...and why not?
The evolution of human beings took so much time just on the basis of the received wisdom, that they were afraid of accepting CHANGE or acclimatisation. A mind preoccupied with fear from thunderstruck at night, howls of beasts nearby and the fearsome sight of the fire was too hard to be convinced to accept the change, and which is why evolution of human being (as i concur) was prolonged process.
This quote is reminiscent of that evolution.
Men were confined in their sense of fear, and therefore they were unable to acclimatize to the new surroundings posing as threat to their long-held superstitions. The cavemen never took the trouble of coming out of their hideouts at nights filled with spooky sounds of thunders/wolves/lions or wild animals.
And when they mustered up courage, tried to evolve with the things around, they learnt that many things or beliefs they were so blithely held up were counterfactual credo.
So, they busted many myths later through the course of their evolution by learning reality of their superstitions.
The moot point is - your mind caged by a thought not allowing you to think broadly or out of the box will make you a narrow-minded fellow who will judge anything as per your own assumed reality.