Other Evidence for Evolution

Modern-day organisms can provide clues about evolution. By
comparing organisms, scientists can infer how closely related the
organisms are in an evolutionary sense. Scientists compare body
structures, development before birth, and DNA sequences to
determine the evolutionary relationships among organisms.
An organism’s body structure is its basic body plan, such as how its bones
are arranged. Fishes, amphibians, reptiles, birds, and mammals, for example,
all have a similar body structure—an internal skeleton with a backbone. This
is why scientists classify all five groups of animals together as vertebrates.
Presumably, these groups all inherited these similarities in structure from an
early vertebrate ancestor that they shared. Similar structures that related
species have inherited from a common ancestor are called homologous
structures. Sometimes scientists find fossil evidence that supports the
evidence provided by homologous structures. For example, fossils show that
the ancestors of today’s whales had legs and walked on land. This supports
other evidence that whales and humans share a common ancestor.
Scientists can also make inferences about evolutionary relationships
by comparing the early development of different organisms. For
example, an adult turtle, a chicken, and a rat look quite different. But
during early development these three organisms go through similar
stages. These similarities suggest that these three vertebrate species are
related and share a common ancestor.
Scientists infer that species with similar body structures and
development patterns inherited many of the same genes from a common
ancestor. Recall that genes are made of DNA. By comparing the sequence
of nitrogen bases in the DNA of different species, scientists can infer how
closely related the species are. The more similar the sequences, the more
closely related the species are. Recall also that the DNA bases along a gene
specify what type of protein will be produced. Thus, scientists can also
compare the order of amino acids in a protein to see how closely related
two species are. Recently, scientists have developed techniques that allow
them to extract, or remove, DNA from fossils. The DNA from fossils has
provided new evidence about evolution.
Scientists have combined evidence from fossils, body structures, early
development, and DNA and protein sequences to determine the
evolutionary relationships among species. In most cases, DNA and
protein sequences have confirmed conclusions based on earlier evidence.
Scientists use such combined evidence to construct branching trees. A
branching tree is a diagram that shows how scientists think different
groups of organisms are related.