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setembro 09, 2000

Comparative Anatomy and Physiology

Are Humans Natural Frugivores/Vegetarians,
or Omnivores/Faunivores?

by Tom Billings
Copyright © 1999 by Thomas E. Billings. All rights reserved.
Contact author for permission to republish.

A detailed TABLE OF CONTENTS linked to all portions of the article can be
found at the bottom of this first page of introduction

An Invitation to Readers

If you are--or have ever been--involved in alternative diets or vegetarianism, you have probably heard or read claims that comparative anatomy and/or comparative physiology "proves" or (in more conservative language) "provides powerful evidence" that humans are "natural" fruitarians, vegetarians, or even omnivores. This paper will assess such claims and the evidence supporting them, but we first need to ask:

  • Whether comparative anatomy and physiology provide actual hard proof of the precise composition of the "natural" human diet, or if they merely provide general indications of possible diets.
  • Then, we want to go beyond the typical simplistic analyses presented in the vegetarian and alternative diet lore, and reexamine what information comparative anatomy and physiology actually provide regarding the natural diet of humans.
  • Further, a number of related claims are often made as part of such comparative "proofs." Some of these claims addressed here are:
    • Does "instinct" (whatever that is) "prove" that we are natural vegetarians?
    • Does research that shows the typical Western meat-based diet is unhealthy prove that all omnivore diets are unhealthy?
    • And, since it is mentioned in the subtitle, just what is a faunivore, anyway?
If these questions interest you, I invite you along for the ride. But first, open your mind and fasten your seat belt--the ride may be bumpier than you expect!

If your time is limited, or you have specific interests...

As this paper addresses numerous, diverse topics by necessity (some in depth), it is lengthy and this may present obstacles for some readers. Also, considerable background information must be covered before the main claims of the major comparative "proofs" of diet can be addressed.
To help navigate the numerous topics, to gain a quick bird's-eye view, or to read the paper in sections as time permits, the comprehensive Table of Contents below provides direct links to all the major sections and subsections of the paper.

T A B L E   O F   C O N T E N T S

  • PART 1: Brief Overview: What is the Relevance of Comparative Anatomical and Physiological "Proofs"?
  • PART 2: Looking at Ape Diets: Myths, Realities, and Rationalizations
  • PART 3: The Fossil-Record Evidence about Human Diet
  • PART 4: Intelligence, Evolution of the Human Brain, and Diet
  • PART 5: Limitations on Comparative Dietary Proofs
  • PART 6: What Comparative Anatomy Does and Doesn't Tell Us about Human Diet
  • PART 7: Insights about Human Nutrition and Digestion from Comparative Physiology
  • PART 8: Further Issues in the Debate over Omnivorous vs. Vegetarian Diets
  • PART 9: Conclusions: The End, or The Beginning of a New Approach to Your Diet?

Brief Overview: What is the
Relevance of Comparative Anatomical
and Physiological Proofs?

Looking at Ape Diets--Myths, Realities,
and Rationalizations



The Fossil Record Evidence
about Human Diet

Intelligence, Evolution of the
Human Brain, and Diet

  • Introduction: claims of the comparative "proofs"
    • Human intelligence ignored or rationalized
    • Brain size discounted
    • Hidden, contradictory views on the value of intelligence
    • Recent evolutionary research now emphasizes the interaction of diet and brain development
  • Encephalization
    • "Expected" vs. actual brain size
    • Kleiber's Law
    • Brain and digestive system compete for limited share of metabolic energy budget
    • A comparative anatomy analysis of primate brains
      • Humans at top of primate scale
      • Large gap between humans and great apes
      • Brain enlargement disproportional
  • Factors in encephalization: energy (metabolism) and diet
    • Extensive energy required for brain growth
    • Dietary quality is correlated with brain size
    • Dietary shift beginning with Homo
    • How dietary quality relates to the brain's share of total metabolic budget
      • Dramatic changes in last 4 million years
      • Human brain's metabolic budget significantly different from apes
      • Human brain MR (metabolic rate) 3.5 times higher than apes
      • Humans depart from normal dietary quality (DQ)/body-weight relationship
    • The paradox: Where does the energy for the large human brain come from?
  • The relationship of dietary quality and gut efficiency to brain size
  • Fruitarian evolution: science fact or science fiction?
    • Vague claims about an ancient frugivorous primate ancestor
    • Crank science and logical fallacies used in support of claims of fruitarian evolution
    • Fruitarian denial of physiological evolution/adaptation
  • Analysis of the fruitarian claims
    • Claim: Morphological evolution is "easy," physiological evolution is nearly impossible.
    • Claim: The digestive system is extremely complex. For it to evolve in a mere 2.5 million years is simply not possible.
    • Claim: (twisted quote) Expert opinion is that physiological evolution is highly unlikely.
    • Claim: There is no convincing teleonomy proof that humans have adapted to a diet that includes meat!
    • Claim: There are no examples of animals evolving backwards from a vegetarian to an omnivorous diet!
    • Claim: If humans are adapted to eating meat, what exactly are those adaptations?
  • Further evidence against the claims of fruitarian evolution
    • Quadrupedal vs. bipedal adaptations and tree-climbing/fruit-picking
    • Actual fruit availability/dependability fluctuates with location and time
    • Different adaptations, different tradeoffs
    • Adaptations must be interpreted in context
    • Body size and tree-climbing
    • Fruitarian nutrition vs. brain evolution
      • Vitamins, minerals, and fats
      • Evolution/survival on pure fruit diets not possible in nature given fluctuations in fruit availability
  • Synopsis and section summary

Limitations on Comparative
Dietary Proofs

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

  • Primate body size
    • Link between body size and diet is not strict
    • Correlation of body size and diet in primates
    • Humans are an exception to the rule
    • Specific human features imply dramatic breakthrough in diet
  • Analysis of head, oral features, and hands
    • Logical problems of the comparative proofs
      • Focusing purely on carnivorous adaptations rather than omnivorous ones
      • Invalid black-and-white views
      • Various oversimplistic assumptions
      • Overlooked differences in adaptive behavior
      • Impact of tool use and language on morphology disregarded
      • Obvious explanations rationalized away as "illegitimate"
    • Examining comparative claims about the head, oral features, and hands
      • Cheek pouches: an example of vague claims
      • Hands vs. claws
        • Simple tool-based technology part of human evolutionary adaptation
        • Special tree-climbing adaptations lacking in humans
      • Jaw joints and jaw motion
        • Oversimplifications in comparative "proofs"
        • Considering detailed features in isolation is misleading
      • Teeth
      • Salivary glands and saliva
      • Teeth and jaw: structure and mastication muscles
        • Hominid dental system is small relative to apes and has decreased in size over evolutionary time
        • Potential effect of primitive food processing technology
        • Universal cultural/technological innovations can reduce/change selection pressures
        • Appearance of modern human form corresponds with reduced dentition
  • Other selection pressures on head and oral features: brain size, posture, and language
    • Selection pressures are multiple and competing rather than solitary
    • Increasing brain size reduces space available for oral features
    • The influence of language on the oral system
    • Selection pressures on humans due to language are unique
    • Design tradeoffs in evolution of the human oral system due to speech/language
    • Linkage of language and brain development in evolution of the human oral system
  • Interaction of evolutionary selection pressures on the human oral system (flowchart)
    • Human oral system forged as an evolutionary compromise
  • Overview of gut (digestive system) morphology in primates and humans
    • Diet categories in comparative proofs are typically narrow
    • Other shortcomings of typical comparative proofs
    • Humans are unique, and the human gut is (nearly) unique
      • Human gut small compared to apes
      • Gut characteristics reflect dietary quality
  • Quantitative analysis of gut morphology in primates and humans
    • The research of Chivers and Hladik [1980, 1984]
      • Dietary categories reflect a continuum, not sharp divisions
      • Typical gut of faunivores
      • Folivore gut characteristics
      • Frugivore guts are variable
      • Faunivory and folivory are the endpoints with frugivory intermediate
      • Determining/measuring relevant gut characteristics is non-trivial
      • Gut differentiation coefficients calculated
      • Index of gut specialization
      • Dietary index: what the animal actually eats
    • A footnote on Chivers and Hladik: human gut morphology
    • The research of Martin et al. [1985]
      • Improved methodology
      • GI quotients for 4 different digestive system components
      • Human GI quotient pattern typical of faunivores
      • Meaningful dietary groupings based on statistical analysis of GI quotients
    • The research of MacLarnon et al. [1986]
      • Refinement needed in analytical techniques used in earlier study
      • Data were reanalyzed in their original, untransformed logarithmic form
      • Primates only analysis: humans and Capuchin monkeys are outliers
      • MDS analysis of a more complete data set: humans grouped (again) with faunivores
      • Human GI tract shows possible faunivore adaptations
    • Assessing Martin et al. [1985], Maclarnon et al. [1986], and related studies
      • Further study needed: small sample size, gut variability
      • Different analytical techniques, different results: the confusion factor
    • The note of Hladik et al. [1999]
      • Humans are frugivores by one measure, faunivores by another
      • Gut surface areas might not support Expensive Tissue Hypothesis
      • Humans fail on raw, ape-style frugivore diets, but thrive on faunivore diets
      • Insects (or comparable animal foods) a part of natural human diet
    • On the term "omnivore," and misuse of quotes
      • Definition of "omnivore" a critical point
        • Omnivore: the common definition
        • Omnivore: Chivers uses the term differently
        • "Omnivore" a vague term lacking in relevance for GI tract functions
        • Contradictory claims about omnivores: which is correct?
        • More examples of out-of-context quoting by dietary extremists
      • Misuse of other quotes

    PART 7
    Insights about Human Nutrition and Digestion
    from Comparative Physiology

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

    • Preface and overview
    • Two fruitarian claims
      • Anti-protein theories: prime examples of crank science
      • Fruitarian straw argument: a pristine preindustrial world
    • Hunter-gatherers: examples of healthy omnivores
      • Environmental factors
        • Malnutrition and starvation
        • Occupational hazards of hunter-gatherer lifestyles
      • Chronic diseases rare in hunter-gatherer societies
      • Incidence of specific diseases
      • Biomarkers
      • Health status of the Aborigines of Australia: O'Dea [1991]
        • Health of Australian Aborigines as hunter-gatherers (chart)
        • Comparison of lifestyle and diet of Australian Aborigines as hunter-gatherers and after Westernization (chart)
    • Which omnivore diet? The "omnivorism = Western diet" fallacy.
      • Fallacy prevalent among scientifically oriented vegans as well as extremists
      • Clinical studies implicitly use those eating Western diets as the "omnivorous control" group
      • Omnivorous hunter-gatherer diets differ from the standard Western diet in important respects
    • Logical fallacies and the misinterpretation of research
      • Common logical fallacies in studies that compare omnivorous vs. vegetarian diets
      • Results from studies using domesticated/feedlot meat cannot be generalized to all omnivore diets
      • Logically invalid extrapolation from SAD/SWD diets to all omnivore diets
      • How reliable are animal studies that use domesticated/feedlot meat?
      • Clinical studies based on the standard Western diet
        • Fallacious claim: One type of veg*n diet vs. all omnivore/faunivore diets
        • Unconscious double standard
      • Examples of incorrectly citing clinical studies based on the SAD/SWD diet
    • Drawbacks to relying exclusively on clinical studies of diet
      • Narrow-minded and discounts other important evidence
      • Clinical studies are based on statistical averages of group data and thus vulnerable to uncontrolled-for individual (genetic) differences
      • Successful long-term veg*ns are self-selecting; dropouts or examples of "failure to thrive" automatically excluded in longitudinal studies of pre-existing veg*n groups
      • Widespread prevalence of moral ostracism by successful veg*ns results in self-censoring of negative results by dropouts
      • Anecdotal observations suggest dietary (animal-food) "exceptions" not unheard of among veg*ns (commonly overlooked in clinical protocols with respect to deficiency issues)
      • Other factors in evaluating clinical studies
    • The Cornell China Project: authoritative proof, or misinterpretation by dietary advocates? Examining the vegan claims.
      • Statistical and other limitations of the China Study
      • Attempts to use the China Study to prove that all omnivore diets are bad is yet another logical fallacy
      • Cancer, veg*n diets, and the China Study
    • Instinct vs. intelligence in diet: where is the line?
      • The difficulty of distinguishing between instinct and intelligence
      • Eating animal foods, part 1: instinct or intelligence?
      • Eating animal foods, part 2: an evolutionary view
      • Eating animal foods, part 3: morality and naturalism
    • Examining fruitarian claims about instinct in food selection
      • Claim: Humans are limited to a narrow diet (nearly 100% fruit) by our genetic code.
      • Claim: The "instinct" to hunt and kill animals is not found in every human, hence it cannot be an instinct.
      • Claim: Children instinctively choose sweet foods, like fruit.
      • Claim: Eating meat is a learned behavior, and not instinctive. Humans have been eating meat for "only" 2 million years.
      • Claim: Some humans are disgusted at the thought of eating meat. How could that happen to a true carnivore?
      • Claim: True carnivores often eat (only) their prey's internal organs and leave the muscle for the vultures. Why don't human meat-eaters behave this way?
      • Claim: Primates have adaptations exclusively for fruit eating--vision, hands, etc. True carnivores usually do not have adaptations for fruit-eating.
      • Claim: True carnivores hunt by smell alone; they don't need technology.
      • Claim: The great apes, except for chimps, are strict vegetarians.
      • Claim: Hunting by chimps is not instinctive.
      • Claim: Meat-eating by humans cannot be instinctive because humans don't eat the specific monkeys that chimps hunt.
      • Claim: Humans cannot eat meat because we lack fangs, claws, sharp teeth.
      • Claim: Humans are fully upright and bipedal, and this makes us ineffective at hunting.
      • Claim: Due to body size "rules," large mammals (like humans) don't have to eat flesh.
      • Claim: Cooking was needed because eating raw meat introduced parasites.
      • Claim: (twisted misquote of expert) Humans are not adapted to be omnivores.

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

    • Contrary facts vs. vegan dogma: facing the honesty problem
      • Synopsis of the primary evidence (conclusions): Humans can be regarded as natural omnivores, and we are not natural vegetarians
      • Veg*n diets are not the natural diet of humans
    • Failure to thrive (FTT)
      • Your health is more important than raw/veg*n dietary dogma
      • Feelings of superiority and recitation of mantras result in blocking of contrary information
      • Excuses and rationalizations the "party line" reaction to FTT
      • How often are diet gurus honest and humble?
      • For some diet gurus, dietary dogma is more important than the health of their followers, and victims may be blamed if the diet doesnot work
      • Difficulty of resolving the question with nutritional calculations done "on paper," opinions from nutritionists, or with clinical studies done to date
    • Rationalizations about the evidence for omnivorous diets from the fossil record and comparative anatomy
      • Diet gurus excel at spinning rationalizations.
      • Rationalization: The fossil record is irrelevant because we were created, rather than evolved--evolution is nonsense!
      • Rationalization: Our prehistoric ancestors were acting "against their nature" by eating meat for 2.5 million years!
      • Rationalization: The evidence of modern ape diets is irrelevant because we are a unique species, and we evolved in a different environment (the African savanna) than the forest-dwelling great apes.
      • Rationalization: The human gut is far too elastic for comparative anatomy to tell us anything about our natural diet.
      • Rationalization: The evidence of comparative physiology is irrelevant because: we can get B-12 from unwashed produce; low bioavailability of some nutrients from plants is good because they are toxic except in tiny quantities; physiological measurements made on meat-eaters are invalid!
    • Rationalizations in response to the evolutionary and hunter-gatherer evidence for omnivorous diets
      • Rationalization: What happened back in the Paleolithic age doesn't really matter. We are different people today.
      • Rationalization: Evolution is concerned with reproductive success, not longevity. By the latter measure, vegetarian diets are an improvement on evolution.
      • Rationalization: Hunter-gatherers may eat some meat but they are not that far from being vegetarians. Plant foods predominate, and animal food is sporadic because hunting is usually unsuccessful/inefficient compared to gathering of plant food.
      • Rationalization: There are no vegan gatherer tribes because they have not been exposed to the "enlightened" philosophy of veg*nism.
      • Rationalization: The hunter-gatherer diet is not feasible for people living in modern times.



    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 and 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?

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