Brad (and then Juli) ask: Aren't humans natural omnivores? Aren't they equipped for eating both plants and animals? I mean, we have these "fangs"...
The Nerd Responds:
Obviously humans are omnivores. The most basic definition of an omnivore is an animal that eats both plants and animals as its primary sources of food. Most humans certainly fit that definition.
The question of whether humans are natural omnivores is far more difficult to answer. Firstly, we have to decide what we mean by natural. The definition of omnivore does not require that an omnivorous animal be perfectly suited to that method of nourishment. Examples in nature show us that animals do not develop omnivorous adaptations in order to more exactly meet all their nourishment requirements. Indeed, outside of humanity, omnivores are nearly all opportunistic feeders, selecting between plant food and meat mostly by availability (I leave aside the question of whether or not a fast-food window is a form of opportunistic feeding) and not by any particular drive to reach a balanced diet of proteins, vitamins, and minerals. That is to say, an omnivore such as a crow will eat meat when meat is available, and will eat plant food when that is available. Nothing in the biology of a crow demands that they should eat a 1/2 meat - 1/2 plant diet and I'm not aware of any studies that determine the optimal omnivore ratio of carnivore to herbivore intake for crow health.
In his paper on the hunting behavior of chimpanzees, Craig Stanford describes the basic reason for omnivore adaptation:
Since neither humans or chimpanzees are truly carnivorous - most traditional human societies eat a diet made up mostly of plant foods - we are considered omnivores. The important decisions about what to eat and when to eat it should therefore be based on the nutritional costs and benefits of obtaining that food compared to the essential nutrients that the food provides.
In other words, being an omnivore is all about "making do" with what is available. It is not so definitive a condition as say being a cold-blooded creature. A crocodile needs a certain amount of sunlight and warmth in a day or its body will shut down. It simply does not possess the necessary chemistry to survive without a direct external heat source. Many people believe that the human diet is similarly rigid: as omnivores we need to be part carnivore/part herbivore in order to get the nutrition an omnivore needs to survive. But this is actually an inverse reasoning on the definition of an omnivore. It is more accurate to say that as living beings, we require certain nutrients, and as omnivores we have the ability to derive these nutrients from both animal and plant sources.
A chimp's large canines dwarf the human's, and yet it is 95% vegetarian
Many people see human "fangs" as proof that we should naturally include vast amounts of animal matter for gathering nutrition. Again, this is an example of reverse thinking which is clearly debunked with a quick comparison of chimp and human teeth. If the size of fangs determines what we "should" be eating, then comparing our teeth to a chimp's, the largest thing we should ever be eating would be maybe a rat. Chimpanzees hunt for approximately 3% of their food, eating mostly monkeys, birds, and smaller mammals (another 2% from termites and other insects). The rest of their diet consists of fruit and other vegetation of which they eat a great variety--up to 20 different species in a day and over 300 species in the course of a year. So its clear that size of fangs doesn't have to be proportional to amount of meat-intake. And the reverse of this is also true: Just because an animal has small fangs, doesn't mean it wasn't "meant" (more properly stated "cannot be well-suited") to eat meat. A crow eats proportionately more meat than a chimp, and it has no teeth!
To me the question becomes interesting when we move beyond the question "are humans naturally omnivorous?" (to which the answer seems to be an resounding "yes") to what most people usually mean by that which is: "Humans are omnivorous by nature so my diet makes perfect sense." People saying this are usually confusing omnivore adaptation with physiological necessities (again the idea that humans need certain amounts of meat a day the way crocodiles need certain amounts of sunlight). The science of nutrition is far better suited for determining if your diet makes sense than glancing casually over the anthropological records.