Someone I know says he believes that we are entirely nature and nurture, no free will, and that he is therefore nothing more than a biological automaton. I doubt very much he believes it, but he says it. Why does he say this? Because nature and nurture have been proven, and free will has not. The obvious logical hole in this reasoning is: does this mean we were not influenced by nature at all before the discovery of DNA? Or were we not influenced at all by our environment before the formalised field of psychology? In other words, discovery of the mechanism for something is not the condition required for that thing to exist. This does not prove the existence of free will, however; it merely rules out that sort of half-formed "logic".
Another person I know claims that we can't rationally say free will exists until we know the cause and the mechanism by which it functions. We can observe the effects of gravity and make calculations describing how its effects operate, but as far as I know there is not yet any definitive explanation of a cause or mechanism for it. Thus, by this chap's reasoning, free will and gravity are equally valid or invalid, so tie yourselves down because we're all about to drift away. Obviously this half-logic does not hold water either.
When it comes down to it, the evidence we believe is electrical signals interpreted by the brain. Common concensus conveyed to us through more such signals shows us that the evidence seems correct. And it all relies on us trusting in those electrical signals. Yes, unfortunately, there is trust, or faith, involved even in empirical science. It is the first stage from which all acceptance of any "reality" is formed; that we trust our perceptions, that we are really receiving data from outside our own minds. In short, we make a decision, to be solopsistic or to trust that the world beyond our mind really exists and that the signals coming in are real. If the latter, then we accept that those signals from other people, from our own senses, from experiments and such are valid, at least within the limits of our abilities and knowledge so far.
But free will? Well, people occasionally ascrifice themselves for other people, or for political causes or religions or such. It's quite a stretch to explain away such acts in terms of preserving one's genes for the future. Life generally wants to live. It's also quite a stretch to explain it in terms of environmental factors alone, especially in cultures which actively discourage suicide and such. Why would you pick up an apple to eat for a snack instead of a carrot? Why the chair you're on and not the one next to it? Is there any signal from outside, or any DNA sequence, which makes you do one or the other?