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# 9.1: Earth Forms and Life Begins

Difficulty Level: At Grade Created by: CK-12

## Lesson 9.1: True or False

Name___________________ Class______________ Date________

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

_____ 1. Life first appeared on Earth about 4 million years ago.

_____ 2. Much of what we know about the history of life on Earth is based on the fossil record.

_____ 3. Absolute dating is often based on the amount of radioactive carbon-12.

_____ 4. The geologic time scale is based on major changes in biology, chemistry, and the evolution of life.

_____ 5. In the early Earth, the oceans formed first, followed by the atmosphere.

_____ 6. Did DNA or proteins evolve first? Scientists believe proteins evolved first.

_____ 7. The oxygen catastrophe killed off many early cells.

_____ 8. The earliest cells were probably autotrophs – that is, they made their own food through photosynthesis.

_____ 9. A digital clock uses DNA sequences to estimate how long ago related species diverged from a common ancestor.

_____ 10. As organic molecules evolved before cells, the molecules must have evolved about 4.5 billion years ago.

_____ 11. The earliest cells may have been just nucleic acid inside a lipid membrane.

_____ 12. Did DNA or RNA evolve first? Some scientists believe RNA evolved first.

_____ 13. The solar system evolved from stardust.

_____ 14. Species with few differences in their DNA sequences are closely related.

_____ 15. In order for fossils to provide useful information, they must be dated.

Name___________________ Class______________ Date________

Read these passages from the text and answer the questions that follow.

The First Cells

How organic molecules such as RNA developed into cells is not known for certain. Scientists speculate that lipid membranes grew around the organic molecules. The membranes prevented the molecules from reacting with other molecules, so they did not form new compounds. In this way, the organic molecules persisted, and the first cells may have formed.

LUCA

No doubt there were many early cells of this type. However, scientists think that only one early cell (or group of cells) eventually gave rise to all subsequent life on Earth. That one cell is called the Last Universal Common Ancestor (LUCA). It probably existed around 3.5 billion years ago. LUCA was one of the earliest prokaryotic cells. It would have lacked a nucleus and other membrane-bound organelles.

Photosynthesis and Cellular Respiration

The earliest cells were probably heterotrophs. Most likely they got their energy from other molecules in the organic “soup.” However, by about 3 billion years ago, a new way of obtaining energy evolved. This new way was photosynthesis. Through photosynthesis, organisms could use sunlight to make food from carbon dioxide and water. These organisms were the first autotrophs. They provided food for themselves and for other organisms that began to consume them. After photosynthesis evolved, oxygen started to accumulate in the atmosphere. This has been dubbed the “oxygen catastrophe.” Why? Oxygen was toxic to most early cells because they had evolved in its absence. As a result, many of them died out. The few that survived evolved a new way to take advantage of the oxygen. This second major innovation was cellular respiration. It allowed cells to use oxygen to obtain more energy from organic molecules.

Questions

1. Describe the first cells.

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2. What was LUCA?

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3. Why were the first cells heterotrophs?

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4. How long did it take for photosynthesis to evolve?

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5. What was the oxygen catastrophe?

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## Lesson 9.1: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

1. Place the following in the order in which they evolved: eukaryotic cell, prokaryotic cell, photosynthesis, organic molecules.
1. eukaryotic cell - prokaryotic cell - photosynthesis - organic molecules
2. prokaryotic cell - eukaryotic cell - photosynthesis - organic molecules
3. organic molecules - prokaryotic cell - photosynthesis - eukaryotic cell
4. organic molecules - photosynthesis - prokaryotic cell - eukaryotic cell
2. Which of the following statements is true concerning LUCA? (1) LUCA was a cell. (2) All life on Earth evolved from LUCA. (3) LUCA probably existed probably around 4.5 billion years ago.
1. 1 only
2. 2 and 2
3. 1 and 2
4. 1, 2, and 3
3. The RNA world hypothesis states that
1. early life was based on RNA as the first organic molecule.
2. RNA evolved soon after the formation of the world.
3. the first cells were made of RNA and lipids.
4. all of the above
4. The “soup” of molecules refers to
1. an ocean full of a mixture of many different substances.
2. organic molecules created from inorganic chemicals in Earth's early atmosphere.
3. a Earth full of volcanic eruptions, thunder, and lightning.
4. the classic evolution experiments of Campbell and Chunky.
5. Early Earth
1. had a primitive atmosphere of ammonia, methane, water vapor, and carbon dioxide.
2. lacked much oxygen gas.
3. probably had a very hot environment.
4. all of the above
6. Which of the following can be considered fossils?
1. a 1 billion year-old rock.
2. a 1 billion year-old piece of amber.
3. a 1 billion year-old piece of amber with a primitive insect inside.
4. all of the above
7. "We are made of stardust" refers to
1. the dust of dead skin cells we shed every day.
2. the rotating cloud of stardust that formed the planets.
3. the gases in the stars that formed the gases in the atmosphere.
4. the rotating cloud of stardust that formed LUCA and all the organisms that evolved later.
8. Place the mouse, fruit fly, duck, and gorilla in order of their relatedness to humans, from least related to most related.
1. mouse - fruit fly - duck - gorilla
2. fruit fly - mouse - duck - gorilla
3. gorilla - mouse - duck - fruit fly
4. fruit fly - duck - mouse - gorilla

## Lesson 9.1: Vocabulary I

Name___________________ Class______________ Date________

Match the vocabulary word with the proper definition.

Definitions

_____ 1. the preserved remains or traces of organisms that lived in the past

_____ 2. divides Earth’s history into divisions that are based on major changes in geology, climate, and the evolution of life

_____ 3. uses DNA sequences to estimate how long it has been since related species diverged

_____ 4. explains how the first eukaryotic cells probably evolved

_____ 5. says that early life was based solely on RNA

_____ 6. occurs when a species completely dies out

_____ 7. process in which organisms could use sunlight to make food from carbon dioxide and water

_____ 8. determines about how long ago a fossil organism lived

_____ 9. when oxygen started to accumulate in the atmosphere

_____ 10. has provided lots of information about the history of life on Earth

_____ 11. determines which of two fossils is older or younger than the other

_____ 12. the one early cell that eventually gave rise to all subsequent life on Earth

Terms

a. absolute dating

b. endosymbiotic theory

c. extinction

d. fossil

e. fossil record

f. geologic time scale

g. Last Universal Common Ancestor

h. molecular clock

i. oxygen catastrophe

j. photosynthesis

k. relative dating

l. RNA world hypothesis

## Lesson 9.1: Vocabulary II

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

1. Life first appeared on Earth about ____________ billion years ago.

2. A ____________ clock uses DNA sequences to estimate how long ago related species diverged from a common ancestor.

3. It is likely that organic molecules evolved before ____________.

4. Fossils are the ____________ remains or traces of organisms that lived in the past.

5. Scientists think that one early cell gave rise to all subsequent life on Earth. That one cell is called the ____________.

6. Absolute dating determines about how long ago a fossil organism ____________.

7. Human DNA is most similar to ____________ DNA.

8. Some scientists speculate that ____________ may have been the first organic molecule to evolve.

9. ____________ dating determines which of two fossils is older or younger than the other.

10. The ____________ theory explains how eukaryotic cells evolved.

11. ____________ and Urey demonstrated that organic molecules could form under simulated conditions on early Earth.

12. If we think of Earth’s history as a 24-hour day, humans would have appeared only during the last ____________ of that day.

## Lesson 9.1: Critical Writing

Name___________________ Class______________ Date________

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

Describe how the first organic molecules arose.

Feb 23, 2012

Jul 25, 2015