Childrens Illustrated Encyclopedia

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422

Prehistoric life 422-

WHEN PLANET EARTH FORMED more than 4.5 billion years ago,

there was no life. Torrential storms raged, lightning bolts

flashed, volcanoes poured out poisonous gases, and there was no

atmosphere to protect Earth from the sun’s radiation. Slowly,

warm shallow seas formed. In these seas the first forms of life

appeared, protected by the water. We call these early

beginnings “prehistory” because they happened before

written history. Fossils—the preserved remains of plants

and animals—provide the only records of prehistoric life.

We know from fossils more than two billion years old that

some of the earliest forms of life were bacteria. Gradually,

plants called blue-green algae evolved, or developed. These

produced oxygen—the gas that plants and animals need for

life. Oxygen was released into the air from the sea and

formed a protective blanket of ozone in the atmosphere.

The ozone screened out the sun’s radiation, and living

things began to invade the land and take to the air. Millions

of kinds of animals and plants have existed since the first

signs of life—some, such as insects, have thrived; others,

such as the dinosaurs, have died out as Earth’s

environment has changed.

TWO bILLION yEARS AGO

The earliest forms of life were

bacteria and blue-green algae.

The algae grew in rings or short

columns called stromatolites,

which are fossilized in rocks.

Today, stromatolites still form

in shallow tropical seas.

390 MILLION yEARS AGO

Fish were the first creatures with backbones.

They evolved quickly into many different

kinds. Gradually, they developed jaws

and fins. The first small land plants,

such as mosses, appeared on the

swampy shores.

How we know tHe age of fossils

Prehistoric time is divided into different stages,

called eras, which are further divided into

periods. Each of these stages lasted for many

millions of years. If you dig deep down into

Earth’s surface, you can find fossils of animals

and plants that lived during the different periods.

Scientists called paleontologists

find out how old a fossil is from

the age of the rocks around it.

This is called relative dating.

They also measure the amounts

of radioactive chemicals in the

rocks and fossils to find out when

they formed. This is called

absolute dating.

350 MILLION yEARS AGO

As plants became established on land, they

were soon followed by the first land animals,

such as millipedes and insects. Woody trees

that looked like conifers stood more than

100 ft (30 m) high. Sharks and many other

fish swam in the seas.

450 MILLION yEARS AGO

Fossils from this time are much

more common, because animals had

developed hard shells that preserved

well. They include trilobites, nautiloids,

sea urchins, and giant eurypterids, or sea

scorpions, more than 8 ft (2.5 m) long.

Some of the

earliest

remains of

life on

Earth are

fossils called

stromatolites.

Sea pens

existed

600 million

years ago.

Trilobites

were common

450 million

years ago.They

are ancient

relatives

of crabs.

Cooksonia was one of

the first land plants to

appear on Earth.

Quaternary period

Devonian period

Ordovician and

Silurian periods

Cambrian

period

Pre-

cambrian

period

4,560–543

543–490

490–418

418–354

354–252

252–199.5

195.5–65

65–1,8

1.8–today

Stages Million years ago (mya)

Carboniferous and

Permian periods

Triassic period

Jurassic and

Cretaceous periods

Tertiary period

One of the first fish, about

390 million years old

600 MILLION yEARS AGO

Rare fossils of soft-bodied

creatures show us that many

different animals had

evolved by this time. They

included the first kinds of

jellyfish, corals, sea pens,

and worms.

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