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Prehistoric Life

The period between the appearance of the first organisms,

3.8 billion years ago (bya), and the first written record of

history, several thousand years ago, is called prehistoric.

It included sudden increases in life, and mass extinctions,

for example during an. ICE AGE.

WHAT WERE THE FIRST FORMS OF LIFE?

Bacteria and other single-celled organisms lived in the

sea and remained the only life forms for billions of

years. Some of these released oxygen into the air,

allowing the evolution of organisms that could use

oxygen. The first animals probably appeared 600

million years ago (mya), those with hard shells and

body cases about 550 mya, and vertebrates (animals

with backbones), such as fish, 500 mya.

WHAT WERE THE FIRST LIFE FORMS ON LAND?

The first land plants evolved from green algae found

at the edge of the sea and rivers 440 mya. In time

they gave rise to horsetails and clubmosses. Forests

then evolved and were the home to the first land

animals. Scorpions and centipedes, as well as

earthworms and leeches, first appeared about

400 mya. They were followed by the first land

vertebrates, which evolved from fish and were

the four-legged ancestors of amphibians.

Anomalocaris was a predator

that at 60 cm (2 ft) long was

one of the largest Burgess Shale

animals – it had a circular

mouth and grasping appendages

Hallucigenia

moved on its spiny legs across

the ocean floor

ICE AGES

4 WOOLLY MAMMOTH

With its thick, insulating coat,

the huge woolly mammoth was

well adapted to living in an ice

age. It lived between two million

and 10,000 years ago.

EARLY MARINE LIFE 3

Based on Canadian fossil findings

from Burgess Shale rock, this

illustration shows what marine

life may have looked like over

500 mya. This period, known as

the Cambrian period, saw an

explosion in animal species

and populations.

An ice age is a period in the Earth’s

history when the climate is far

colder than usual, and large areas

of the Earth’s surface are covered

by ice sheets. There have been 20

ice ages in the past two and a half

million years, each lasting about

100,000 years.

HOW DID ICE AGES OCCUR?

Changes in the Earth’s orbit around the Sun produced

cooler summers, so winter snows did not melt. As ice

sheets formed, they reflected sunlight back into space

so it did not warm the Earth. These changes also

affected the oceans, increasing numbers of plant-like

plankton, which took in carbon dioxide from the

atmosphere. Since this gas helps to retain heat around

the Earth, a drop in its levels accelerated cooling.

FIND OUT MORE. Dinosaurs 78–79 • Evolution 74–75 • Life on Earth 70–71 • Vertebrates 102

prehistoric

life