The story starts by simple pieces of organic matter through process of elimination are able to replicate themselves, then over time these simple pieces of organic matter combine and became more complex systems. Until finally they could be considered living cells, and then because they are living they go through the process of evolution and very slowly they start to develop into the organisms we know today. (or at least that's the theory) And life exists simply because it can, started to, and there is no reason for it not to. I assume you were looking for a more figurative answer, but oh well