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12.4: Lesson 12.3: History of Earth’s Life Forms

Difficulty Level: At Grade Created by: CK-12
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Key Concepts

  • Biological diversity
  • Variation and adaptation in living things
  • Biological evolution
  • Studying the fossil record
  • Eras of the Phanerozoic Eon
  • Mass extinctions

Lesson Objectives

  • Describe how adaptations develop.
  • Explain how the fossil record shows us that species evolve over time.
  • Describe the general development of Earth’s life forms over the last 540 million years.

Lesson Vocabulary

  • adaptation: inherited trait that helps an organism survive or reproduce in a given environment
  • evolution: change in the genetic makeup of a population or species over time
  • paleontologist: scientist who finds and studies fossils to learn about the history of life
  • tropical: type of climate that is warm year-round and may be wet
  • variation: difference in an inherited trait in a population

Teaching Strategies

Introducing the Lesson

Introduce evolution by giving students examples of common uses of the term evolve. For example, say: “My ideas on this subject have evolved.” “Our plan of action is still evolving.”

Question: Based on these examples, how would you define evolution?
Answer: Evolution means “change.”
Question: What does evolution mean in biology?
Answer: Evolution means a change in traits (or genes) over time.

Tell students they will learn in this lesson how life on Earth evolved since it first began.


Demonstrate how continents moved throughout the Phanerozoic with the interactive timeline below. You can relate the positions of the continents to changing climates during the eon.



Point out that the Mesozoic was the age of reptiles. Giant marine reptiles were the top predators in Mesozoic marine food chains. They ruled the sea just as dinosaurs ruled the land. Students are likely to be familiar with dinosaurs already. They can explore some of the marine reptiles from the Mesozoic with the interactive activity “Marine Reptiles” at this URL: http://nature.ca/discover/exf/mrnrptls/index_e.cfm.


Students are sometimes confused about how mutations, which occur randomly, can lead to the evolution of adaptive traits. Explain that the appearance of new genes through mutation is random but that the increase (or decrease) in the frequency of a mutation is not. A mutation will increase in frequency when it makes individuals who have the mutation more likely to survive and reproduce. This is natural selection, and what nature selects depends on the environment.

Differentiated Instruction

Kinesthetic learners and students with less proficient language or reading skills may have a better understanding of natural selection if they do a hands-on simulation of natural selection, such as the one at this URL: http://www.indiana.edu/~ensiweb/lessons/ns.chips.html.


Refer interested students to the amazing evolution animation at the following URL. It shows how human hands, bat wings and whale flippers all evolved from the generalized forelimb of an early mammalian common ancestor. The demonstration may give the students a more sophisticated insight into evolution and adaptation.


Science Inquiry

Have students do one or more of the inquiry activities described in the article at the URL below. All of the activities allow students to practice science process skills, collaborate with others, and model how scientists work while solving problems relating to evolution and the fossil record.


Common Misconceptions

Many people believe that evolution is a theory about the origin of life rather than a theory about how living things change through time by natural selection. Explain that while scientists may still disagree about how life began, there is virtually no disagreement about how evolution changes species through time.

Reinforce and Review

Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Earth Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.

Lesson Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.

Lesson Quiz

Check students’ mastery of the lesson with Lesson 12.3 Quiz in CK-12 Earth Science for Middle School Quizzes and Tests.

Points to Consider

The processes of evolution are fundamental to much of biology. Why do people have such a hard time understanding them?

A lot of organisms are dying out now due to changes in climate and effects of human activities. How does what's happening now resemble a mass extinction?

The amount of biodiversity on Earth is staggering. Why are there so many different types of organisms?

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