In their vast diversity humans vary about as much as biological minds can vary.
Can you be more precise about your reasons? I'm sure they're in the linking articles somewhere but there's a bit too much there for me to comprehend.
Sorry, I didn't mean to imply that there were already existent strong arguments for that; it's just my personal speculation. I think it's one of those things where I could type for a few days straight and still not accurately describe my intuitions and their origins. Better to just read a ton of vaguely relevant stuff, I think.
One of the most important points raised by the sequences is that not all minds are like humans. In quite a few places, people have discussed minds with slight changes from human minds, which seem altogether different. However, a lot of this discussion has been related to AI, as opposed to minds created by evolution. I'm trying to think of ways that minds which evolved, and are effective enough to start a civilization, could differ from humans'.
Three Worlds Collide would seem like an excellent starting point, but isn't actually very useful. As far as I recall, the Babyeaters might have learned their baby eating habits as a result of societal pressure. The main difference in their society seemed to be the assumption that people who disagreed with you were simply mistaken: this contrasts to humans' tendency to form rival groups, and assume everyone in the rival groups is evil. The Super-Happies had self modified, and so don't provide an example of an evolved mind.
So here are my ideas so far.