Example: Science didn't give an animal the ability to reproduce, the animal had that ability to start with. Science simply figured out exactly how it took place and filled in most of the blanks.
It's the same with evolution. What did, or did not cause life to come into existance and develop over time has already happened. There IS a correct answer to all of our questions. Science is simply trying to find that answer.
To get to the original posters point about starting with an answer, I see no problem with it because there IS an answer. We might not know what it is, but it still exists.
Actually, this isn't part of science, but is an article of faith by many scientists (including me). However, we have no (strictly rigorous) reason to believe that the answers already exist: all we have is a reasonable expectation that this is so, because we've found this to be frequently true--and never, ever, proven untrue--in the past. Either you have to accept this (like the axiom of choice) or get nowhere. But it is an ontological proposition which strictly speaking is outside of science.
Note also, we have no reason to believe that the same answers will exist in the future, and no reason to believe that the same reasons existed in the past. There is no self-sufficent cause to believe that the laws of physics or metaphysics are constant in time.