Are evolutionary mutations spontaneous?
Mutations occur spontaneously because all molecular processes are stochastic in nature: DNA replication is an imperfect process. However, certain external factors can increase the probability of a mutation to occur or even cause specific types of mutations.

Both spontaneous mutations and this kind of externally driven mutation contribute to the genetic variability that is the basis for evolution. It is sufficient that variation exist for evolution to be possible, no matter the origin of this variation. X-rays are probably not common in nature, but there is plenty UV on the surface of earth, and there probably exist chemical mutagens that are formed without human contribution.