Do you seriously believe there is a genetic bias for rationality?

I seriously believe that some people have genetic defects that make some of the important arational systems to malfunction. They then compensate for those weaknesses using the (for most practical purposes inferior) general intelligence capabilities.

Do you think some paleolithic humans were more 'rational' than others because of a few gene tweaks here or there?

There is genetic variation in the extent people use different styles of thought. Preference for near mode vs far mode for example. A side effect of this is the kind of 'rational' thinking that is relevant to, for example, signing up for cryonics or caring about existential risks.