Well there's a lot of answer for this. On religious view, God created the world according to his view. He created life and everything that surrounds it. On scientific view, I guess there's a lot of theories that would support to this. We have the evolution theory and other's that im not aware of. A lot of people discussed about this but the true answer is still unknown. It's really hard to answer this question because there's no basis or proof where life started.