Humans are Omnivores
Introduction
There are a number of popular myths about vegetarianism that have no scientific basis in fact. One of these myths is that man is naturally a vegetarian because our bodies resemble plant eaters, not carnivores. In fact we are omnivores, capable of either eating meat or plant foods. The following addresses the unscientific theory of man being only a plant eater.
Confusion between Taxonomy and Diet
Much of the misinformation on the issue of man's being a natural vegetarian arises from confusion between taxonomic (in biology, the procedure of classifying organisms in established categories) and dietary characteristics.
Members of the mammalian Order Carnivora may or may not be exclusive meat eaters. Those which eat only meat are carnivores. Dietary adaptations are not limited by a simple dichotomy between herbivores (strict vegetarians) and carnivores (strict meat-eaters), but include frugivores (predominantly fruit), gramnivores (nuts, seeds, etc.), folivores (leaves), insectivores (carnivore-insects and small vertebrates), etc. Is is also important to remember that the relation between the form (anatomy/physiology) and function (behavior) is not always one to one. Individual anatomical structures can serve one or more functions and similar functions can be served by several forms.
Omnivorism
The key category in the discussion of human diet is omnivores, which are defined as generalized feeders, with neither carnivore nor herbivore specializations for acquiring or processing food, and who are capable of consuming and do consume both animal protein and vegetation. They are basically opportunistic feeders (survive by eating what is available) with more generalized anatomical and physiological traits, especially the dentition (teeth). All the available evidence indicates that the natural human diet is omnivorous and would include meat. We are not, however, required to consume animal protein. We have a choice.
The Great Apes
There are very few frugivores amongst the mammals in general, and primates in particular. The only apes that are predominantly fruit eaters (gibbons and siamangs) are atypical for apes in many behavioral and ecological respects and eat substantial amounts of vegetation. Orangutans are similar, with no observations in the wild of eating meat.
Gorillas are more typically vegetarian, with less emphasis on fruit. Several years ago a very elegant study was done on the relationship between body size and diet in primates (and some other mammal groups). The only primates on the list with pure diets were the very small species (which are entirely insectivorous) and the largest (which specialize in vegetarian diet). However, the spectrum of dietary preferences reflect the daily food intake needs of each body size and the relative availability of food resources in a tropical forest. Our closest relatives among the apes are the chimpanzees (i.e., anatomically, behaviorally, genetically, and evolutionarily), who frequently kill and eat other mammals (including other primates).
Evidence of Humans as Omnivores
Archeological Record
As far back as it can be traced, clearly the archeological record indicates an omnivorous diet for humans that included meat. Our ancestry is among the hunter/gatherers from the beginning. Once domestication of food sources began, it included both animals and plants.
Cell Types
Relative number and distribution of cell types, as well as structural specializations, are more important than overall length of the intestine to determining a typical diet. Dogs are typical carnivores, but their intestinal characteristics have more in common with omnivores. Wolves eat quite a lot of plant material.
Fermenting Vats
Nearly all plant eaters have fermenting vats (enlarged chambers where foods sits and microbes attack it). Ruminants like cattle and deer have forward sacs derived from remodeled esophagus and stomach. Horses, rhinos, and colobine monkeys have posterior, hindgut sacs. Humans have no such specializations.
Jaws
Although evidence on the structure and function of human hands and jaws, behavior, and evolutionary history also either support an omnivorous diet or fail to support strict vegetarianism, the best evidence comes from our teeth.
The short canines in humans are a functional consequence of the enlarged cranium and associated reduction of the size of the jaws. In primates, canines function as both defense weapons and visual threat devices. Interestingly, the primates with the largest canines (gorillas and gelada baboons) both have basically vegetarian diets. In archeological sites, broken human molars are most often confused with broken premolars and molars of pigs, a classic omnivore. On the other hand, some herbivores have well-developed incisors that are often mistaken for those of human teeth when found in archeological excavations.
Salivary Glands
These indicate we could be omnivores. Saliva and urine data vary, depending on diet, not taxonomic group.
Intestines
Intestinal absorption is a surface area, not linear problem. Dogs (which are carnivores) have intestinal specializations more characteristic of omnivores than carnivores such as cats. The relative number of crypts and cell types is a better indication of diet than simple length. We are intermediate between the two groups.
Conclusion
Humans are classic examples of omnivores in all relevant anatomical traits. There is no basis in anatomy or physiology for the assumption that humans are pre-adapted to the vegetarian diet. For that reason, the best arguments in support of a meat-free diet remain ecological, ethical, and health concerns.
[Dr. McArdle is a vegetarian and currently Scientific Advisor to The American Anti-Vivisection Society. He is an anatomist and a primatologist.]
Source: The Vegetarian Resource Group.