Geology Reference
In-Depth Information
The fossils that are preserved represent only a very small portion of all the
creatures that have existed on earth. The history of life in the rocks is biased to-
ward those creatures who were in the right place at the right time and had the
right features.
Add to that fact the many dramatic geologic events that shift, crack, stretch, and uplift
the rocks on earth's surface, exposing existing fossils to be weathered and disappear
forever. You can see why the fossil record illustrates only a small fraction of the ex-
traordinary variety of life that has inhabited the planet in the last 4 billion years. Scient-
ists recognize this challenge and keep it in mind when using the fossil record to answer
questions about the long history of life on earth.
Hypothesizing Relationships: Cladistics
Scientists who study fossils, called
paleontologists,
have developed a method for predict-
ing (and then testing) hypotheses about evolutionary relationships among different or-
ganisms. This method is called
cladistics.
Through cladistics, paleontologists and biolo-
gists are able to classify organisms in the present and the past through an understand-
ing of evolutionary change.
The classification of organisms in cladistics is based on
shared, derived traits:
character-
istics of each animal that are present now but were not present in their distant, common
ancestors. Shared, derived traits are also called
synapomorphies.
When classifying animals through cladistics, scientists make a few assumptions:
Organisms are related by descent to a common ancestor.
Characteristics of a
lineage
(a line of related organisms) change through time.
Each time change occurs, the organisms are split into two groups: one with the old
characteristic, and one with the new characteristic. (The split is called a
bifurcat-
ing pattern
in the lineage.)
It is important to note that while these are the assumptions made by cladistics, many
scientists propose alternate or opposing hypotheses as well. For example, some scient-
ists do not accept that lineages must split in two, displaying a bifurcating pattern. They
suggest that multiple groups with different characteristics may arise at the same time, in
which case cladistics would not be as useful in classifying evolutionary relationships.
