But ... (and now I'm genuinely curious) why aren't we living in a period way after rationality became the optimal choice? JulianMorrison and my suggestion provides the lower bound, but what is the upper bound?
Are you asking an explanation for why anthropic reasoning is bunk?
Related on OB: Taboo Your Words
I realize this seems odd on a blog about rationality, but I'd like to strongly suggest that commenters make an effort to avoid using the words "rational," "rationality," or "rationalist" when other phrases will do. I think we've been stretching the words to cover too much meaning, and it's starting to show.
Here are some suggested substitutions to start you off.
Rationality:
Rationalist:
Are there any others?