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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 5A)

Limitations on Comparative
Dietary Proofs

This section discusses the limitations--structural and logical--inherent in the various comparative "proofs" of particular diets. Such "proofs" typically focus on comparative anatomy and comparative physiology, though they often include other elements as well. The first part of this section provides a list, with explanations, of the various limitations. The second part discusses some real-world examples that illustrate the difficulties in using comparative studies as "proof."

Editorial note: In the following, the term "comparative proofs" refers to the comparative proofs for particular diets. It is not a general reference to any/all comparative studies.

Logical and Structural Limitations

Comparative "proofs" of diet are subjective and indeterminate; i.e., they do not provide actual proof of diet.

The "proof" in Fit Food for Humanity that claims humans are natural vegetarians was discussed in an earlier section. This "proof" groups all the anthropoid apes into the vegetarian category. However, we now know that the diet of the great apes is not strictly vegetarian, so let's have some fun and consider the following logical exercise.

First, let's modify the "proof" in Fit Food for Humanity as follows. Based on the more recent knowledge now available, replace the collective category for vegetarian apes with two more-well-defined (and accurate) ones: first, with a category consisting solely of the gorilla (a folivore); then second, with a category consisting solely of the chimpanzee (a frugivore/omnivore). The comparative information in Groves [1986] and Aiello and Dean [1990] might be helpful in producing the new comparison tables. This yields two new comparative "proofs" similar to the one in Fit Food for Humanity: one "proof" that shows humans are folivores, and yet another "proof" that humans are frugivores/omnivores.

Next, note that Mills (The Comparative Anatomy of Eating) provides a lengthy "proof" that purports to show humans are herbivores. Finally, note that Voegtlin [1975] as cited in Fallon and Enig [1997], provides a comparative anatomy analysis that indicates humans are actually natural carnivores. See Functional and Structural Comparison of Man's Digestive Tract with that of a Dog or Sheep (offsite) for a comparative anatomy analysis from Voegtlin [1975].

Four "proofs"--but what do they prove? Thus, as a result of producing and collecting the "proofs" cited above, we will have 4 different types of comparative "proofs":

Then, we simply note that the above cannot all be true as they are contradictory. This points to the conclusion that comparative "proofs" of diet are subjective and indeterminate--that is, they are not hard proof of anything.

It should be mentioned here that some who present comparative "proofs" openly admit their subjective nature (though the point is often not given proper emphasis, for the obvious reasons), whereas others are not so candid.

There is no logical validation of the list comparison process.

That is, there is no logical validation for the implicit assumption that comparing two or more species via a list of features constitutes proof that a particular diet is natural or optimal. Comparative studies can be powerful tools for analysis, but there is so much variation in nature that one cannot logically derive a "should" (i.e., you should eat this diet) from an "is" derived from other species' diets (i.e., that other, allegedly similar species, eat the same type of diet). The presence of substantial morphological similarity between two different species does not prove, or even imply, that their diets must be similar. The idea that in nature a "should" can be derived from an "is" (of another species) in such a fashion is a major logical fallacy of the comparative "proofs" for particular diets.

Additionally, one other point bears repeating here. Let us pretend that comparison of a list of features somehow constitutes hard "proof" for a specific diet. It then follows that one must ask relevant questions, listed below:

For the obvious reasons (they are too heavily subjective), the above questions are unanswered. The advocates of comparative "proofs" are assuming that a matching list somehow "proves" their claims. In reality, it proves nothing--a matching list is evidence of possible association or similarity; it helps to define hypotheses, but it does not provide definitive proof of a hypothesis. As Ridley [1983, p. 34] points out:

The comparative method is used to look for associations (between the characters of organisms) which natural selection, or some other causal principle, might be expected to give rise to. It can test hypotheses about the cause of adaptive patterns. The result of this test is an association or correlation. The mere association of two characters, A and B, does not tell us whether in nature A was the cause of B, or B of A.

The comparative "proofs" focus only on similarities while ignoring differences.

In legitimate applications of comparative anatomy and/or physiology, the differences--those features that make each species unique--are of central interest. In the comparative "proofs" for diets, the differences are often ignored or rationalized away. (That is, any differences that inhibit promotion of the target diet will be rationalized or ignored.)

By ignoring or rationalizing away the differences, the comparative "proofs" of diet render themselves logically invalid. In a discussion of the topic of comparative studies of adaptation, Ridley [1983, p. 8] observes:

The most rudimentary kinds of comparative study are those which, after looking through the literature more or less thoroughly, present a list of only the species (or some of them) which conform to the hypothesis... Such a list may usefully demonstrate that a trait is taxonomically widespread. Or it may suggest a hypothesis. But it cannot test one. If the method does not also list the species that do not conform to the hypothesis it is analogous to selecting only the supporting part of the results from a mass of experimental data...

The very first requirement, then, for a test of a comparative hypothesis is that the criterion for including a species in the test is independent of whether it supports the hypothesis. This criterion may seem obvious; it may seem that it does not require statement. But it is enough to rule out the entire [comparative] literature (with one exception) from before about 1960. That exception is the French biologist Etienne Rabaud.

The basic problem of studying similarities while deliberately ignoring differences is that it constitutes picking-and-choosing the data to conform to the particular hypotheses of interest. Note: the studies of Rabaud (mentioned above) are discussed further, later herein.

The comparative "proofs" ignore the fossil record and evolution.

The comparative "proofs" are dated and do not reflect current knowledge of ape diets.

The comparative "proofs" assume dietary categories are discrete (distinct), while in nature diets form a continuum.

The above points were covered in previous sections.

The comparative "proofs" ignore the important features that make humans unique in nature.

In particular, these features are generally ignored:

As discussed in the previous section, the topic of evolution of the brain vs. diet is usually not covered. Our brain allows us to develop technology (e.g., tools and cooking), which can impact our morphology via the behavior/culture evolutionary feedback loop. We will see later that human intelligence, expressed via tools and behavior, allows humans to easily overcome many of the alleged limits on adaptation that the comparative "proofs" discuss.

The "Expensive Tissue Hypothesis" [Aiello and Wheeler 1995] and related research discussed in the previous section suggest that the evolution of our brain and higher human intelligence is in part the result of a diet that included significant amounts of animal foods. The energy budget and life history events of humans are unique, relevant, and should be covered in a legitimate comparative study. The comparative "proofs" ignore these facts.

Comparative "proofs" assume the form/function connection is strict and necessary.

Here the word "necessary" is used in its logical sense; i.e., that function necessarily follows form, and that the form/function linkage is strict. This is a critical assumption underlying the comparative "proofs" that promote specific diets. However, the assumption is incorrect and logically invalid.

Similar functions can be served by dissimilar forms. John McArdle, Ph.D., a scientific advisor to the American Anti-Vivisection Society notes [McArdle 1996, p. 173]:

It is also important to remember that the relation between 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.

The form/function relationship can be dissimilar even in closely related species. Sussman [1987] comments that (p. 153):

When comparing species-specific dietary patterns between different primate species, we often find that these patterns are not dependent upon taxonomic relationships. Closely related species often have very different diets, whereas many unrelated species have convergent dietary patterns. However, we would assume that at some level there is a relationship between dietary patterns, foraging patterns, and anatomical and physiological adaptations.

Overall systemic constraints as important as individual functions/forms. Finally, Zuckerman [1933] reminds us of important systematic restraints (pp. 12-13):

Function cannot be stressed more than form. Their separation from each other, and their individual isolation from the intact working organism, are both essential, but to a large extent necessarily incomplete, operations. The same is true of the abstraction of biochemical facts. Form, chemistry, and function are indissolubly united, and it may truly be said that from the taxonomic standpoint all characters of the body, whatever their nature, have a fundamental equivalency... To discuss them on the basis of ex cathedra judgements [i.e., via subjective assessments only] of their relationship to heritage or habitus is to attempt the impossible, and to ignore in greater part the lesson taught by the experimental study of genetics and growth.

Research on the form/function linkage

Some examples of research studies that tried to document a form/function linkage are as follows:

Thus we see that a narrowly focused study, within a closely related group of species, might find possible evidence of a form/function linkage. Of course, in sharp contrast, the comparative "proofs" of diet are overly broad, and their conclusions dubious.

As the form/function linkage assumption is such a critical part of the comparative "proofs" of diets, we will next examine additional aspects of the form/function relationship, including specific examples of forms that can serve more than one function.


(Limitations on Comparative Dietary Proofs, cont., plus Counterexamples)

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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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