Because of this, its existence can be proven by experimentation. Simply be rigorous in your application of them. If evolution is occurring in the present, that doesn't mean that is how life came to exist on planet earth. That is extrapolating beyond the data. The question of how life came to exist on planet Earth is outside the scope of evolution.
Evolution only kicks in once that life is already here.
The evidence for what has happened since then is overwhelming. Extrapolating from that evidence into a theory of evolution only requires a belief that the laws of science apply to evolution just as they do to everything else we have ever observed. Any scientific theory also has predictive power. In other words, it should be able to predict results of later experiments or observations. When evolution predicts which types of fossils will be found in which layers of sediment and that simpler forms will evolve into more complex ones, and every subsequent experiment or observation produces results consistent with those predictions, the theory gains strength.