This thread is stimulated by a thread by Oleander on the Platypus. The Platypus is a monotreme, an earliest type of mammal that has hair, and produces milk, but has the genital, urinary and anal tracts of the female exiting the body in a single opening, the cloaca. This is the opening through which eggs are laid, as is also true of the reptiles and birds. What evolutionary disadvantage is present that would have given an evolutionary advantage to the marsupials [and later carried forward by the placentals] to have three separate openings, instead of just one? Was there a stage in which there were two openings, then later three? This is part of the murky origins of mammals of which we are still lacking knowledge, I believe. Any insight?