Why do humans possess a twisted birth canal?

The relatively narrow human birth canal presumably evolved as a 'compromise' between its abilities for parturition, support of the inner organs, and upright walking. But not only the size of the birth canal, also its complex, 'twisted' shape is an evolutionary puzzle. Researchers now present new insights into why the human birth canal evolved to have this complex shape. They suggest that the longitudinally oval shape of the lower birth canal is beneficial for the stability of the pelvic floor mu...