Lines of evidence: The science of evolution
At the heart of evolutionary theory is the basic idea that life has existed for
billions of years and has changed over time.
Overwhelming evidence supports this fact. Scientists continue to argue
about details of evolution, but the question of whether life has a long history
or not was answered in the affirmative at least two centuries ago.
The history of living things is documented through multiple lines of evidence
that converge to tell the story of life through time. In this section, we will
explore the lines of evidence that are used to reconstruct this story.
These lines of evidence include:
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Fossil evidence
Homologies
Distribution in time and space
Evidence by example
Fossil evidence
The fossil record provides snapshots of
the past that, when assembled, illustrate
a panorama of evolutionary change over
the past four billion years. The picture
may be smudged in places and may have
bits missing, but fossil evidence clearly
shows that life is old and has changed
over time.
Early fossil discoveries
In the 17th century, Nicholas Steno
shook the world of science, noting the
similarity between shark teeth and the
rocks commonly known as "tongue
stones." This was our first understanding
that fossils were a record of past life.
Nicholas Steno's anatomical
drawing of an extant shark (left)
and a fossil shark tooth (right).
Steno made the leap and
declared that the fossil teeth
indeed came from the mouths of
once-living sharks.
Two centuries later, Mary Ann Mantell
picked up a tooth, which her husband
Gideon thought to be of a large iguana, but it turned out to be the tooth of a
dinosaur, Iguanodon. This discovery sent the powerful message that many
fossils represented forms of life that are no longer with us today.
Additional clues from fossils
Today we may take fossils for granted, but we continue to learn from them.
Each new fossil contains additional clues that increase our understanding of
life's history and help us to answer questions about their evolutionary story.
Examples include:
Indication of
Clues at the
interactions
cellular level
This ammonite
Fossils can tell us
fossil (see right)
about growth
shows punctures
patterns in
that some
ancient animals.
scientists have
The picture at
interpreted as the
right is a crossbite mark of a
section through a
mosasaur, a type of predatory
sub-adult thigh bone of the duckbill
marine reptile that lived at the
dinosaur Maiasaura. The white
same time as the ammonite.
spaces show that there were lots of
Damage to the ammonite has been
blood vessels running through the
correlated to the shapes and
bone, which indicates that it was a
capabilities of mosasaur teeth and
fast-growing bone. The black wavy
jaws. Others have argued that the
horizontal line in mid-picture is a
holes were created by limpets that
growth line, reflecting a seasonal
attached to the ammonite.
pause in the animal's growth.
Researchers examine ammonite
fossils, as well as mosasaur fossils
and the behaviors of limpets, in
order to explore these hypotheses.
Transitional forms
Fossils or organisms that show the intermediate states between an ancestral
form and that of its descendants are referred to as transitional forms. There
are numerous examples of transitional forms in the fossil record, providing
an abundance of evidence for change over time.
Pakicetus (below left), is described as an early ancestor to modern whales.
Although pakicetids were land mammals, it is clear that they are related to
whales and dolphins based on a number of specializations of the ear, relating
to hearing. The skull shown here displays nostrils at the front of the skull.
A skull of the gray whale that roams the seas today (below right) has its
nostrils placed at the top of its skull. It would appear from these two
specimens that the position of the nostril has changed over time and thus
we would expect to see intermediate forms.
Note that the nostril placement in Aetiocetus is intermediate between the
ancestral form Pakicetus and the modern gray whale — an excellent example
of a transitional form in the fossil record!
Our understanding of the evolution of horse feet,
so often depicted in textbooks, is derived from a
scattered sampling of horse fossils within the
multi-branched horse evolutionary tree. These
fossil organisms represent branches on the tree
and not a direct line of descent leading to modern
horses.
But, the standard diagram does clearly show
transitional stages whereby the four-toed foot of
Hyracotherium, otherwise known as Eohippus,
became the single-toed foot of Equus. Fossils
show that the transitional forms predicted by
evolution did indeed exist.
As you can see to the left, each branch tip on the
tree of horse evolution indicates a different
genus, though the feet of only a few genera are
illustrated to show the reduction of toes through
time.
Homologies
Evolutionary theory predicts that related organisms will share similarities
that are derived from common ancestors. Similar characteristics due to
relatedness are known as homologies. Homologies can be revealed by
comparing the anatomies of different living things, looking at cellular
similarities and differences, studying embryological development, and
studying vestigial structures within individual organisms.
In the following photos of plants, the leaves are quite different from the
"normal" leaves we envision.
Each leaf has a very different shape and function, yet all are homologous
structures, derived from a common ancestral form. The pitcher plant and
Venus' flytrap use leaves to trap and digest insects. The bright red leaves of
the poinsettia look like flower petals. The cactus leaves are modified into
small spines which reduce water loss and can protect the cactus from
herbivory.
Another example of homology is the forelimb of tetrapods (vertebrates with
legs).
Frogs, birds, rabbits and lizards all have different forelimbs, reflecting their
different lifestyles. But those different forelimbs all share the same set of
bones - the humerus, the radius, and the ulna. These are the same bones
seen in fossils of the extinct transitional animal, Eusthenopteron, which
demonstrates their common ancestry.
Homologies: anatomy
Individual organisms contain, within their bodies, abundant
evidence of their histories. The existence of these features is
best explained by evolution.
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Several animals, including pigs, cattle, deer, and dogs
have reduced, nonfunctional digits, referred to as
dewclaws. The foot of the pig has lost digit 1
completely, digits 2 and 5 have been greatly reduced,
and only digits 3 and 4 support the body. Evolution
best explains such vestigial features. They are the
remnants of ancestors with a larger number of
functional digits.
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People (and apes) have chests that are broader than they are deep,
with the shoulder blades flat in back. This is because we, like apes, are
descended from an ancestor who was able to suspend itself using the
upper limbs. On the other hand, monkeys and other quadrupeds have
a different form of locomotion. Quadrupeds have narrow, deep chests
with shoulder blades on the sides.
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Hoatzin chicks have claws on their wings, as do some chickens and
ostriches. This reflects the fact that bird ancestors had clawed hands.
Homologies: comparative anatomy
Organisms that are closely related to one another share many anatomical
similarities. Sometimes the similarities are conspicuous, as between
crocodiles and alligators, but in other cases considerable study is needed for
a full appreciation of relationships.
Modification of the tetrapod skeleton
Whales and hummingbirds have tetrapod skeletons inherited from a common
ancestor. Their bodies have been modified and parts have been lost through
natural selection, resulting in adaptation to their respective lifestyles over
millions of years. On the surface, these animals look very different, but the
relationship between them is easy to demonstrate. Except for those bones
that have been lost over time, nearly every bone in each corresponds to an
equivalent bone in the other.
Homologies: developmental biology
Studying the embryological development of living things provides clues to
the evolution of present-day organisms. During some stages of
development, organisms exhibit ancestral features in whole or incomplete
form.
Snakes have legged ancestors.
Some species of living snakes have hind limb-buds as early embryos but
rapidly lose the buds and develop into legless adults. The study of
developmental stages of snakes, combined with fossil evidence of snakes
with hind limbs, supports the hypothesis that snakes evolved from a limbed
ancestor.
Above left, the Cretaceous snake Pachyrhachis
problematicus clearly had small hindlimbs. The
drawing at right shows a reconstruction of the pelvis
and hindlimb of Pachyrhachis.
Baleen whales have toothed ancestors.
Toothed whales have full sets of teeth throughout their lives. Baleen whales,
however, only possess teeth in the early fetal stage and lose them before
birth. The possession of teeth in fetal baleen whales provides evidence of
common ancestry with toothed whales and other mammals. In addition,
fossil evidence indicates that the late Oligocene whale Aetiocetus (below),
from Oregon, which is considered to be the earliest example of baleen
whales, also bore a full set of teeth.
Again, these observations make most sense in an evolutionary framework
where snakes have legged ancestors and whales have toothed ancestors.
Homologies: cellular/molecular evidence
All living things are fundamentally alike. At the cellular and molecular level
living things are remarkably similar to each other. These fundamental
similarities are most easily explained by evolutionary theory: life shares a
common ancestor.
The cellular level
All organisms are made of cells, which consist of membranes filled with
water containing genetic material, proteins, lipids, carbohydrates, salts and
other substances. The cells of most living things use sugar for fuel while
producing proteins as building blocks and messengers. Notice the similarity
between the typical animal and plant cells pictured below — only three
structures are unique to one or the other.
The molecular level
Different species share genetic homologies as well as anatomical ones.
Roundworms, for example, share 25% of their genes with humans. These
genes are slightly different in each species, but their striking similarites
nevertheless reveal their common ancestry. In fact, the DNA code itself is a
homology that links all life on Earth to a common ancestor. DNA and RNA
possess a simple four-base code that provides the recipe for all living things.
In some cases, if we were to transfer genetic material from the cell of one
living thing to the cell of another, the recipient would follow the new
instructions as if they were its own.
These characteristics of life demonstrate the fundamental sameness of all
living things on Earth and serve as the basis of today's efforts at genetic
engineering.