114 Evolution and the Fossil Record
same basic building blocks in their anatomy but had modified those parts in remarkably dif-
ferent ways. For example, the basic vertebrate forelimb (fig. 4.8) has the same basic elements:
a single large bone in the upper arm (the humerus), a pair of two long bones in the forearm
(the radius and ulna), a number of wristbones (carpals and metacarpals), and multiple bones
(phalanges) supporting five digits (fingers). But look at the wide array of ways that some
animals use this basic body plan! Whales have modified them into a flipper, while bats have
extended the fingers out to support a wing membrane. Birds also developed a wing, but in
an entirely different way, with most of the hand and wrist bones reduced or fused together,
and feather shafts providing the wing support instead of finger bones. Horses have lost
their side toes and walk on one large finger, the middle finger. None of this makes any sense
unless these animals inherited a standard body plan in place from their distant ancestors
and had to modify it to suit their present-day function and ecology. These common elements
(bones, muscles, nerves) that serve different functions despite being built from the same
basic parts are known as homologous structures. For example, the finger bones of a bat wing
are homologous with our finger bones, and so on.
If the system had been divinely created by an “intelligent designer,” why would there
be this underlying similarity? A good engineer would create all wings the same best possible
way from scratch, rather than jury-rig the structure using bones that the animal inherited
from its ancestors. In fact, nature uses a variety of nonhomologous ways to build a wing.
We have already seen how vertebrates build wings in two completely different ways, even
though bats and birds started out with the same bones from a common ancestor—and neither
FIGURE 4.7. Branching diagram of the similarities in cytochrome c among various organisms. Nearly every bio-
chemical system shows a similar branching pattern, which is identical to the branching pattern of life during
its evolution. (From Fitch and Margoliash 1967. Copyright © 1967 American Association for the Advancement
of Science. Reprinted with permission.)