The chemicals in our brains that create "sentience" are the result of evolution. Evolution has occurred, it is what brought us from single celled organisms to people arguing over the value of fWAR on a magical box.
http://en.wikipedia.org/wiki/Evolution_as_fact_and_theory#Fact (yeah, I know, citing Wikipedia, but this isn't a term paper...)
"Evolution is a fact in the sense that it is overwhelmingly validated by the evidence. Frequently, evolution is said to be a fact in the same way as the Earth revolving around the Sun is a fact. The following quotation from H. J. Muller, "One Hundred Years Without Darwin Are Enough" explains the point.
There is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea. When we say a thing is a fact, then, we only mean that its probability is an extremely high one: so high that we are not bothered by doubt about it and are ready to act accordingly. Now in this use of the term fact, the only proper one, evolution is a fact."
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"[stephen Jay] Gould also points out that "Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory—natural selection—to explain the mechanism of evolution."