Animals have lived on Earth for millions of years. Scientists know about them because their remains are found embedded in rocks, turned to stone. These stony remains are called fossils.
Nobody knows exactly when life first appeared, because there are very few fossils older than about 570 million years, the start of what is called the Palaeozoic Era. Palaeozoic comes from Greek words meaning 'old life'. Scientists can put approximate dates to fossils because they can work out the age of the rocks in which they are found. Fossils show how plants and animals have changed, or volved, throughout the Earth's long history.
The Pageant of Life
The first animals were invertebrates, animals that have no backbones, and they lived in the sea. They were soft, primitive creatures, rather like the jellyfish and sponges of today. The first animals with backbones - vertebrales - appeared about 450 million years ago. They were primitive forms of fish. After a long time some fish became able to breathe air and developed lungs (there are still some lungfishes today). From them came the first animals that could live both on land and in water. These amphibians were the ancestors of present-day frogs. They lived about 350 million years ago.
Many kinds of life evolved on land, including insects and reptiles. By about 200 million years ago giant reptiles, the dinosaurs, ruled the Earth. They died out suddenly, about 65 million years ago. By then birds and mammals had evolved, and mammals became the dominant animals. Man is one of the most recent mammals to evolve. Our ancestors appeared about two million years ago.
