2. The process of fossilization takes a very long period of time. In order to study the fossil, the land is excavated.
3. Fossils are collected from different levels of depths. A systematic study of these fossils and its occurance revealed that the deepest layers were found to have fossils of invertebrates.
4. In layers above them were found prehistoric fish-like animals, amphibians, reptiles, birds and mammals respectively in that order.
5. This suggests that perhaps this was the order in which the animals appeared on the earth.