We only have theories, but there is a lot of evidence to support them. Conditions for the formation of life were favorable and there were organic molecules. Life then evolved.
What do you mean, “what really happened”? Your phrasing makes it sound as if you find some answer objectionable. Which theory do you find suspicious? Are you looking to references for research on the subject?
what really happened you ask? well there are lots of different peoples views on this subject. what i really should have asked is how do you think humans evolved. my mistake
if you’re really interested in this stuff then read the selfish gene by Richard dawkins. Its heavy going at times but gives you a really good idea how it is believed evolution takes place. Had quite a profound impact on me.