Early Paleozoic animals were similar to
the jellyfish, sponges, and worms that
live in oceans today. Other animals in
the oceans were arthropods—the early
ancestors of crabs, insects, and spiders.
Arthropods called trilobites were common
sea animals. They were oval and
often measured 6 to 7 inches (15 to 18
centimeters) long. Trilobites became
extinct by the end of the Paleozoic era.
The first fish appeared about 450 million
years ago. These animals did not
look much like fish today. They had
armorlike shells and no jaws. Later fish
developed jaws, a backbone, and fins.
Life on Land
By about 400 million years ago early
plants had started growing on land. By
the end of the Paleozoic era there were
forests of large, treelike ferns. Huge
swamps also developed near the equator.
In general, northern lands were dry and
warm. Ice sheets covered the south.
Meanwhile, some of the arthropods in
the oceans went through changes that let
them live on land. They eventually
evolved, or developed, into the first centipedes,
insects, and spiders. Some fish
also developed body parts that allowed
them to live on land. They became
amphibians, or animals that live both in
water and on land.
Reptiles evolved from amphibians by
about 300 million years ago. Reptiles
were the first animals with a backbone
to live entirely on land.
Mesozoic Life
The Mesozoic era followed the Paleozoic
era. It lasted from about 248 million
years ago to about 65 million years ago.
The first life-forms were very simple. Later forms were more complicated and more varied.
BRITANNICA STUDENT ENCYCLOPEDIA Prehistoric Life 145
Large reptiles called dinosaurs dominated
the land during this era. Some dinosaurs
fed on palmlike plants and evergreen
trees, which grew well in the mild to
warm climate.Other dinosaurs preyed
on other animals. Large reptiles also lived
in the oceans and flew in the air.
Mammals evolved from reptiles during
the Mesozoic era. The earliest mammals
appeared by 200 million years ago. They
were small, shrewlike creatures.
Birds also appeared during the Mesozoic
era. They evolved from reptiles by about
150 million years ago. The earliest birds
had wings and feathers like modern
birds. But they also had teeth and claws.
All dinosaurs became extinct by about 65
million years ago. But other reptiles,
including the ancestors of snakes, turtles,
crocodiles, and lizards, survived. Birds,
mammals, insects, flowering plants, and
other life-forms also survived.
Cenozoic Life
The next period in Earth’s history is the
Cenozoic era. It began about 65 million
years ago and continues today.
Many mammals developed during this
era. The first horses, rhinoceroses, pigs,
hippopotamuses, camels, deer, giraffes,
sheep, cattle, and apes appeared. By
about 4 million years ago the ancestors
of humans, called hominins, had
appeared. Early forms of humans developed
between 2 million and 1.5 million
years ago.
Between 1.8 million and 10,000 years
ago large ice sheets covered much of
Earth. During four periods, known as
ice ages, glaciers moved across the northern
half of the planet. Long warm periods
separated the ice ages. During this
time such huge mammals as woolly
mammoths, mastodons, and sabertoothed
cats roamed the land. But by
the end of the ice ages, most of the huge
mammals had died out.
Early humans hunted mammoths and
other animals. They made and used
stone weapons and tools. For this reason
this period of prehistory is known as the
Stone Age. The Stone Age ended at different
times in different places. It ended
as humans began to farm, settle in villages,
and make tools out of metal.
About 5,000 years ago humans began to
write. Prehistory then ended, and
human history began.
#More to explore
Dinosaur • Earth • Evolution • Fossil
• Human Origins • Mastodon and
Mammoth • Stone Age
Presley, Elvis
Elvis Presley was a famous popularmusic
star in the United States. He was
one of the first musicians to sing and
play rock music. For this he is often
called the King of Rock and Roll.
Elvis Presley was born on January 8,
1935, in Tupelo, Mississippi. His family
was very poor. In the late 1940s the
Presleys moved to Memphis, Tennessee.
As a young man Elvis enjoyed many
Many animals
died out in
historical
times. They
include the
dodo bird of
Mauritius and
the passenger
pigeon of
North
America.
146 Presley, Elvis BRITANNICA STUDENT ENCYCLOPEDIA
different kinds of music, including gospel,
blues, and country.
In 1953 Presley went to Sam Phillips’
music studio to make a recording for
his mother. Phillips had recently started
the Sun Records label. He liked
Presley’s singing. In 1954 Presley made
his first recordings for Sun, including
“That’s All Right Mama,” which
became a local hit.
In 1955 Presley began recording with
RCA Victor, a major record label. Presley’s
early hits included “Don’t Be
Cruel,” “All Shook Up,” and “Hound
Dog.” He released his first number-one
record, “Heartbreak Hotel,” in 1956.
Presley became an actor in 1956. His
first movie, Love Me Tender, was released
in that year. He went on to star in more
than 30 films.
In 1958 Presley entered the U.S. Army.
In 1960 he returned to the United States
