The huge level of bacteria, and the lower amount of food product, added to the high level of toxins would appear to make waste-eating humans unlikely. Offhand, there are few species that willingly devour their own wastes, and those that do so usually have quite specific needs that are so served.
Two examples for the non-squeamish amongst you: 1) Termites eat one another's feces on a fairly regularbasis. This is apparently to keep their intestinal protozoa up to par. They don't rely on feces as a foodstuff, though.
2) Rabbits regularly eat their own pellets. But this is misleading, because rabbits actually produce two types of pellets: soft and hard. The soft pellets are on their first trip through the digestive tract, and are _always_ eaten, if possible. The hard pellets, which are inedible and ignored, have finished their second trip through the bunny. Rabbits don't chew their cud, but by passing each mouthful through their digestion twice, they're able to get the maximum food value out of plant matter.
In general, plants are hard to digest. That's the main reason that herbivores have larger body cavities than carnivores -- because herbivores need complex and lengthy digestive tracts. Compare a deer and a mountain lion, or a goose and a hawk. Or a scarab beetle and a wasp. It's also the reason why most herbivores can digest meat if fed to them, whereas few carnivores can do this.
The usual claim that humans, as omnivores, can eat both meat and plants is a bit misleading. The fact is that humans can't really eat all that wide a range of plants. We can't digest most types of grass and grass leaves, and very few tree leaves are edible to us. We _can_ eat lettuce and a few other types of leaves, but most of these actually have negative food value (i.e., it takes more calories to digest a piece of lettuce than we derive from it -- that's why it's such great diet food). The majority of plants that we eat are either (1) Seeds -- not all seeds, though. Just grass seeds and nuts, really. And not all nuts. Note that the grass seeds (frex: wheat) take significant processing before they're edible. (2) Fruit -- a special organ of the plant that's _intended_ to be easy to eat. (3) Vegetables -- starch-storing roots, a few types of leaves, and that's about it.
Take the lordly cattail as an example. It's renowned among survivalists as edible to humans. A true herbivore would just gulp it down. For a human to get nourishment from the thing, we have to peel and expose the internal stems of first-year cattails (second-year or older cattail stems are inedible). We can also pull off the heads of cattails when they're real new and roast them. At least that's the story. I've never met anyone who actually tried roasting cattail heads, but the stems aren't bad.
Humans also have reasonably sensitive digestive tracts. We can eat carrion, but if it's too old, it smells bad to us, and very few humans will eat it. This is not just a cultural thing, either -- old dead meat contains toxins that are bad for us, and so it's adaptive for us to avoid it. This was well-known even in the good old days. For instance, the Borgias would always add the steeped juices from a rotten toad to poison, as it significantly increased deadliness. On the other hand, buzzards can eat even real putrid matter. As a side effect, buzzards are exceedingly difficult to poison because of their digestive tracts.
NOTE ON LANGUAGE: I use the word "buzzard" in the American sense of the word. I am well aware that the critter termed a "buzzard" in England is not identical. For those unaware of this further evidence of Our Common Tongue, here's the scoop.
In Europe, certain hawks are called "buzzards". In the USA, a hawk is called a "hawk". However, vultures are called "buzzards". No doubt when early colonists came to the USA, and commonly saw vultures (unknown in Great Britain), they simply called them "buzzards" after the beasties they knew back home. The name stuck and adhered to them, and so now vultures are universally buzzards here in the States. We do know the word "vulture", too, and it is considered a synonym.
I don't mean to start up another elk/moose debate. The former one was absurd -- how anyone can declare that a particular word's usage is "wrong" (especially in ENGLISH, for Goddess' sake!) is beyond me. But just as in proper US English, the term for Alces alces is "moose", here the word "buzzard" is improper for a hawk, but correct for a vulture.
Incidentally, I'm just assuming that vultures are unknown in Great Britain because I've never seem 'em there, and never read about 'em either. I always figured that vultures only made it to Southern Europe, at best. They're all over the south and western USA, and are probably the commonest large flying bird, unless you count Canada Geese, which are really only visible part of the year.
Sandy
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