Well, what do you expect. We don't have direct evidence, apart from the fact that we evolved from single cells to a man typing response to a clueless comment which wants to see everything laid out nicely and every part covered 100% because thats how it should be.

You cannot point out exact steps how we evolved till now yet here we are. "We don't know" is a perfectly fine answer even if it somehow triggers you. Universe is not arranged around such folks, universe doesn't care about some tantrums, universe just is, in ways we still don't fully understand and thats fine.