Anatomy and Development
Comparisons of the anatomy of different types of organisms often
reveal basic similarities in body structures even though the structure’s
functions may differ between organisms. For example, sometimes
bones are present in an organism but are reduced in size and either
have no use or have a less important function than they do in other,
related organisms. Such structures, which are considered to be
evidence of an organism’s evolutionary past, are called (vehs
TIJ ee uhl). For example, the hind limbs of whales are ves-
tigial structures.
As different groups of vertebrates evolved, their bodies evolved
differently. But similarities in bone structure can still be seen,
suggesting that all vertebrates share a relatively recent common
ancestor. As you can see inFigure 9,the forelimbs of the vertebrates
shown are composed of the same basic groups of bones. Such struc-
tures are referred to as homologous (hoh MAHL uh guhs).
are structures that share a common ances-
try. That is, a similar structure in two organisms can be found in the
common ancestor of the organisms.
The evolutionary history of organisms is also seen in the devel-
opment of embryos. At some time in their development, all
vertebrate embryos have a tail, buds that become limbs, and pha-
ryngeal (fuh RIN jee uhl)pouches. The tail remains in most adult
vertebrates. Only adult fish and immature amphibians retain
pharyngeal pouches (which contain their gills). In humans, the tail
disappears during fetal development, and pharyngeal pouches
develop into structures in the throat.Homologous structuresstructuresvestigial286 CHAPTER 13The Theory of Evolution
The word vestigialcomes
from the Latin word ves-
tigium, meaning “footprint.”
Homologousis from the
Greek word homologos,
meaning “agreeing.”The forelimbs of vertebrates contain the same kinds of bones, which form in the
same way during embryological development.
Figure 9 Homologous structures
HumerusRadiusUlnaCarpalsMetacarpalsPhalangesPenguin Alligator
Bat Human