In this video Darwin explains the origin of feces. A body could, in theory, completely assimilate all organic molecules. After all, there is plenty of food value left in feces; just ask your local scarab beetle or bacterium. But the body of each species only digests the food as far as it is economically worthwhile to do so. To completely assimilate all organic material, an animal would need to haul around a gigantic digestive system. It isn’t worth it.
I got to thinking about this when I read the French science fiction novel Le Voyageur Imprudent (The Careless Traveler), by Rene Barjavel. The traveler visited a future in which humans had evolved into huge females with thousands of breasts, and small males with a hole instead of a mouth, into which a female breast could precisely fit. The males had no excretory openings; they completely assimilated the milk. Sounds crazy? It is, but it made me think.
Enjoy the video!