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Content Benchmark L.12.D.4
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Life Science
Structure of Life
Organisms and Their Environment
Diversity of Life
Content Areas
Nature of Science (NOS)
Life Science
Earth Science
Physical Science

Students know the extinction of species can be a natural process. E/S

When teaching all of the L.12.D benchmarks, it is imperative to help students understand the process of science. Most objections and misconceptions about evolution are directly related to the misunderstanding of how science works. When students understand the nature of science, they will understand how scientists have studied the process of evolution. As questions arise about a “supernatural” creation of Earth and the Universe, students who understand the nature of science will understand why supernatural forces cannot be studied as part of scientific processes.

Extinctions occur “often” in evolutionary history. Scientists estimate that more than 90% of the species that have lived on earth are probably extinct. Most extinctions occur due to selective nature of the species. That is, if an organism does not have or does not develop adaptations that allow it to flourish in an environment, that species will eventually die out. These are called background extinctions. The fossil record provides evidence that background extinctions regularly occur throughout time. However, the number of species which have become extinct at any one point in time is relatively low. Scientific studies show another type of extinction in which massive numbers of species become extinct in a short period of time. These are called mass extinctions. There are 5 recognizable periods of mass extinction in the earth’s history. There are several theories as to the cause of the mass extinctions, most of which hypothesize that major climatic and environmental changes occurred globally. These changes were most likely due to impact of extraterrestrial object or extreme geological activity such as many active volcanoes and crustal movements. Many species that were not adapted to the resulting changes, died out. The following sites provide more information about both mass and background extinctions:

One of the largest mass extinctions was the Permo-Triassic (PT) extinction which occurred between the Permian Period and the Triassic Period about 248 million years ago. Based upon fossil records and other evidence, it is estimated, that up to 90 percent of all existing species were lost during the P-T extinction. To learn more about the P-T extinction, see

The most familiar mass extinction is associated with the demise of the dinosaurs. This Cretacious-Tertiary (KT) extinction occurred about 65 million years ago, between the Cretaceous Period of the Mesozoic Era and the Tertiary Period of the Cenozoic Era. To learn more about the KT extinction, see

Most scientists believe that the KT extinction was most likely caused by the impact of a large asteroid. Evidence of this asteroid includes deep sea core samples taken near the suspected impact area. The core samples provide evidence of ash and ejecta material as well as distinctive fossil variants above and below the ash layer.

Figure 1. This is a diagram of a deep sea core that supports the theory of an asteroid impact on earth at the KT time boundary. Diagram is from: .

Other evidence of asteroid impacts near the K-T time boundary includes the presence of an unusually high concentration of Iridium in layers of clay at the K-T boundary. Iridium’s sources are cosmic dust and the earth’s core. The high Iridium concentration therefore indicates that either an asteroid struck the earth or a massive volcanic eruption occurred at that time. The iridium layer was first discovered by Luis and Walter Alvarez in Italy in the 1970’s, but has also been observed in several other sites around the world.

The exact effects of an asteroid impact on life are debated by scientists. To learn more about the uncertainty of the cause of the KT extinction, see

The following graph shows peaks of invertebrate extinctions over the last 600 million years. As the graph shows, there have been 5 major peaks indicating mass extinction episodes.

Figure 2. A graph of invertebrate extinctions over the last 600 million years.
The mass extinctions appear as periodic peaks rising above the background extinction levels. This data is from the work of D. M. Raup and J. J. Sepkoski.
From, (

This figure can be seen at

Background extinctions can occur as a result of many things. The extinction of a species can occur through the process of evolution. Extinction by natural causes may be due either to actual death of a species or due to evolution of the species into one or more different species. Organisms become new species through modification over time. If the ancestral species disappears and is replaced by the new species, extinction has occurred. Evidence from the fossil records substantiates that many species have become extinct and new species have developed over time.

Many scientists believe that Earth may be in a 6th mass extinction phase, as many species are going extinct every day. Humans are not the sole cause of extinctions. However, since our appearance on earth, humans have had an impact on extinction of several species. This has occurred via hunting, habitat destruction, and other environmental impacts.

For more information on the human impact on extinctions, go to

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Performance Benchmark L.12.D.4

Students know the extinction of species can be a natural process. E/S

Common misconceptions associated with this benchmark:

1. Humans cause all extinction misconceptions

Students incorrectly believe that humans have caused all extinctions. Although humans do play a role in modern extinctions due to alteration and destruction of habitat, pollution, overexploitation, and disease there are other causes that contribute to the extinction of species. Some scientists believe that we may be in the midst of a sixth period of mass extinction. The rate of extinction is currently higher than the average background extinction rates, and some of this may be due to human influence. However humans are not the only factor affecting extinctions today. In fact, most extinctions that we know about today, occurred prior to the appearance of humans. As extinctions have occurred naturally throughout the history of life, they continue naturally today.

To learn how several esteemed scientists responded to questions about whether or not we are in the midst of mass extinction and how humans might be involved, see:

2. Dinosaur extinction represents the failure of an entire branch of life

Although extinct now, dinosaurs represent one of the greatest successes of adaptation and survival. They existed on Earth for more than 150 million years, which is longer than any other land animal. Birds evolved from dinosaurs. To learn more about the success of dinosaurs, see

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Content Benchmark L.12.D.4

Students know the extinction of species can be a natural process. E/S

Sample Test Questions

1st Item Specification: Recognize that most species that have lived on Earth are now extinct.

Depth of Knowledge Level 1

  1. Natural selection can best be defined as the
    1. endangered.
    2. extinct.
    3. evolved.
    4. unchanged.
  1. When there are no more living members of a species, that species is said to be
    1. endangered.
    2. extinct.
    3. evolved.
    4. unchanged.

Depth of Knowledge Level 2

  1. Extinction can be beneficial because when
    1. some species become extinct, it allows others to thrive.
    2. some species become extinct, other species will become extinct.
    3. species become extinct, it always leads to evolution.
    4. species become extinct, biodiversity is decreased.

2nd Item Specification: Explain that extinction can be slow or rapid.

Depth of Knowledge Level 1

  1. The Paleozoic Era ended with the disappearance of many land and marine species. This type of event in geological time is known a(n)
    1. endangered species.
    2. boundary transition.
    3. mass extinction.
    4. transitional change.
  1. Which of the following statements about extinction are true?
    1. Extinction can occur rapidly when natural disasters occur or slowly through natural selection and evolution.
    2. Extinction always occurs rapidly and is caused by natural disasters, natural selection, and evolution.
    3. Extinction happened slowly through most of history, but recently sped up due to human activity.
    4. Extinction rates were rapid in the past due to natural selection and evolution caused extinction but today they are slow.

Depth of Knowledge Level 2

  1. The graph below shows extinction rates through the past 600 million years.

The graph demonstrates that extinction rates

  1. have not changed much through time.
  2. steadily increase through time.
  3. steadily decrease through time.
  4. have been both gradual and rapid through time.
  1. The graph below shows the percent of genera that became extinct through the past 550 million years.

What can you infer based on this graph?

  1. In general, less species become extinct today than in the past 600 million years.
  2. In general, more species become extinct today than in the past 600 million years.
  3. The greatest mass extinction occurred between 300-400 million years ago.
  4. The greatest mass extinction occurred between 500-600 million years ag

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Content Benchmark L.12.D.4

Students know the extinction of species can be a natural process. E/S

Answers to Sample Test Questions

  1. B, DOK Level 1
  2. B, DOK Level 1
  3. A, DOK Level 1
  4. C, DOK Level 2
  5. A, DOK Level 2
  6. D, DOK Level 2
  7. A, DOK Level 1

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Content Benchmark L.12.D.4

Students know the extinction of species can be a natural process. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Hypothesis of Dinosaur Extinction
To study several hypotheses about dinosaur extinction and the evidence used to support the hypotheses, see this site:

2. Panel discussion of Scientists Addressing Mass Extinction
Students can listen to several scientists as they address questions about whether or not we are currently in another period of mass extinction.

3. Video clip to stimulate class discussion on human caused extinction.
To stimulate a discussion about human impact on extinction, this short video clip that discusses how an invasive species causes extinction.

4. Ocean Core Evidence of Asteroid Impact
A recently recovered deep sea core contains evidence of an asteroid impact approximately 65 million years ago. Students and teachers can study the evidence in this sample.

5. Humans Role in Extinction
An activity that causes students to think about the role of humans in extinctions can be accessed at

6. AAAS Recommended Evolution Education
For additional information on what is recommended in evolution education, see: Evolution on the Front Line: An Abbreviated Guide for Teaching Evolution from Project 2061 at AAAS.

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