The scale is different, since obviously there have been far fewer dice rolls for spawning a civilization on Earth than galaxy-wide. Anyway, I would bet that th

The scale is different, since obviously there have been far fewer dice rolls for spawning a civilization on Earth than galaxy-wide. Anyway, I would bet that the 'terrestrial great filter' is evolutionary selection for large brains. Big brain costs a lot of energy and makes birth difficult to the point that humans have to be born prematurely and are as newborns utterly incompetent compared to other animals. And marginally better brain gives very little advantage to gene spreading.