Content Benchmark L.8.D.2
Students know fossils provide evidence of how life and environmental conditions have changed throughout geologic time. E/S
The term fossil originates from the Latin for “dug up.” Fossils are evidence from organisms that once lived on Earth. Examples of fossils include bones, teeth, shells, imprints of leaves, and footprints. Fossils can also be of entirely preserved organisms such as a mosquito preserved in amber, a mammoth frozen for thousands of years in a Siberian field, or a saber-tooth tiger preserved in the LaBrea tar pits. Fossils can form in a variety of ways and provide paleontologists (scientists who study fossils) with a glimpse of the organisms that once lived in a certain area. Paleontologists can use this information to infer how the environmental conditions and life have changed over time.
Figure 1. Examples of Fossils.
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Formation of Fossils
Most fossils are found in sedimentary rock, which are the most common rocks on Earth’s surface. Sedimentary rocks form when particles of existing rock (sediment) settle in layers, typically in the bottom of lakes, rivers, and oceans. The sediment becomes compacted over a long time, as the layers get deposited, the weight of the top layers press on the bottom layers causing sedimentary rockto form. Fossils can form in a variety of ways. Fossil formation usually occurs when organisms experience rapid burial such as an underwater land slide or being covered by volcanic ash. When organisms get trapped in the layers of sediment, they have the possibility of becoming fossilized. The soft structures such as skin, internal organs, and muscles are rarely preserved in fossils due to the fact that soft body structures typically decompose before they can be preserved. However, it is possible for small particles of rock to surround an organism and preserve an imprint of its shape, including its soft tissue. Typically,only the hard structures of plants and animals will fossilize.In some cases, the hard body structures are replaced with mineral compounds and preserve the remains of the organism.
To observe an animation of how fossils form follow the link
Another description of the formation of fossils is available at
Not all organisms die under conditions where their bodies will become fossilized. Since fossilization is a relatively rare event, fossils can be difficult to find and unearth. The fossil record provides information to scientists on the transitional forms of organisms that may have existed in their evolutionary past. In fact, scientists have discovered many transitional fossils which they proposed existed based upon the information in the fossil record. One example is the transitional fossil, Tiktaalik roseae, which fills the evolutionary gap between fish and amphibians. Evolutionary biologists proposed such a species existed prior to the fossil being discovered. As more fossils are added to the fossil record, the gaps in the evolutionary history of many organisms are being filled. However, the fossil record will never be entirely complete.
For more information on Tiktaalik roseae, go to
Fossils Providing Evidence of Past Life
The study of paleontology and geology provide vast amounts of evidence to show how life and environmental conditions on Earth have changed over time. The layers of sedimentary rock (strata), represent layers of the Earth’s history. In an undisturbed portion of rock, the newer strata form above the existing layer of rock. Therefore, if a cross section of the rock layers were viewed, the older layers would be closer to the bottom of the sample and the newer layers would be closer to the top. Scientists use this principle of superposition to relatively date rock strata and the fossils within them. Figure 2 illustrates the principle of superposition as it is used to determine the relative age of fossils.
Figure 2. Diagram showing the principle of superposition.
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The layers within sedimentary rock also represent a timeline. The information contained in the fossil record allows scientists to conclude that environmental conditions on Earth have changed throughout its 4.54 billion year history. Fossil evidence suggests that life originated on Earth 3.9 billion years ago and the organisms that inhibited the Earth have varied greatly over its long history. The fossil record serves as a vast database that documents past life that has existed on Earth. Fossils also provide evidence for major events in Earth’s history such as the extinction of the dinosaurs 65 million years ago and the evolution of land plants420 million years ago. The information from the fossil record can be organized into a timeline showing time periods of great biodiversity. This timeline is known as the geologic time scale, which divides major events of the Earth’s history into eras, which are divided into periods, then into epochs. Figure 3 illustrates a portion of the geologic time scale from 650 million years ago to the present. The fossil record also shows that 99% of organisms that lived on Earth are now extinct, meaning their species has completely died out.
Figure 3. Geologic Time Scale from 650 mya to present.
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Dating Fossils
Scientists can determine the age of fossils by comparing them to the age of other fossils. This technique is known as relative dating, which combines the use of index fossils and the principle of superposition. Index fossils are fossils of organisms that existed in large quantities, for a relatively short period of time in Earth’s history, and were geographically widely distributed. The extinct, trilobite, is a common example of an index fossil and one is illustrated in Figure 4.
a. b.
Figure 4. A trilobite is a common index fossil.
(a.) Illustration of a fossilized trilobite and (b.) a sketch of the profile of a trilobite.
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Visit the “Evolution and the Fossil Record”website for greater detail on relative dating, paleontology, geology and evolution at
Relative dating is useful, but it only gives scientists an approximate age of a specimen compared to the age of another fossil. In order to obtain an estimate of the actual age of a fossil, scientists can use absolute (radioactive) dating. By utilizing this technique, scientists determine the age of the rock in which the fossil is embedded. Radioactive dating techniques use the half-life of a known element found in the sample. A half-life is the amount of time it takes half of the radioactive atoms in a sample to decay. Therefore, after one half-life, one-half of the original amount of the radioactive atoms have decayed. By using radioactive dating, scientists can calculate the age of a sample based upon the amount of radioactive isotopes that remain in the sample.
Figure 5. General graph of the half-life of a radioactive isotope.
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Each radioactive isotope has a unique half-life. Table 1 shows the half-life of a few isotopes that are commonly used in absolute dating and the daughter products into which they decay. For example, Carbon-14, a naturally occurring isotope of Carbon-12, has a half-life of approximately 5,730 years. By comparing the amount of Carbon-12 and Carbon-14 in a specimen, scientists can determine the age of the sample. Due to the relatively short half-life of Carbon-14, it only useful for dating fossils that existed less than 100,000 years ago. In order to date older samples, scientists use elements that have a longer half-life. For example, Potassium-40 has a half-life of 1.28 billion years which makes it useful in dating very old rock samples. Paleontologists often use a combination of relative and absolute dating to determine the age of fossils.
Table 1: Isotopes Commonly used for Radiometric Dating
(years) / Effective Dating Range
(years)
Dating Sample / Key Fission Product
Lutetium-176 / Hafnium-176 / 37.8 billion / early Earth
Uranium-238 / Lead-206 / 4.468 billion / 10 million to origin of Earth
Uranium-235 / Lead-207 / 704 million / 10 million to origin of Earth
Rubidium-87 / Strontium-87 / 48.8 billion / 10 million to origin of Earth
Potassium-40 / Argon-40 / 1.277 billion / 100,000 to origin of Earth
Carbon-14 / Nitrogen-14 / 5730 ± 40 / 0-100,000
Note: the half-life durations listed in the text sections of this tutorial are rounded off for
uranium-238 and potassium-40.
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For more information on absolute dating, an online tutorial is available at Virtual Dating
A summary of the process of dating fossils is explained in “Evolution and the Fossil Record”
By studying the fossil record, scientists obtain clues on how organisms have changed over time and the transitions that organisms have undergone. The fossil record provides a database of evidence for biological and geological evolution.
Content Benchmark L.8.D.2
Students know fossils provide evidence of how life and environmental conditions have changed throughout geologic time. E/S
Common misconceptions associated with this benchmark
The Understanding Evolution for Teachers website presents thorough information explaining the misconceptions that students (and the general public) have regarding evolution. Several of the misconceptions from this webpage are explained in detail below.
The Understanding Evolution website is available at
- Students misuse the term theory and state that evolution is “just a theory.”
The use of the term theory in science differs from its use in the general public. In science, theory is defined as a well-substantiated explanation of some aspect of the natural world that incorporates facts, laws, predictions, and tested hypotheses. The use of the term theory in everyday life refers to an unproved idea or theoretical speculation. It is important for teachers to stress the scientific terms in science class.
A list of evolution misconceptions including the “just a theory” example is found at
- Students incorrectly think that gaps in the fossil record disprove evolution.
Scientists understand that there will be gaps in the fossil record because specific conditions are necessary for an organism to become fossilized. Many transitional fossils have been discovered such as the transitional fossils which show a link between whales and their terrestrial ancestors. Scientists discovered the transitional fossil, Tiktaalik roseae, which fills the evolutionary gap between fish and amphibians.
For more information on Tiktaalik roseae, go to
- Students confuse the topic of evolution with religion and the origin of life.
Students may reject the theory of evolution by stating, “I believe God created life.” Evolution explains how life has changed since its origin, but it does not explain the origin of life. This also places students in a situation where they feel that science contradicts their religious beliefs. In fact, many scientists are devoutly religious and accept the process of evolution.
For an explanation of the misconception that science and religion are incompatible, see
- Students may incorrectly suggest that “both sides” of religion and evolution should be taught in the classroom.
Due to the number of religions and their accounts of creationism, there are more than two sides. Not all the students in the classroom may share the same religious beliefs. Therefore, focusing on one religion’s account of creation is biased. Furthermore, debating scientific and religious ideas has no place in the science classroom and misleadingly suggests that a choice must be made. Topics such as Intelligent Design, which is a type of creationism, also have no place in science curriculum. The Kitzmiller v. Dover trial in Dover, Pennsylvania focused on this misconception.
For more information on the Dovertrial see
For a list of religions and their views on teaching evolution in public schools, see
- Students incorrectly believe that organisms change over time to become a more perfect species.
Students view evolution as a ladder where the organisms are trying to reach the top rung rather than thinking of the tree of life as a branching tree. Species do not evolve to become better or more complex. Some organisms such as jellyfish, crayfish, mosses, and fungi have changed very little in the past million years. Students may also believe that humans are at the top of the ladder and evolution is now complete.
For more information on the ladder versus the branching tree concept of evolution, see
Content Benchmark L.8.D.2
Students know fossils provide evidence of how life and environmental conditions have changed throughout geologic time. E/S
Sample Test Questions
Questions and Answers to follow on a separate document
Content Benchmark L.8.D.2
Students know fossils provide evidence of how life and environmental conditions have changed throughout geologic time. E/S
Answers to Sample Test Questions
Questions and Answers to follow on a separate document
Content Benchmark L.8.D.2
Students know fossils provide evidence of how life and environmental conditions have changed throughout geologic time. E/S
Intervention Strategies and Resources
The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.
- Understanding Geologic Time
On this interactive website, students are guided through Earth’s history by surveying major events in geologic time. Students can use this web-based module to gain a basic understanding of geologic time, Earth’s history, andrelative and absolute dating techniques.
The website is available at
- How Big is a Billion?
The “How Big is a Billion” activity is an extension to another activity that has the students investigate the geologic time scale. This website provides examples that help students visualize and comprehend the value of a billion. Students are asked to calculate values such as if you saved $100/day, how long would it take you to become a billionaire. Also, how many times would you circle the Earth if you took a billion step hike?
To access the activities go to
- Comprehending Geologic Time
This website provides an alternative to the classic analogy of relating geologic time to a meter stick or football field. This website gives the user the opportunity to enter a measurement of length and a timeline of important events in Earth’s history is calculated and scaled according to the measurement that was entered.
This activity is available at
- Becoming Whales Activity
TheBecoming Whales websiteprovides a list of activities that illustrate the evolution of whales. The first activity, Whales in the Making,involves having the students arrange cut-outs of fossils from the oldest land-based whale ancestor to the most recent relative of modern whales. You can also have the students predict the transition fossils between the discovered fossils.
This activity is available at
- The AmericanMuseum of Natural History
This website provides a list of resources and activities related to fossils and evolution.The list is provided for educators who are searching for activities to add to their evolution/fossil unit. Many of the activities are appropriate for students in grades 4-8, however, some are geared for the primary grades. The activities are interactive and encourage students to learn about fossils.
Their website with the links to the list of education activities can be found at
- Getting into the Fossil Record
Getting into the Fossil Recordis an interactive website sponsored by the National Science Foundation. The purpose of Getting into the Fossil Record is for students to gain a basic understanding of what a fossil is and the factors involved in becoming part of the fossil record. Level one is designed for middle school students and level two is for high school.
The website is available at the following link
- Stories from the Fossil Record
This online module is designed to help students understand that fossils can provide a great deal of information about the past and the present. The website introduces students to four different aspects of fossil interpretation. Students explore how fossils provide evidence of past biodiversity, geologic age and geologic events, the abiotic and biotic factors of past ecosystems, and the behavior and interactions of individuals of the past and the evolution of structures and functions.
The link to the website is