Despite being furry, gorilla bums don't need wiping. Source

Standing on their own two feet required development of a radically different muscular structure compared to other primates. But the development of huge muscles on both sides of the human anus upsized our risk of soiling. In response, humans developed additional inspection and grooming behaviours:

Intricate self-grooming is made possible by the radical re-working of spine, hip and pelvis that enables bipedalism. Source: NY Post

Human spines and pelvises are unlike those of primates and pigs. Following list from Eugene McCarthy's MacroEvolution:

• Short, dorsal spines on first six cervical vertebra
• Seventh cervical vertebrae:
  • long dorsal spine
  • transverse foramens
• Fewer floating and more non-floating ribs
• More lumbar vertebrae
• Fewer sacral vertebrae
• More coccygeal vertebrae (long “tail bone”)
• Centralized spine
• Short pelvis relative to body length
• Sides of pelvis turn forward
• Sharp lumbo-sacral promontory

• Large arse (Massive gluteal muscles)

  Why?

  Because being bipedal requires more and bigger muscles attached to a more complex pelvis. Which forces those muscles to the rear. Taking humans from this:

  Obviously, this is disrespectful to the pig. Source

  to this:

  Female demonstrates human gluteal muscle size and mobility. Source: Pornkai

For the technical compromises that enabled this feat of bioengineering, see How did human butts evolve to look that way?.

• Curved sacrum with short dorsal spines
• Hind limbs longer than forelimbs
• Femur:
  • Condyles equal in size
  • Knock-kneed
  • Elliptical condyles
  • Deep intercondylar notch at lower end of femur
  • Deep patellar groove with high lateral lip
  • Crescent-shaped lateral meniscus with two tibial insertions
• Short malleolus medialis
• Talus suited strictly for extension and flexion of the foot
• Long calcaneus relative to foot (metatarsal) length
• Short digits (relative to chimpanzee)
• Terminal phalanges blunt (ungual tuberosities)
• Narrow pelvic outlet

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