Wikipedia:Very good articles/Evolution
We know that living things have changed over time, because we can see their remains 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 a fact, because it is overwhelmingly supported by many lines of evidence. At the same time, evolutionary questions are still being actively researched by biologists.
The theory of evolution is the basis of modern biology. Nothing in biology makes sense without it.