I agree with Mike that the rationality of people in the past was relatively similar to today. I clearly understand evolution’s power to create change in genetics, and as well, believe that it is possible some of these changes may be able to enhance our capabilities to think, but I’m not so trusting that complex particular thought processes during only a few thousand years have altered our capability to reason any better. People still to this day rarely choose a mate based on the quality of intelligence. It is more often that we choose mates based on either practical criteria or emotions alone.
I think that religious thinking in ancient times was not necessarily originated in irrational thinking. It was for the desire to make real logical sense of reality and practical considerations that initiated concepts that were likely of a secular mind-set that began the structures of what later became more formalized as religion.