What does that mean? Brains are their own little worlds of thought, but they also communicate-- thinking has a social component as well. Both are absolutely crucial and trivially present in all thinking. The scientific method, for example, is founded upon a tension between independent and social examinations of scientific ideas.
Clearly there's a good way to think independently and a bad way; a good way of thinking as a group and a bad. How is the line drawn? It's certainly not a line that can be drawn in advance.
In your post you said:
> It makes me think that there must be some really powerful evolutionary processes in place which make it so that irrational idolists have more children and live longer than rational people.
That seems rather essentialist to me: Do you really think people are so transparent that you can not only judge whether they are intrinsically rational or irrational but also that quality is hereditary and also that there is a mysterious force that makes us idiots via natural selection?
Better instead to ponder what social and psychological processes can draw someone into a cult and how those processes play out in your own life. We take for granted how thin the margin is between our own lives and the lives of others. It is almost surely the case that all of us hold absolutely absurd delusions about our lives and how we should go about them.