You know, I suspect that the need for wiping one's behind may not have existed for adults among our prehistoric ancestors.
I suspect our modern diet (especially lack of fiber), obesity (fat cheeks), and not squatting to defecate is what creates messy defecation in most modern humans.
That is, we may not be necessarily doomed to messy defecation because of our large buttock muscles that evolved to support our bipedal ability.