Steve Jenkins - Life on Earth: The Story of Evolution

LIFE
ON EARTH

THE STORY OF EVOLUTION

BY STEVE JENKINS
HOUGHTON MIFFLIN BOSTON

Rabbits, cacti, bumblebees, jellyfish, penguins, sunflowersall living things on the earth are the descendants of simple, single-celled organisms that lived more than 3 billion years ago.

Millions and millions. There are millions of different kinds of plants and animals living on the earth. Many millions more lived here in the past. Where did they all come from? Why have some died out and others lived on?

In the beginning. The earth is more than 4 billion years old. For a long time, life couldn't exist here. The ground was hot enough to melt rocks. There was no liquid water. Comets and asteroids frequently crashed into the surface, and volcanoes erupted constantly, filling the air with poisonous gases.

The first life. Living things differ from those that aren't alive in several ways. Something that is alive uses energy, gets rid of wastes, responds to its environment, and makes more of itself by reproducing. Because conditions on the early earth were so harsh, it took a long time for the first life to develop. By the time the earth was a few hundred million years old, its surface had cooled enough for water to collect and form oceans. By 3 billion years ago, microscopic bacteria were thriving in these oceans.

No one knows when or where life began. Perhaps it formed in a warm, shallow sea or a mud puddle. It might have developed first in sea spray, deep in the ocean, or underground. Life might even have arrived on the earth inside an asteroid.

A brief history of life. For more than a billion years, primitive bacteria were the only form of life on the earth. As time went by, new living things appeared, at first in the seas and later on land. Eventually, life could be found almost everywhere on the planet.

3.8 billion years ago. The first primitive, single-celled organisms appear. They are much simpler than the forms of life that exist today.

600 million years ago. The first simple animals are living in the oceans. They include plantlike animals attached to the sea floor, worms, and jellyfish. They are all small, probably less than an inch long.

520 million years ago. Many new kinds of animals develop, including strange wormlike creatures and hard-shelled trilobites. The largest of these animals grow to more than two feet in length. All of them live in water.

470 million years ago. Armored fish, the first animals with backbones, swim in the oceans. These early fish are less than a foot long.

420 million years ago. Shellfish, nautiluses, and corals swim in the water or live on the ocean floor.

400 million years ago. Mosses and ferns move from the water onto the land, followed by centipedes, insects, and the first amphibians. The first plants and animals to live on land are quite small.

330 million years ago. Giant ferns and club mosses form huge swampy forests, and insects become the first flying animals. Dragonflies with wings two feet across fly among plants that grow to heights of fifty feet or more.

290 million years ago. The age of reptiles begins. The first reptiles are small animals, about a foot long, and look much like today's lizards.

230 million years ago. One or more groups of reptiles evolve into dinosaurs. They range from bird-like animals a few inches tall to giants more than 90 feet long. Dinosaurs will be the dominant animals on earth for the next 160 million years.

215 million years ago. A different group of reptiles develops into the first mammals. For the next 150 million years, mammals remain small in size and are active mainly at night. This helps them avoid being eaten by dinosaurs.

150 million years ago. The first birds, probably relatives of tree-climbing dinosaurs, take to the air.

65 million years ago. The last of the dinosaurs become extinct, and large flightless birds, some more than seven feet tall, become the dominant meat eaters on land.

40 million years ago. With the dinosaurs gone, mammals are able to evolve into large plant eaters and predators. This rhinolike animal, called "battering ram beast," is nearly as big as an elephant.