This assumes that you will only be convinced by a rational. cogent and scientifically-provable argument.
I believe that we humans tend to be convinced rather easily by totally irrational arguments that are more convenient to our personal well-being and pride.
Why did we take it 500 years ago as an incontrovertible truth that the earth was the centre of the universe? Because it made us feel good about ourselves.
Don't you think that perhaps the good feeling we get from our free wills making us better than the merely instinct-driven animal world has a part to play in convincing us of its veracity?