What is the answer too this question?

By the fossils that dead animals or plants left

Scientist look at various evidence like fossils, rocks, and glaciers to determine what it is like millions of years ago. Evidence like rocks, movement of the tectonic plates and glaciers can determine the landforms and climate of an area. Evidence like fossils, combined with the current knowledge of evolution and the climate at that time allows scientists to hypothesize what it looks like millions of years ago.