Fossil evidence!
Fossils are mineralised bone.
When something something dies its bones are left behind in the Earth, because of all of the minerals in the ground the bones get “enriched” and become preserved. If we dig deep enough we can dig them up and see what the skeletons of older organisms were like. A similar thing happens with footprints and organisms trapped in Amber.
When we find the preserved remains of dead organisms. These remains are called fossils.
I hope that’s clear, let me know if you have more questions in the comments