While we do not truly know how life began, there is some general belief that it probably started in warm, sunlit, shallow water. Most likely, the earliest forms of life were small and soft. As such, they would have left no trace of their passing existence. It was only when living creatures started developing skeletons, shells, and other hard parts that they were finally able to leave a mark in the Record of the Rocks.

In examining these layers of rocks, the oldest layer to show any fossil marks of life is the Precambrian layer. Mostly what is found is Algae and some microscopic creatures known as Radiolaria. The next layer of rock is associated with the Paleozoic period, and the fossils in the rock reflect an increasing diversity of life forms, mostly small sea creatures and things that crawl. It is in the next layer, the Mesozoic, that we finally start seeing not only a record of fossils, but reptilian bones as well. Also known as The Age of Reptiles, the Mesozoic Period is best known for its dinosaurs.

The final layer of the Cenozoic is still very much in progress, and is the time period in which Mammals and eventually Humans arrived. As will be related in The Age of Mammals.

<< The Earth in Space Timeline The Age of Reptiles >>