Evolution
It is known that living things have changed over time, because their remains can be seen in the rocks. These remains are called 'fossils'. This proves that the animals and plants of today are different from those of long ago.  The older the fossils, the bigger the differences from modern forms. This has happened because 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.