Whale Hunting ■ 201(Figure 11.11). For example, whales are so differ-
ent from humans that it can be difficult to iden-
tify similarities, but we evolved from a common
mammalian ancestor and do share homologous
traits: humans and whales both nurse their
young and have a single lower jawbone because
our common ancestor had those traits.
Vestigial traits are another type of trait that
many organisms have because of a common
ancestor. These features are a piece of the evolu-
tionary past, inherited from a common ances-
tor but no longer used. Vestigial traits may
appear as reduced or degenerated parts whose
function is hard to discern (Figure 11.12). For
example, many modern whales have vestiges of
thighbones, also called femurs, embedded in the
skin next to the pelvis. In land mammals, birds,
and other tetrapod vertebrates, these bones
are critical for walking, running, and jumping.
Aquatic whales have no need of this bone, yet its
traces remain.
Whales also have small muscles devoted to
nonexistent external ears, likely from a time
when they were able to move their ears as land
animals such as dogs do for directional hear-
ing. Vestigial traits are not adaptations. In
fact, they can be detrimental. Most humans no
longer need wisdom teeth to replace lost teeth
during adolescence, yet most people still have
them. They tend to erupt around the twen-
tieth year of life, often causing severe pain
and displacing other teeth, and they usually
require removal.
Clues in the Code
Within the cells of every organism is one of
the strongest pieces of evidence for evolution:
DNA. Living things universally use DNA as
hereditary, or genetic, material (see Chap-
ter 8 for review). The fact that all organisms
on Earth—even those as different as bacte-
ria, redwood trees, and humans—use the
same genetic code is further evidence that the
great diversity of living things evolved from a
common ancestor.
Researchers have analyzed the DNA
sequences of whales and other animals and
shown that, of all animals, whales are most
Figure 11.11
Homologous traits are shared
characteristics inherited from a
common ancestor
The human arm, the whale flipper, and the
bat wing are homologous structures due
to common descent. All three structures
have a matching set of five digits and
a matching set of arm bones that have
been altered by evolution for different
functions.Q1: What is meant by the term “common
ancestor”? Give an example.Q2: Why are homologous structures among
organisms evidence for evolution?Q3: Aside from skeletal structural
similarities, what other commonalities
among organisms might be considered
homologous?Bones of the same origin
are shown in the same color.HumanWhaleBat