The "present is the key to the past," refers to an important
principle in paleontology. Paleontologists infer that the way evolution
works today is the same as it worked in the past history of the earth.
Evolution, in paleontological terms, is the change in the morphology of
fossils through time. The presence and extinction of small organisms,
especially the invertebrates and protozoa, show many examples of evolution.
The larger the organism, the harder it is to see evolution. For instance,
complete invertebrate skeletons are frequently preserved, but complete
dinosaur skeletons are virtually nonexistent. For this reason (among many
others), human evolution is not as well documented as the many other
organisms that lived on Earth.
One of the best known examples of change through time of a vertebrate is the
evolution of the horse. The oldest known horse is found in both North
American and Europe. According to the fossil record, the evolution to the
modern horse consisted of a variety of changes, including an overall
increase in size, development of a single hoof from several toes,
development of large molar teeth for grazing, and an increase in brain size.
Most of these changes were caused by changes in the environment where the
ancestral horses lived. For example, the molar teeth grew larger in order to
chew grass, which is a very tough plant food. Horse evolution was not a
linear trend, but more like a branching tree; many types of horse evolved
and went extinct before the appearance of modern horses.
- The worksheet shows the skeleton of a Mesohippus, an early form
of horse that lived during Oligocene Epoch (about 38-26 million years
ago). Have the students recreate what they think Mesohippus might
have looked like, using the bones as a guide. Do not tell the students
it is a horse, because they will not think about what the bones are
- Paleontologist have found these fossils. The Hyracotherium is
found below Mesohippus, below Merychippus, and so on.
Point out to students the development of the brain and the hoofs.
Paleontologists can only interpret the record of their fossils, which
leads them to believe that horses have evolved through the last 50
- Explain to the students that horses are just one of many fossil
examples that paleontologists have used to demonstrate a slow, but
constant change of life on the Earth. This change is called evolution.