The real difference between so-called "evolutionists" and other (usually religious-based) theorists is not in the answer, per se. The difference is in the process.

Science-based approaches start with a blank sheet of paper and build a paradigm based on the evidence that they collect. If the evidence changes, the paradigm changes. Creationists (as an example) start with a pre-conceived notion, and look for evidence to support it. What doesn't fit, is chalked up to probabilities and the unknowable.

-anthony