I argue that the average intellect of humans has risen a lot in the course of history - due to evolution: both biological and social.
Why? Simply because intellect is a somewhat desirable trait when finding a spouse, and as such it's likely to get passed to children: as a result, each generation is slightly smarter than previous.
As far as social evolution goes, most people back in biblical days were illiterate peasants. Even the brightest minds were uneducated by today's standards.