Philosophers through time have had many kinds of theories about where life in nature originated. All the earliest philosophers believed that there had to be a certain basic substance at the root of all change. There had to be "something" that all things came from and returned to. They posed questions relating to the transformations they could observe in the natural world. It can be said that these philosophers took the first step in the direction of scientific reasoning. Unfortunately, only fragments of what the natural philosophers said and wrote have survived. The little we know about their thoughts can be found in the writings of Aristole who lived two centuries later. .
There were three philosophers from Miletus who believed in one single substance as the origin of all life. These three philosophers were Thales, Anaximander and Anaximenes. Thales, who came from a Greek colony in Asia Minor, thought that the source of everything is water and all life will be returned to water again when it dissolves. Just like when water turns to ice or vapor and then turns back into water again. Anaximander, who lived at the same time as Thales, did not think a known substance was the source of all life. He believed that the source was something other than the things created. All created things are limited so they must be "boundless". Anaximenes, who was familiar with Thane's theory of water, thought that the source of all things must be air or vapor and that water was condensed air. Air was therefore the origin of earth, water, and fire. Earth, air, water, and fire are all necessary for the creation of life but the source of life was vapor or air.
In the Greek colony of Elea, there was a group of philosophers who were interested in the problem of change i.e. "How something can change into something else?" The most important of these philosophers was Parmenides. According to him everything that exists had always existed.