I don't have an average human, and I don't think the universe does either. I think there's a lot to be said for not having a mental image of an average human.

Furthermore, since there are nearly equal numbers of male and female humans, gender is trait where the idea of an average human is especially inaccurate.

I think the best substitute is "Take typical humans. They're thus and such." Your average alert listener will be ready to check on just how typical (modal?) those humans are.