Are humans vegetarians or omnivores?

Thomas M. Greiner, Assistant Professor of Anatomy / Physical Anthropology

Those terms are not strict biological designations, and so the question
needs to be rephrased a bit before it can be answered. The confusion stems
from the use of the word "carnivore" to mean "meat eater" - instead the
word carnivorous should be used. To be biologically strict here, a
carnivore is an order of mammal (recognized by the presence of the
carnassial tooth, among other things) that includes cats (felids), dogs
(canids), bears (ursids), and a number of other mammalian families.
Carnivores are not strictly meat eaters - most will eat some type of plants
as a part (sometimes even the main part) of their diet.

The term "vegetarian" is strictly a human construct. Vegetarians shun
animal products for food, usually for religious or ethical reasons. Even
still, there are degrees of vegetarian. Some vegetarians will never
consciously eat any food that comes from an animal. Other will consume
dairy products (an animal food even though it isn't meat). Others will
allow themselves to eat eggs (the pre-chicken, so to speak), or fish.
Vegetarianism is more of a dietary philosophy then an ecological food
preference, so it's hard to speak of it in biological terms.

Biologists rarely (never in my experience) categorize non-carnivorous
animals into one "plant eating" group. That is because different dietary
specializations are required to eat different types of plants. Thus, you
will hear of herbivores (specialized to eat shoots and growing tips),
folivores (specialized to eat leaves) and frugivores (specialized to eat
fruit) among other adaptations. Even within these broad groupings there are
further specializations, such as grazers that eat grass and browsers that
eat shrubs (both are types of herbivore). And even among the "plant eating"
animals there is no animal that I am aware of that will not consume some
type of animal protein when given the opportunity.

An omnivore is an animal that will draw its food from all aspects of the
ecosystem (plants, animals - whatever). As I've already pointed out, you
could claim that nearly every animal is omnivorous since a purely meat
eater or purely plant eater is very rare. Nonetheless, you can look at the
animal's anatomy to look at how they are adapted to process food.

First you can look at the teeth.

Molars are broad flat teeth that are useful in grinding up tough fibrous
material - such as plants. Animals that are primarily plant eaters have
very large molars with six pairs (three uppers and three lowers) on each
side. Animals that do not specialize in eating plants tend to have a
reduced number of molars. Humans are equipped with six pairs of molars.
Although they are not very large, this would suggest that humans have the
ability to process fibrous plant food.

Premolars (bicuspids) are the slicing teeth. Mammals originally had eight
pairs of premolars, although most mammals alive today have fewer. Premolars
are the primary teeth used by meat eating specialists, and are frequently
missing in animals that specialize in plant foods. Humans have four pairs
of premolars, which suggests that humans have the ability to process animal
food.

Incisors are grasping teeth, that change shape depending upon how they are
used. The broad flat surfaces of human incisors is most associated with
animals that specialize in eating fruit.

So, if we just look at our teeth - humans are clearly built to be
omnivorous. But, of course there is more data. We can look at how nutrients
are processes and absorbed in the body.

Meat and fruit are high quality foods that are not difficult to assimilate.
Animals that specialize in these types of food tend to have a short
digestive tract, with a very short large intestine. Plant foods can be
nutritious, but take longer to absorb. Therefore, animals that specialize
in plant eating tend to have long and elaborate digestive tracts. Humans
are clearly intermediate here. We have a long large intestine (more common
in plant eaters), but we lack the elaborations that would allow us to
digest and assimilate nutrients from high fiber plant foods (such as
grass or leaves). So, again, the human digestive tract can be used to argue
that we are omnivorous.

Finally, you need to look at nutritional requirements. There are some
B-complex vitamins that are available only by eating other animals. The
human body requires this nutrient, but does not synthesize it the way some
other animals do. Therefore, if humans truly ate no animal foods, and had
no artificial vitamin supplements, they would sicken and die. In nature,
there are no true "human vegetarians."

Humans are omnivores. The order of mammals that includes humans (the
primates) are all omnivores. To be sure, the modern American diet includes
a lot more meat than is healthy. And the human animal can be very healthy
by being a lot more vegetarian. But to never eat meat is both unnatural and
unhealthy.

Finally, you ask about my credentials to answer this question. Well, I am
employed as an anatomy professor (and am therefore a specialist in human
anatomy). I teach at a college that specializes in training health care
workers (so I am familiar with issues of human health an nutrition).
Finally, I earned by Ph.D. in Physical Anthropology (which sort of makes me
an expert in how humans adapt both biologically and culturally to the
environment).

Some References relevant to this questions:

Harding, RSO & Teleki, G (1981) Omnivorous Primates. Columbia University
Press: New York.

Romer, AS & Parsons, TS (1986) The Vertebrate Body. Saunders College
Publishing: New York

Oxnard, C. (1987) Fossils, Teeth and Sex. University of Washington Press:
Seattle