Evidence of Change: The Fossil Record
As the paleontologist in your group, your job is to present the evidence of the fossil record and how
it supports evolution. You have been provided with some readings and diagrams that will help your
case, along with some guiding questions that will help the other scientists understand your field.
Start by looking through the diagrams and articles and in your book (p. 382, 418-420), then answer
the questions that highlight the important points of the fossil record and how it provides evidence
for evolution. Be sure that you are able to explain these main points to your group members.
1. What is the fossil record?
The record of fossils – by comparing fossils from older rock layers with fossils from younger
rock layers, scientists could document the fact that life on Earth has changed over time.
Similar organisms are grouped together and arranged in the order in which they lived, from
oldest to most recent.
2. What is a fossil?
Fossils are traces of once-living organisms.
3. How are fossils formed?
Most fossils form in sedimentary rock. Fossils can also be trapped in resin or frozen in ice.
4. How do we know how old a fossil is? What types of dating techniques are used? Is one
type of technique more powerful than another?
There are two types of dating techniques used to determine how old a fossil is. First, in
relative dating, the age of a fossil is determined by comparing its placement with that of
fossils in other layers of rock. In this, scientists use index fossils to compare the relative ages
of fossils. Relative dating allows paleontologists to estimate a fossil’s age compared with that
of other fossils. However, it provides little to no information about its absolute age.
Second, in radioactive dating, scientists calculate the age of a sample based on the amount of
remaining radioactive isotopes it contains. In this, scientists use radioactive decay to assign
absolute ages to rocks. In other words, radioactive dating is the use of half-lives to
determine the age of a sample. Radioactive dating is more powerful than relative dating
because an actual age can be determined.
5. What is an index fossil?
An index fossil is a distinctive fossil used to compare the relative ages of fossils in relative
dating techniques.
6. Give an example that shows the fossil record being used to show how a species has changed
over time.
The fossil record provides evidence about the history of life on Earth. It also shows how
different groups of organisms, including species, have changed over time. Sometimes the
fossil record includes similar, intermediate forms of a group of organisms that together
suggest gradual modification over time. Cephalapods all have a coiled shell. Fossil evidence
indicates that during a period of their evolution, certain groups of cephalapods evolved a
longer, more coiled shell. You can see this progression by comparing the fossils.
7. What about gaps in the fossil record? Does this weaken the argument for evolution?
There are gaps in the fossil records of many species, although a lot of them shrink each year
as new fossils are discovered. These gaps do not indicate weakness in the theory of
evolution itself. Rather, they point out uncertainties in our understanding of exactly how
some species evolved.