1.) Man is the only animal with a butt. We developed monstrously huge gluteal muscles (compared to other simians) in order to pull us upright. Other apes knuckle-walk. They can stand upright, but not for long. Ditto for bears. Other creatures that are bipedal have either redesigned their bodies so their legs are under a balanced center of mass (think of ducks and geese) or else have grown a counterbalancing tail (dinosaurs like T. rex. Allosaurs have their tails stiffened and held out by a network of tendons so tough that they fossilize.) I don’t know of any other biped besides man that has those two huge masses of gluteus maximus muscle, overlaid with subcutaneous fat. In affluent societies,a lot of fat. So buttocks lead to the gluteal fold, better known as the “ass crack”. It impedes the free passage of excrement.
2.)Fortunately, people have also developed hands to wipe with. I am amazed and troubled by this. It’s unhealthy to leave excrement in place on skin – you develop rashes and sores. If you didn’t wipe, you’d have to bathe daily. If we didn’t have hands ith opposable thumbs, hygeine would be harder in any case. Is it possible tha opposable thumbs allowed us to be bipedal, or vice-versa?
3.) Western people sit o a toilet that compresses the buttocks together,which makes relatively cean defecation much harder. As a number of books on the bathroom have shown, the “squat” method is better in that it spreads the buttocks, and also allows the abdominal muscles to bear dwn and expel aste.
4._ Even if you squat to poop and eat a non-Western diet, you still need to wipe. There’s plenty of evidence to support it. The book End Product: The Last Taboo (arguably the best book on the topic, and easily among the weirdest books in my collection) tells about somone who advocates and teacg=hes “paperlesspooping” , matter of diet and mucle control. It doesn’t seem practical for most people.
So, it seems to me that people ar unique by their construction in requiring wiping, and I suspect it’s related to other aspects of our volution. No joke. Western0style toilets and diet may ake the stuation worse, but can’t explain the basi need.