The problem even now is that people's bodies aren't sufficiently hydrated, they really weren't back then either but squatting allowed gravity to help bowel movements.
If you aren't hydrated, your stool will likely be hard and you'll have to push it though...the stool itself will act like a grate against the walls of your lower intestine, rectum and anus causing lesions.. which bleed. The lesions heal eventually but if it continually happens over time..a number of things can happen. Overgrowth of scar tissue, which can create pockets inhibiting the flow of waste, infections, hemorrhoids and possibly cancer.