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12.3: Lesson 12.2: Early Earth

Difficulty Level: At Grade Created by: CK-12

Key Concepts

  • Earth’s continents and supercontinents
  • Ancient life
  • Addition of oxygen to the atmosphere
  • Evolution of multicellular organisms

Lesson Objectives

  • Describe the supercontinents that have existed in Earth history.
  • Discuss how life began and what early life was like.
  • Trace the evolution of life from the first cells to multicellular organisms.

Lesson Vocabulary

  • DNA (deoxyribonucleic acid): type of nucleic acid, an organic compound that stores genetic information and passes it to the next generation
  • eukaryote: type of organism whose cells contain a nucleus
  • nucleic acid: organic compound that stores genetic information and passes it to the next generation; either DNA or RNA
  • prokaryote: type of organism whose cells lack a nucleus
  • RNA (ribonucleic acid): type of nucleic acid, an organic compound that stores genetic information and passes it to the next generation

supercontinent singe, massive continent that formed when all of Earth’s landmasses came together

Teaching Strategies

Introducing the Lesson

Introduce the origin of life by asking students to brainstorm ways that living things differ from non-living things. (Some ways include the ability to use energy, respond to the environment, grow, and reproduce.) Tell students they will learn in this lesson when living things first appeared on Earth and what the earliest living things were like.


In the activity at the URL below, students take a simulated "voyage" back in time to the beginning of our planet. They will "witness" that beginning, the origin of life, and a number of key events from then to the present. This will help students relate physically to the relative timing of events in geological and biological history and to the vastness of that history.



Discuss why life may have existed on Earth for hundreds of millions of years without leaving a trace. Point out that the earliest life forms consisted of single cells, like modern bacteria, which can be seen only with a microscope. In fact, bacteria are so small that there are 10 times more bacteria in and on the human body than there are body cells.

Differentiated Instruction

Outlining a reading passage can help students see the organization of the material, making it easier to master. Have students make an outline of this lesson in the FlexBook® lesson. Suggest that they use the headings and subheadings of the lesson as the framework for their outline and then add important details to the outline as they read the lesson.


The famous 1952 experiment by Miller and Urey showed that organic molecules (amino acids) could form spontaneously under simulated conditions believed to be similar to those on early Earth. Have a few students learn about this famous experiment and then report back to the class. The URLs below are a good place for them to start.



Science Inquiry

Point out how the evolution of photosynthesis added oxygen to Earth’s atmosphere, which had lacked oxygen prior to that time. The FlexBook® lesson stresses how oxygen benefited organisms by forming ozone, which protects living things from harmful solar radiation. Tell the class that the introduction of oxygen to Earth’s atmosphere also harmed many organisms. Challenge students to infer why. (Oxygen was toxic to many organisms because they had evolved in, and adapted to, an oxygen-free atmosphere.)

Real-World Connection

The career of paleontologist may sound exciting to many students. They can learn firsthand what it’s like to be a paleontologist with the interactive “Digging in the Dirt; a Paleontologist’s Field Journal” at the first URL below. The second URL provides links to several other sites that offer more information about the paleontologist career.



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 12.2 Quiz in CK-12 Earth Science for Middle School Quizzes and Tests.

Points to Consider

Early life was very simple by comparison with the biodiversity we see today. How did so much diversity come to be?

How do organisms change through time (how do they evolve)?

Are humans the pinnacle of evolution?

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