Notes to Instructors
Chapter 25 The History of Life on Earth
What is the focus of these activities?
All the evidence we have been able to gather indicates that life on Earth began about 3.5
What are the particular activities designed to do?
Activity 25.1 What do we know about the origin of life on Earth?
Students can use this activity to review the types of evidence that exist for the origin of
Activity 25.2 How can we determine the age of fossils and rocks?
What misconceptions or difficulties can these activities reveal?
Activity 25.1
Students often have difficulty “putting all the pieces together.” This type of activity will help
them integrate the various ideas and theories about the evolution of life on the early Earth.
Activity 25.2
While the math involved here is fairly simple, many students still have difficulty with it.
Answers
Activity 25.1 What do we know about the origin of life on Earth?
Construct a concept map of conditions on the early Earth and the origin of life-forms. Be
sure to include definitions or descriptions of all the terms in the list below. Keep in mind
that there are many ways to construct a concept map.
180 Notes to Instructors
Here is an example:
Terms
Use the understanding you gained from creating the concept map to answer the
questions.
1. Modern theory suggests that the early (pre-life) atmosphere on Earth was a reducing
one. Why (for what reasons) is it believed that oxygen was not present when life
formed on Earth?
One key piece of evidence is the lack of oxidized iron in rocks that are more than
no oxygen
reducing atmosphere
high-oxygen atmosphere
sunlight
electrical discharge (for
example, lightning)
protobiont
micelle
phospholipid bilayer
ammonia
phospholipids
water
low levels of UV radiation
prokaryotes
RNA world
Eukarya
energy source
carbon source
and therefore
The early Earth had
resulting in
a reducing
atmosphere
no oxygen no ozone layer
high levels of
UV radiation
which contained
(and)
2. What proposed energy sources existed on this early (pre-life) Earth?
3. In the 1950s, Stanley Miller performed a set of experiments to determine whether life
could have evolved given the conditions stated in the answers to questions 1 and 2.
a. How was the experiment designed?
Miller created a sealed system. The only compounds in the system were water
b. What were the necessary controls?
The system had to be meticulously cleaned and sterilized to be sure it contained
no organic molecules. This included evacuating all the air out of the system and
c. What was produced in the experiment?
In the first experiment, organic molecules, including some amino acids, were
d. What did the results imply about the possible origin of life on Earth?
Miller’s experiments showed that organic compounds could be produced in an
182 Activity 25.1
e. There is general agreement that life must have evolved in the oceans originally
and only much later invaded land. What factors of the physical environment on
the early and evolving earth support these ideas? Changes in which of these were
essential to allow life to survive on land?
The early Earth’s atmosphere contained little or no oxygen. No ozone layer
f. Most of us can’t imagine a world without oxygen. However, as you learned
earlier, chemically oxygen is a powerful oxidizing compound. What effect(s)
would the increase in oxygen levels of the atmosphere have on the organisms that
existed at that time?
Most anaerobic organisms (e.g., prokaryotes) alive at the time would be unlikely
Activity 25.2 How can we determine the age of fossils and rocks?
To determine the age of fossils and rocks, scientists determine the amounts of radioactive
compounds and their stable daughter products present in the sample. Radioactive
elements are known to decay into stable daughter compounds at specific rates. A number
of radioactive compounds, their stable daughter compounds and their half lifes are shown
in the table below.
Radioactive Compound Stable daughter compound Half life
Carbon 14 Nitrogen 14 5370 years
Activity 25.2 183
For dating rocks, the potassium-argon method is often used because:
• Argon is a gas. When rock is molten, any existing argon gas will escape. As a
result, newly formed rocks contain no argon.
For dating organic material, carbon-nitrogen dating is often used because:
• The ratio of radioactive to nonradioactive carbon dioxide in the atmosphere is
fairly constant over time. As a result, the levels of these elements in organic
tissue remain relatively constant as long as the organism is alive.
1. In one half life, half of the original radioactive compound will decay into its stable
daughter compound.
a. If a newly formed rock contains 100 units of 40potassium (40K), how many units
of 40pottassium would it contain after 1.25 billion years?
b. How many units of 40argon (40Ar) would the rock contain when newly formed vs.
after 1.25 billion years?
c. After the 1.25 billion years, what would be the ratio of 40K to 40Ar in the rock?
d. After 2.5 billion years, what would be the ratio of 40K to 40Ar?
After the next 1.25 billion years, 25 of the 50 remaining 40K molecules would
2. You are fossil hunting and find a trilobite fossil in an old riverbed. You have it
radiometrically dated and are told it is 275 million years old.
a. In this amount of time, how many half lifes of 40K have elapsed?
184 Activity 25.2
b. Given your answer in a, what would be the ratio of 40K to 40Ar found in the fossil
remains?
In 0.2 half-lives, 0.2 of a half-life has elapsed. Therefore, 0.2 of half of the 40K
3. You want to date some fabric that you have discovered at an archeologic dig.
a. What method of dating would be best for this? Describe the general procedure
for dating the cloth.
Since human civilizations arose fairly recently in geologic time, it would be best to
b. Assume the cloth is 2,000 years old. How would the level of radioisotope used
change in this period of time?
Two thousand years is 2000/5730 or about 0.35 of the half-life for carbon 14. As
Activity 25.2 185