Why have we evolved with a capacity for reason? One answer might be because reason is a good way of getting at the truth, and truth is valuable for self-preservation. But reason is often fallible. It isn't on it's own a reliable route to truth. Dan Sperber, who with Hugo Mercier, has researched this issue extensively, explores the question of why we have reason in this episode of the Philosophy Bites podcast. Their research suggests that reason has a social function.
A pdf of Hugo Mercier and Dan Sperber's paper on why humans reason is available from Dan Sperber's website here. Hugo Mercier provides a very useful overview of their research on the Argumentative Theory of Reason (together with further links, including extensive press coverage).
Philosophy Bites is made in association with the Institute of Philosophy