Evidence from Living Organisms
By examining fossils and by determining their relative and absolute ages, scientists have
collected evidence that supports the theory that species changed over time. Further evidence
is derived from living organisms. In order to determine if species change scientists compare
common ancestry, structure, biochemistry, and development of organisms alive today.
As you read this section, study this evidence and critically evaluate whether it indicates that
species may have arisen by descent and modification from ancestral species.
Evidence of Common Ancestry
If species change over time, then scientists should be able to cite examples showing that
a group of living species may have come from a common ancestor. Let us examine one of many
cases for which this seems to be true. Gracing the islands of Hawaii is a family of birds
commonly called the Hawaiian honeycreepers. All Hawaiian honeycreepers have similarities in
skeletal and muscle structure that indicate they are closely related. However, each of the
Hawaiian honeycreeper species has a bill specialized for eating certain foods. Scientists
suggest that all 23 honeycreeper species apparently arose from a single species that migrated
If a bat, a human, an alligator, and a penguin all evolved from a common ancestor,
then they should share common anatomical traits. In fact, they do. Compare the forelimbs
of the human, the bat, the penguin, and the alligator. Find the humerus, radius, ulna, and
carpals in each forelimb. Though the limbs look strikingly different on the outside and
though they vary in function, they are very similar in skeletal structure. More
significantly, they are derived from the same structures in the embryo. Structures that are
embryologically similar, but have different functions, are called homologous structures.
Though these animals look different, a comparison of homologous structures indicates that they
are quite similar. This suggests that these animals evolved from a common ancestor.
Some organisms have structures or organs that seem to serve no useful function. For example,
humans have a tailbone at the end of the spine that is of no apparent use. Some snakes have
tiny pelvic bones and limb bones, and some cave-dwelling salamanders have eyes even though
members of the species are completely blind. Such seemingly functionless parts are called
vestigial organs or structures. Vestigial organs are often homologous to organs that are
useful in other species. The vestigial tailbone in humans is homologous to the functional
tail of other primates. Thus vestigial structures can be viewed as evidence for evolution:
organisms having vestigial structures probably share a common ancestry with organisms in with
organisms in which the homologous structure is functional.
Biochemistry also reveals similarities between organisms of different species. For
example, the metabolism of vastly different organisms is based on the same complex
biochemical compounds. The protein cytochrome c, essential for aerobic respiration,
is one such universal compound. The universality of cytochrome c is evidence that all
aerobic organisms probably descended from a common ancestor that used this compound for
respiration. Certain blood proteins found in almost all organisms give additional evidence
that these organisms descended form a common ancestor. Such biochemical compounds, including
cytochrome c and blood proteins, are so complex it is unlikely that almost identical compounds would have evolved independently in widely different organisms.
Further studies of cytochrome c in different species reveal variations in the amino
acid sequence of this molecule. For example, the cytochrome c of monkeys and cows is more
similar than the cytochrome c of monkeys and fish. Such similarities and differences suggest
that monkeys and cows ate more closely related than are monkeys and fish. Scientists have
similarly compared the biochemistry of universal blood proteins. Their studies reveal
evidence of degrees of relatedness between different species. This evidence implies
that some species share a more recent common ancestor than other species do. From such
evidence scientists have inferred the evolutionary relationships between different
species of organisms.
The similarities described above are not the only ones scientists have noticed
among organisms of different species. The image to the left shows that embryos of certain
species develop almost identically, especially in the early stages. Such physical
similarities indicate that there are genetic similarities between the organisms. These
similarities can be considered evidence that the organisms shown probably descended from
a common ancestor.
The similarities between living species-- in ancestry, in homologous and vestigial structures,
in embryological development, and in biochemical compounds-- all could be explained as
extremely remarkable coincidences. However, a far more probable explanation of these
similarities is that species have arisen by descent and modification from more ancient forms.
Additionally, the fossil record contributes compelling evidence that species have changed
over time. The fossil evidence and evidence from living organisms strongly suggest that