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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 6A)

What Comparative Anatomy Does
& Doesn't Tell Us about Human Diet

Primate Body Size

Link between body size and diet is not strict. To a certain degree, body size may be a predictor of diet in (land) mammals, including primates, as diet and body size are correlated. Note that the relationship is correlation, and not necessarily causation. Contrary to the claims made by some fruitarian extremists, the link between body size and diet is neither strict nor absolute. In primates, body size is, at best, a poor predictor of diets.

Correlation of body size and diet in primates. The concepts behind the claims regarding body size vs. diet are explained by Richard [1995, p. 184]:

Small mammals have high energy requirements per unit body weight, although their total requirements are small. This means that they must eat easily digested foods that can be processed fast; however, these foods do not have to be abundant, since they are not needed in large quantities. Large mammals have lower energy requirements per unit body weight than small ones and can afford to process food more slowly, but their total food requirements are great. This means their food items must be abundant but not necessarily easy to digest.

Richard [1995, figure 5.13, p. 185] also provides frequency distributions of body weights, by diet, for a large set of primates. The frequency distributions indicate the order of generally increasing body sizes, by diet, is:

Humans are an exception to the rule. Given the frequency distributions in Richard [1995], the average weight of humans (65 kg) appears to be at or above the upper limit of the range for frugivores eating leaves, and within the folivore range. This would suggest, if the "rule" relating body size and diet were strict, that the most likely diet for a primate the size of a human would be a diet in which leaves are the primary food, and fruit a minor/secondary food. Of course, that is a sharply different diet from the nearly 100% fruit diet advocated by certain extremists who claim that the body size/diet "rule" indicates a strict fruitarian diet for humans.

Richard [1995, p. 186] provides further insight into the alleged "rule":

Within broad limits, then, body weight predicts the general nature of a primate's diet. Still, there are exceptions to the rule, and body weight is not always a good predictor even of general trends...

[W]hen finer comparisons are made among the species the rule loses its value altogether. Specifically, similarly sized species often have quite different diets.

Specific human features imply dramatic breakthrough in diet. Milton [1987, p. 106] also comments on the "rule":

In contrast [to australopithecines], members of the genus Homo show thinner molar enamel, a dramatic reduction in cheek tooth size, and considerable cranial expansion (Grine 1981; McHenry 1982; S. Ambrose, pers. comm.). In combination, these dental and cranial features, as well as an increase in body size, apparently with no loss of mobility or sociality, strongly imply that early members of the genus Homo made a dramatic breakthrough with respect to diet--a breakthrough that enabled them to circumvent the nutritional constraints imposed on body size increases in the apes.

The above quotes from Richard [1995] and Milton [1987] reflect that the body size "rule" is not strict, and that humans are a specific example of a species that has overcome it.

Additionally, recall that our preceding section on brain evolution discussed how human energy metabolism is dramatically different from all other primates. We humans expend considerably more energy on our brains--energy that is apparently made available by our reduced gut size, which itself is made possible via a higher-quality diet. By changing the internal energy budget to supply our large brains with energy, humans have also overcome the "rule" on primate body size.

Analysis of Head, Oral Features, and Hands

Introduction: the main claims

Oversimplistic either/or views of carnivorous adaptations. The various comparative "proofs" of vegetarianism often rely heavily on comparisons with true carnivores, e.g., lions, tigers, etc. The basic arguments made are that meat cannot be a "natural" food for humans, because humans don't have the same physical features characteristic of the small group of animals that are considered to be "true" carnivores. However, such arguments are based on an oversimplified and flawed view of adaptation; that is, the underlying assumption being that there is only one--or at least one main set of--physical adaptation(s) consistent with eating meat. Such arguments also ignore the high intelligence and adaptive behavior of humans.

Let's examine some of the claims made in Mills' The Comparative Anatomy of Eating, plus a few additional claims made in a recent extremist fruitarian book. (Out of politeness and to avoid what some might construe as inflammatory, the latter book will not be identified other than to say that it is a massive plagiarism of the book Raw Eating, by Arshavir Ter Hovannessian--a book published in English in Iran in the 1960s.)

Flawed comparisons that pit humans against "true" carnivores. The claims are made that the human body lacks the features of certain carnivores, hence we should not eat meat, as it is not "natural." The differences are as follows; carnivores have, but humans do not have:

In The Comparative Anatomy of Eating, Mills summarizes the above analysis with the statement that:

An animal which captures, kills and eats prey must have the physical equipment which makes predation practical and efficient.

We shall see that this claim by Mills is incorrect if one interprets it only in morphological terms. Without further delay, let's now examine the above claims.

Logical problems of the comparative proofs

The above physical comparisons are accurate--clearly, humans do not have the jaws, teeth, or claws of a lion. However, to use this information to conclude that humans "cannot" eat meat, or "must have" the same physical traits as other predators to do so, or did not adapt to meat in the diet, is logically invalid and bogus. A short list of errors in reaching the above conclusion is as follows:

Examining comparative claims about the head, oral features, and hands

Now let's examine, in the subsections that follow, some of the claims of the comparative proofs (regarding the head, hands, oral features, etc.) in light of knowledge of prehistoric diets and evolutionary adaptation.


(Evolution of the Human Oral System and Its Relevance to Diet, cont.)

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GO TO PART 1 - Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?

GO TO PART 2 - Looking at Ape Diets: Myths, Realities, and Rationalizations

GO TO PART 3 - The Fossil-Record Evidence about Human Diet

GO TO PART 4 - Intelligence, Evolution of the Human Brain, and Diet

GO TO PART 5 - Limitations on Comparative Dietary Proofs

GO TO PART 6 - What Comparative Anatomy Does and Doesn't Tell Us about Human Diet

GO TO PART 7 - Insights about Human Nutrition & Digestion from Comparative Physiology

GO TO PART 8 - Further Issues in the Debate over Omnivorous vs. Vegetarian Diets

GO TO PART 9 - Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Back to Research-Based Appraisals of Alternative Diet Lore

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