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

Overview of Gut (Digestive System)
Morphology in Primates and Humans


After the various claims of the comparative proofs that "humans can't eat meat because they lack claws, fangs, etc.," the next set of claims made concerns the digestive system. Typically, a number of features of the digestive system are compared: stomach (type, relative capacity, etc.), length of small intestine, and so on. (See The Comparative Anatomy of Eating, by Mills, for a longer list.)

Diet categories in comparative proofs are typically narrow. The comparisons made are relatively simplistic, and the thinking process involved is also usually rather narrow: herbivores vs. carnivores vs. humans (and omnivores are sometimes added to the list). In referring to narrow thinking here, we mean that the comparative proofs neatly define their dietary categories in a narrow way, and ignore the fact that in nature, diets are usually not strict (i.e., "pure" diets are rare). The comparison of digestive system features comprises the second major component of many comparative proofs of diet.

Other shortcomings of typical comparative proofs. Needless to say, the comparative proofs suffer from many of the limitations identified in previous sections--subjective list construction, assuming the form/function linkage is strict, ignoring the reality of modern knowledge of ape diets, resolution problem (analysis may be on too gross a level), and so on.

Preview of this section. Instead of listing the numerous claims of Mills and the other proofs, and analyzing them directly for possible shortcomings, this paper takes a different approach. First, we'll review the work of Milton [1987], as it nicely illustrates that the human gut is (nearly) unique--even when compared with the great apes. Then we'll review an excellent series of papers: Chivers and Hladik [1980, 1984], Martin et al. [1985], and MacLarnon et al. [1986], which are the most authoritative analyses of gut morphology done to date. Also, Sussman [1987] and Hladik et al. [1999] are discussed, for they provide additional insight into the major papers on gut morphology. Finally, the different definitions of the term "omnivore" are explored, including how those differences have been exploited in an intellectually dishonest way (in my opinion) by a fruitarian extremist.

Humans are unique, and the human gut is (nearly) unique

Since we will be comparing the gut (digestive system) of humans with other animals, especially primates, it is appropriate to begin with an illustration that provides a comparison.

Figure: Gastrointestinal tracts of a chimpanzee, orangutan, an adult human, and human fetus.

Milton [1987, p. 102] notes:

When compared to those of most other mammals, the relative proportions of the human gut are unusual (my calculations, using data from Chivers and Hladik [1980], and Hladik [1967]).

Human gut small compared to apes. Observing that human gut proportions are different from those found in carnivores, herbivores, swine (an omnivore), and even most other primates, including the anthropoid apes, Milton [1987, p. 101] notes that "...the size of the human gut relative to body mass is small in comparison with other anthropoids (R.D. Martin, pers. comm.)." Milton [1987] includes a table (3.2, p. 99) that compares the relative volumes of the different parts of the gut for selected hominid species. The table shows the stomach at 10-24% of total gut volume in humans, while for orangs and chimps it is 17-20%. The small intestine is 56-67% of total gut volume in humans, 23-28% in orangs and chimps. And the colon is 17-23% of total gut volume in humans, while it is 52-54% in orangs and chimps. The percentages quoted in the preceding sentence are unscaled, i.e. are not scaled for inter-specific differences in body size. Despite this, the figures are useful to compare patterns of gut proportions, and the general pattern is clear: humans have "large" intestines, while chimps and orangs have "large" colons.

Additionally, Milton [1987] discusses two primates whose gut proportions appear to roughly match those of humans:

Gut characteristics reflect dietary quality. Like humans, Capuchin monkeys and savanna baboons make extensive use of their hands for pre-processing of food items. Milton concludes that the similarity in gut proportions reflects adaptation to high-quality diets [Milton 1987, p. 103]:

Rather, it appears to represent similar adaptive trends in gut morphology in response to diets made up of unusually high-quality dietary items that are capable of being digested and absorbed primarily in the small intestine.

The plasticity (or elasticity) of the human gut--that is, how the proportions can change to accommodate temporary fluctuations in diet--is discussed in Milton [1987]. (The topic will be addressed later here.) She predicts that further studies will (continue to) show that the human gut is dominated by the small intestine, with high variability in colon size due to temporary changes in diet.


(Quantitative Analysis of Gut Morphology in Primates and Humans)

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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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