Biology (Holt)

Section 3 Life Invaded the Land

264 CHAPTER 12History of Life on Earth

The Ozone Layer

The sun provides both life-giving light and dangerous ultraviolet

radiation. Early in Earth’s history, life formed in the seas, where

early organisms were protected from ultraviolet radiation. These

organisms could not leave the water because ultraviolet radiation

made life on dry ground unsafe. What enabled life-forms to leave

the protection of the seas and live on the land?

Formation of the Ozone Layer

During the Cambrian period and for millions of years afterward,

organisms did not live on the dry, rocky surface of Earth. However, a

slow change was taking place. About 2.5 billion years ago, photosyn-

thesis by cyanobacteria began adding oxygen to Earth’s atmosphere.

As oxygen began to reach the upper atmosphere, the sun’s rays

caused some of the molecules of oxygen, O 2 , to chemically react and

form molecules of ozone, O 3. In the upper atmosphere, ozone blocks

the ultraviolet radiation of the sun, as shown in Figure 11.After mil-

lions of years, enough ozone had accumulated to make the Earth’s

land a safe place to live.

Objectives

Relatethe development of
ozone to the adaptation of
life to the land.

Identifythe first multicellular
organisms to live on land.

Namethe first animals to live
on land.

Listthe first vertebrates to
leave the oceans.

Key Terms

mycorrhizae

mutualism

arthropod

vertebrate

continental drift

Age(in Millions of Years Ago)

Plants, arthropods, and fungi invade land; jawed fishes appear Bony fishes become abundant

440 •••••• 430 410 400 390

SILURIAN PERIOD DEVONIAN PERIOD

Ozone

absent

Ozone

present

Ultraviolet radiation Ultraviolet radiation

As ancient cyanobacteria added oxygen to the atmosphere, ozone began to form.

Figure 11 Ozone shields the Earth

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