Biology (Holt)

Section 1 How Did Life Begin?

252 CHAPTER 12History of Life on Earth

The Age of Earth

When Earth formed, about 4.5 billion years ago, it was a fiery ball

of molten rock. Eventually, the planet’s surface cooled and formed

a rocky crust. Water vapor in the atmosphere condensed to form

vast oceans. Most scientists think life first evolved in these oceans

and that the evolution of life occurred over hundreds of millions of

years. Evidence that Earth has existed long enough for this evolu-

tion to have taken place can be found by measuring the age of rocks

found on Earth.

Measuring Earth’s Age

Scientists have estimated the age of Earth using a technique called

radiometric dating. is the estimation of the age

of an object by measuring its content of certain radioactive isotopes

(IE soh tohps). An isotope is a form of an element whose atomic mass

(the mass of each individual atom) differs from that of other atoms

of the same element. Radioactive isotopes, or , are

unstable isotopes that break down and give off energy in the form of

charged particles (radiation). This breakdown, called radioactive

decay, results in other isotopes that are smaller and more stable.

For example, certain rocks contain traces of potassium-40, an iso-

tope of the element potassium. As Figure 1 shows, the decay of

potassium-40 produces two other isotopes, argon-40 and calcium-

40. The time it takes for one-half of a given amount of a radioisotope
    to decay is called the radioisotope’s. By measuring the
    proportions of certain radioisotopes and their products of decay, sci-
    entists can compute how many half-lives have passed since a rock
    was formed.

half-life

radioisotopes

Radiometric dating

Objectives

Summarizehow radioiso-
topes can be used in
determining Earth’s age.

Comparetwo models that
describe how the chemicals
of life originated.

Describehow cellular
organization might have
begun.

Recognizethe importance
that a mechanism for heredity
has to the development
of life.

Key Terms

radiometric dating

radioisotope

half-life

microsphere

Radioactive Decay

1/16

1/8

1/4

1/2

1/1

1 half-life

1.3

2 half-lives

2.6

3 half-lives

3.9

4 half-lives

5.2

Newly formed rock

Amount

(of remaining

potassium-40 atoms)

Time passed (in billions of years)

Potassium-40

Argon-40 (product)

Calcium-40 (product)

Figure 1Rate of decay for

potassium-40.This graph

shows the rate of decay for the

radioisotope potassium-40.

After one half-lifehas passed,

half of the original amount of

the radioisotope remains.

3E

7A

4A

6A