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Evolution is ascientific theory used bybiologists. It explains howanimals andplants changed over a long time, and how they have come to be the way they are.

Earth is very old, about 4.5billion years. By studying thelayers of rock that make up Earth's crust, scientists can find out about its past. This kind of research is calledhistorical geology.

We know that living things have changed over time, because we can see theirremains in the rocks. These remains are called 'fossils'. So we know that the animals and plants of today are different from those of long ago. And the further we go back, the more different the fossils are.

How has this come about? Evolution has taken place. That evolution has taken place is afact, because it is overwhelmingly supported by many lines ofevidence. At the same time, evolutionary questions are still being actively researched by biologists.

The theory of evolution is the basis of modernbiology. Nothing in biology makes sense without it.