Most of the species that 1. _____ ever lived on Earth faced extinction. When conditions 2. _______, such as when the climate gets 3. ________ or the quantity of food decreases, there is a huge possibility that species 4. _____ die if they cannot adapt to the new conditions. Paleontologists have learned about a variety of extinct creatures from fossils – evidence of prehistoric life preserved 5. _____ rocks. Some fossils are just markings that animals had left behind while moving, such as footprints. Others are emptied out prints of an animal’s entire body that were 6. ______ in the rocks. Others are the saved remains of an animal’s body, such as skeletons, bones or shells.