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12.3: History of Earth's Life Forms

Difficulty Level: At Grade Created by: CK-12

Lesson 12.3: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. More than a million species are known to live on Earth today.

_____ 2. Almost all existing species have now been discovered.

_____ 3. All organisms have a unique set of traits.

_____ 4. All gene mutations are harmful.

_____ 5. Adaptations evolve because they help organisms survive or reproduce.

_____ 6. All organisms alive today evolved from earlier life forms.

_____ 7. Ancient horses were much larger than they are today.

_____ 8. Most species that once lived on Earth are now extinct.

_____ 9. Paleozoic life was more diverse on land than in the oceans.

_____ 10. Moving from water to land required many new adaptations.

Lesson 12.3: Critical Reading

Name___________________ Class______________ Date________

Read this passage based on the text and answer the questions that follow.

Variation and Adaptation

There is variation in the traits of a population. Every organism has its own unique traits. For example, human hair color has a lot of variation. There is an endless number of shades of blonde, brown, black, and red hair. Hair color—like many other traits—is controlled by genes, so genes vary in populations too.

How does genetic variation arise? The origin of variation is mutation. A mutation is a random change in a gene. Mutations are naturally occurring. Some are harmful, many are neutral, and a few are beneficial. A beneficial mutation gives an organism a better chance of surviving and having offspring. Genetic traits that help an organism survive or reproduce in a given environment are called adaptations. Through time, they may increase in frequency and become widespread throughout a population. A population that changes in this way has undergone evolution.

The following example illustrates how adaptations evolve. Think about a population of oak trees in North America. Imagine that a fungus has arrived from Asia and infected the North American trees. Most of the infected trees are killed by the fungus. But a few oak trees have a mutation that allows them to survive the fungus and reproduce. These oak trees are better adapted to the new environment than the others. They pass on their favorable mutation to their offspring. Eventually, the population of oak trees evolves. After many generations, most of the trees have the trait to survive the fungus.


  1. How does genetic variation arise?
  2. What are adaptations?
  3. How do adaptations evolve?

Lesson 12.3: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. The Cambrian Period occurred at the beginning of the
    1. Mesozoic Era.
    2. Paleozoic Era.
    3. Phanerozoic Eon.
    4. two of the above
  2. Our own species evolved during the
    1. Cambrian Period.
    2. Cenozoic Era.
    3. Proterozoic Eon.
    4. two of the above
  3. Which answer best completes the following sentence? “Adaptations help an organism survive ...”
    1. without new mutations.
    2. in a specific environment.
    3. during a given geologic eon.
    4. in any ecosystem on Earth.
  4. Cacti have traits that help them survive in a
    1. tropical rainforest.
    2. very cold climate.
    3. desert.
    4. none of the above
  5. Variation in a genetic trait is first introduced by
    1. mutation.
    2. evolution.
    3. adaptation.
    4. reproduction.
  6. How did Earth’s climate change during the Phanerozoic Eon?
    1. The climate became steadily colder.
    2. The climate became steadily warmer.
    3. The climate cycled between warmer and colder.
    4. none of the above
  7. Which statement about the Cambrian Period is false?
    1. It began about 540 million years ago.
    2. It had a warm, humid tropical climate.
    3. It had an explosion of living things.
    4. It ended with the Permian mass extinction.

Lesson 12.3: Matching

Name___________________ Class______________ Date________

Match each definition with the correct term.


_____ 1. geologic era known as the age of reptiles

_____ 2. trait that helps an organism survive or reproduce

_____ 3. random change in a gene

_____ 4. geologic era in which plants and animals first lived on land

_____ 5. genetic changes in a population or species over time

_____ 6. geologic era during which human beings evolved

_____ 7. difference in a genetic trait in a population


a. mutation

b. evolution

c. Paleozoic

d. variation

e. adaptation

f. Mesozoic

g. Cenozoic

Lesson 12.3: Fill in the Blank

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

  1. The __________ Eon began about 540 million years ago.
  2. The huge increase in life diversity at the start of the Paleozoic is known as the __________.
  3. The first insects evolved during the __________ Era.
  4. The earliest amphibians evolved from a group of __________.
  5. The first flowering plants evolved during the __________ Era.
  6. The largest mass extinction is called the __________ mass extinction.
  7. Dinosaurs died out during the __________ mass extinction.

Lesson 12.3: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Describe causes and consequences of mass extinctions. Use specific examples to illustrate your answer.

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