4-6 The Earth's History

Scientists believe the age of the earth is about 4.6 billion years.  Paleontologists use information from studying rocks and fossils to divide prehistoric time into four periods.  Prehistoric means "before history."  These four periods are referred as eras.

The first era is the longest.  It is known as the Precambrian era.  It began when the earth's crust was first formed, about 4.6 billion years ago.  Single-celled bacteria and animals  without backbones, like sponges, worms, and jellyfish appeared during this era.  It is believed that this era had many violent earthquakes and volcanoes.  The atmosphere was formed; with oxygen being released later by very early forms of plant life.

The Paleozoic era came next.  It began about 600 million years ago.  It is believed that at this time the earth was one large continent located near the South Pole.  This large continent was covered with ice.  Simple animals with backbones (called vertebrates) and plants lived and developed during this time.  Fish and crablike animals called trilobites evolved at the beginning of this era.

About 400 million years ago during this same era, it is believed that the large continent drifted toward the equator.  The ice began to melt forming swamps and shallow lakes.  The climate became humid and moist.  Mountains and layers of sedimentary rocks began to form.  Some plants and seaweed adapted to this change.  Some fish developed lungs and fins and were able to leave the water and spend time on land.  These animals may have become amphibians (animals that live in the water and on land).  Air-breathing creatures like scorpions, millipedes, and insects developed during this time.  Later reptiles followed.

When the Precambrian era ended, trilobites no longer lived.  However, the evolution of fish during this era was very important.  Some scientists believe fish are the ancestors of all vertebrates, including amphibians, birds, reptiles, and mammals.

Directions: Answer the questions about the Precambrian and Paleozoic Eras.

Paleontologists believe the age of the earth is 4.6 years.

Which was the longest era?

Precambrian
Paleozoic

Which era began about 600 million years ago?

Precambrian
Paleozoic

Which era had many earthquakes and volcanoes?

Precambrian
Paleozoic

During which era did the large continent move toward the equator?

Precambrian
Paleozoic

Animals with backbones are called .

During which era was the atmosphere formed?

Precambrian
Paleozoic

During which era did single-celled bacteria and animals  without backbones, like sponges, worms, and jellyfish appear?

Precambrian
Paleozoic

Animals that live in water and on land are called .

Some scientists believe are the ancestors of all vertebrates, including amphibians, birds, reptiles, and mammals.