Evolution is defined as genetically based change from generation to generation. That means the only way evolution has not taken place is if you look exactly the same as both of your parents, who would look exactly like their parents and so on. So, even going back to the beginning of life (however you define that beginning), one cannot honestly say that evolution has not occurred. Assuming that all humans were born on this planet and are genetically based rather than an organism that reproduces by budding or a similar method, humans are for the most part genetically based. Therefore, it would be impossible for one to deny that some type of evolution has not occurred. Evolution is not merely a theory. It is a fact.
Often, when evolution is discussed, people are confused at exactly what the specifics of the subject matter are. How long evolution has been going on, under what conditions, and what starts it are theories.
When a scientist mentions the "theory of evolution," they are not talking about the theory that evolution occurred; they are talking about the theory of how evolution occurred.
Even with that being taken into consideration, a scientific theory is way more than just a guess. In order to be defined as a scientific theory, something must be falsifiable and not disproved after an investigation. If something is falsifiable, you can disprove it without resorting to some type of supernatural phenomenon. So far, the theory of evolution by means of natural selection (Charles Darwin's Theory) has been shown to be the best explanation for the path that evolution has taken.
The hypotheses or "sub-theories" of the exact nature of how the process of natural selection works are consistently being updated as scientists learn more about the world. Darwin's hypotheses are...