic girdle (hind

legs attach to these)

coccyx (bones

where many

other mammals

have a tail)

thighbone

attached to

pelvic girdle

small bone

attached to

pelvic girdle

A B

Figure 23.8 Comparing embryos also provides evidence

of evolutionary relationships. A reveals that the early

embryos of vertebrates are similar, even though the adults

are very different. This is evidence of change in a common

program of development. B Fishlike structures still form

in the early embryos of reptiles, birds, and mammals. For

example, a two-chambered heart (orange), certain veins

(blue), and portions of arteries called aortic arches (red)

develop in the embryos of sharks and other fishes. Adult

fishes have them, too. These structures also form in an

early human embryo.

fish

A

B

reptile bird

mammal

(human)

adult

shark

human

embryo

(3 milli-

meters

long)

© Cengage Learning

© Cengage Learning

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