On Nature was a philosophical poem which details Anaximander's theories about the evolution of the Earth, plants, animals and humankind. Anaximander described his theory that humans and other animals descended from fish once the world's oceans began to dry up. Also he described a theory of abiogenesis in his book in the way that he believed that the first life forms formed from mist. We know little about his book because it has been lost or destroyed, however it still remains important today because it describes one of the world's earliest theories of evolution.