The world is full of all sorts of
animals. Dogs, birds, snakes, frogs, whales, and ants are just a few kinds of
animals alive today. But the animals we have today did not always exist. They
developed from other animals that lived long ago. Those early animals included
dinosaurs, tiny ancestors of horses, and gigantic flightless birds.
Those animals and many others became extinct, or
died out. They are called prehistoric animals because they lived in prehistoric
times. Prehistoric times are all of time until about 5,500 years ago. That is
when people began to invent writing and record history. But most extinct
animals lived much longer ago than that.
Not all animals that lived in prehistoric
times are extinct. Cockroaches, for example, have existed almost unchanged for
320 million years. Crocodiles and sharks are also much the same as their
ancestors. Such animals are often called living fossils.
WHAT WERE THE FIRST ANIMALS?
The first animals on Earth lived more than 1
billion (1,000,000,000) years ago. They all lived in the ocean. They included
creatures similar to jellyfish and worms.
Other kinds of sea animals slowly evolved, or
developed, from those early animals. Some types evolved shells. One common shelled
animal was the trilobite, a small, crablike creature that crawled about on the
ocean floor. By 400 million years ago, many sea animals had developed
backbones. Those were the first fishes.
By 360 million years ago, animals called
amphibians had evolved. Amphibians could live both in water and on land. Some
amphibians started living more on land than in water. They began to evolve into
other kinds of animals. By that time, the land was covered with trees and other
plants. Early insects and spiders crawled through the ancient plants.
THE AGE OF DINOSAURS
About 330 million years ago, some amphibians
evolved into creatures called reptiles. Today’s reptiles include snakes,
alligators, and lizards. The reptiles of long ago included flying reptiles that
looked like huge, strange birds. They also included gigantic, sharp-toothed sea
lizards up to 30 feet (9 meters) long. But the most famous reptiles of long ago
were the dinosaurs.
The first dinosaurs appeared about 230 million
years ago. They evolved from animals that looked a lot like crocodiles. Some
dinosaurs ate plants. Others ate meat. One of the most ferocious meat eaters
was the Tyrannosaurus rex, a terrifying animal up to 41 feet (12 meters)
with a huge head and long, pointed teeth.
Some dinosaurs were as small as chickens, but
others were even bigger than the Tyrannosaurus. The biggest dinosaurs
were plant eaters. The largest of these weighed 80 metric tons or more. That’s
heavier than 15 elephants!
THE END OF THE DINOSAURS
Dinosaurs ruled the Earth until about 65 million
years ago. Then they all died out. Why that happened is not known for sure.
Many scientists think their extinction was caused when a huge rock called an
asteroid struck our planet.
But dinosaurs may have left some descendants. Many
scientists believe that modern birds evolved from dinosaurs.
THE MAMMALS TAKE OVER
After the dinosaurs disappeared, a group of animals
called mammals spread all over the planet. Mammals feed their young with
mother’s milk. Cats, dogs, cows, horses, people, and many other animals alive
today are mammals.
The first mammals were small animals just a few
centimeters long. As long as the dinosaurs were stomping around, mammals stayed
small and came out mainly at night. When the dinosaurs were finally gone,
mammals started evolving into bigger animals. Some mammals returned to the sea.
Those animals evolved into dolphins and whales.
Most mammals, however, stayed on land. Some types got
really big. One type of mammal, which lived about 30 million years ago, was a
leaf-eater called Indricotherium. It was about 18 feet (5.5 meters) high
at the shoulder and weighed about 20 metric tons. The Indricotherium may
have been the largest mammal that ever lived on land.
Another large mammal was the woolly mammoth. It
lived during the last Ice Age. Woolly mammoths were closely related to today’s
elephants but were bigger. Some were more than 14 feet (4.3 meters) high at the
shoulder. They had tusks up to 13 feet (4 meters) long. Most mammoths died out
about 11,000 years ago. They and other large Ice Age animals may have been
hunted to extinction by another mammal: human beings.