Chris Mooney's theory:
There would surely have been a survival value to getting other people in your hunter-gatherer group to listen to you and do what you want them to do — in short, a value to being persuasive. And for the listeners, there would have been just as much a premium on being able to determine whether a given speaker is reliable and trustworthy, and should be heeded. Thus, everybody in the group would have benefited from an airing of different views, so that their strengths and weaknesses could be debated — regarding, say, where it would be a good place to hunt today or whether the seasons are changing.
Considered in this light, reasoning wouldn't be expected to make us good logicians, but rather, good rhetoricians.