Near as the men in white coats can figure, certain carbon-based molecules, when arranged in a certain way, spontaneously self replicated. This self replication takes energy and subsequently organisms have grown around these molecules in order to gather energy. The organisms compete for different energy sources. Some of them cooperate in order to better exploit resources and complex life is formed. Because of this ongoing process of competition and cooperation life tends towards complexity until one day sentient, mobile, pools of water filled with billions of madly self-replicating carbon based molecules think up words like existential and crisis and wonder why they bother to reproduce. Well it?s because: Near as the men in white coats can figure, certain carbon-based molecules, when arranged in a certain way, spontaneously self-replicate. There are of course other theories, mostly propagated by men in dresses. They make just about as much sense but can be more entertaining.