Proof of the past
You put some food in the freezer and it becomes rock solid. Then you put it in the refrigerator to thaw out, and a day or two later it is soft. What happened? How did it thaw out? How did it get soft? Did it thaw out slowly via the thermodynamic transfer of heat energy, or did it miraculously in an instant, poof, go from solid to soft, by the intervention of angels?
How can we answer such questions about the past?
All you can do is assume that whatever happened between its current observed condition (soft), and your present observation of a fossilized past or remembering of a past event (putting it in the freezer and noticing it was rock solid); all you can do is assume that natural processes occurred during this interval of time to change it from rock hard to soft.
We can observe things that are old, such as fossils, layers and folding in sedimentary rock formations, ancient light reaching a telescope from distant galaxies, the mere existence of DNA in all its incredible intricacy with the molecular mechanisms to copy it and to build proteins from it.
Those who oppose evolution want to say that you can't have knowledge of the past by looking at things as they are now (but then creationists go on to explain how the things now can be explained by a global flood).
There is no natural way for a design intelligence to magically create the current biological mechanisms of life. It is all too intricate and tangled together in a web of chemical interactions and dependencies. And there is no evidence whatsoever that someone in the past magically created a new species fully developed and dumped it somewhere for it to thrive and flourish.
The only sensible conclusion is evolution.
Therefore, it is valid in science to look for coherent explanations for all the evidence we see of past conditions. Obviously, there are no experiments possible to redo what happened. Redoing and repeating experiments are very useful for science, but this is not an option for the past. Skeptics of evolution claim that not doing repeatable experiments means it's not science and, therefore, evolution is not scientific.
But you can prove that the laws of physics are everywhere and always in operation and so, therefore, it is those laws which converted the past into its present form.
In developing a theory about the past (such as evolution), you can predict what else you might find and what you definitely won't find if these predictions hold true. These are not predictions such as with experiments, but should perhaps be called past-dictions (as opposed to pre-dictions). You are not claiming what will happen but, rather, what you will discover or what you will not discover.
What would conditions have to be like in the past so that, using only the laws of physics, we would see what we see today? And what are the mechanisms that would change things from the way they were in the past to the way they are now?
So therefore, God used the natural laws and evolution to create all of biological life. This theological view is called "theistic evolution". (But what is God's role in this at all — does he poke at it as it evolves? Well, that is another question for another time.)