I always thought as a distinct possibility and as another theory says (can't remember which):
We evolved from a Meteor that came crashing down on earth, years after dinosaurs (it was even contemplated that it was the same meteor that caused the dinosaurs to become extinct).
Basically, at that time, there was huge amounts of chain chemical reactions after the impact, alongside with the already roaming sulfuric and nitrogen gases that were reacting with the environment.
Over time, This caused the first micro-organism to be created, this micro-organism (perhaps along with other different ones) then began to evolve through its cycle, eventually coming to the divergent path of Natural Selection/mutation, where it began to undergo dramatic alterations in its biological form.
Thus evolution.
However, where did the meteor come from?
and so, with the theories, come the questions, and then more and more questions to have some kind of exact/precise knowledge of the subject, which is a long way, away from our lifetime.
just my