Science operates on determining what the most likely case is. Given the fossil evidence, and the accurate predictions that we can construct from evolutionary theory, we arrive at the inescapable conclusion that evolution via natural selection is the most likely cause of the complex life. If you demanded that I put that probability into numbers, it would be something on the order of 99.9999~ad infinitum.
Well for one thing you made up that conclusion. But most likely is not good enough and most likely can be found to be wrong.