Quadrupedal vs. bipedal adaptations and tree-climbing. First, humans are fully upright and bipedal, while apes are quadrupedal (though chimps, bonobos, and gorillas, of course, do have limited bipedal abilities). A fully upright bipedal posture is a major disadvantage in tree-climbing compared to quadrupedal motion--it is easier and safer to navigate trees when one can grasp and hold with four extremities rather than two, as humans must. That is, a fully upright bipedal posture is a disadvantage, overall, in collecting fruit--and hence would reduce survival of the alleged "pure fruitarian" prehistoric humans or proto-humans.

Bipedalism/quadrupedalism and fruit-picking. The hypothesis has been advanced that bipedalism evolved because it is more efficient to pick small, low-growing fruits [Hunt 1994]. Standing up to pick low-growing fruits allows use of both hands to pick fruit, and may allow more efficient harvesting of small fruit from small trees. However, this hypothesis [Hunt 1994] that bipedalism evolved as a fruit-picking strategy, specifically refers to chimps and australopithecines. Even then, it is well-known that chimp diet is not exclusively fruit; their diet also includes small but nutritionally significant amounts of fauna (insects, meat), along with other foods such as leaves and pith. Also, chimps retain quadrupedal motion and arboreal capabilities which they use for efficient fruit collection in larger trees. Hunt [1994] also discusses special muscular adaptations in chimps for vertical climbing (trees) and hanging (from trees).

As for Australopithecus, radio-isotope studies indicate that Australopithecus was an omnivore/faunivore, and not a fruitarian. (See Sillen [1992], Lee-Thorp et al. [1994]; also Sillen et al. [1995].) Thus even if bipedalism did evolve initially as a specialized fruit-picking mechanism, the evolution occurred within the framework of an omnivorous/faunivorous diet (or a frugivorous diet that specifically included nutritionally significant amounts of fauna), not a strict fruitarian diet.

Actual fruit availability/dependability fluctuates with location and time. Note that the hypothesis of Hunt [1994] does not suggest or imply that chimps or Australopithecus had a fruit-only diet. Also note that even in a savanna environment, fruit is not limited to low-growing trees. Consider the major restrictions placed on the survival of our prehistoric ancestors (or even modern-day chimps) if their diet were limited to only fruit picked within 2-3 meters of the ground. Such a restriction would greatly reduce survival, not increase it.

A pure fruitarian needs a year-round supply of large amounts of fruit, and the ability to efficiently harvest as much of the available fruit as possible. That is, one needs to be able to efficiently pick most of an entire, tall tree--hence needs effective tree-climbing ability, that is, quadrupedal motion and arboreal adaptations.

Different adaptations, different tradeoffs. In contrast, an omnivore/faunivore for whom fruit is only a part of the diet, and who effectively hunts/harvests other high-calorie foods (e.g., animal foods) can easily afford the loss of overall efficiency in fruit harvesting that fully upright bipedalism (as in humans) involves. This is particularly true if bipedalism enhances the ability to harvest high-calorie foods (fauna) by increasing hunting efficiency. (Hunting efficiency is discussed in a later section.)

The set of adaptations of chimps--limited bipedal ability that improves efficiency in picking fruit from small trees, combined with quadrupedal/arboreal adaptations to efficiently harvest fruit from tall trees, appears to be the optimal adaptation set for a primarily frugivorous animal. However, humans clearly do not have all of these important survival adaptations. In regard to humans, Hunt [1994] argues that (p. 198):

Accordingly, scavenging, hunting, provisioning and carrying arguments for the origin of bipedalism (Shipmam, 1986; Sinclair et al., 1986; Carrier, 1984; Wheeler, 1984; Lovejoy, 1981; Jungers, 1991) are more convincing explanations for the refinement of locomotor bipedalism in Homo erectus, as are heat stress models (Wheeler, 1993).

Aiello [1992, p. 43] points out that:

In fact, at walking speeds human bipedalism is significantly more efficient than ape quadrupedalism.

Body size and tree-climbing. Finally, the body size of humans may be very questionable for a pure fruitarian. Humans are larger than orangutans, the most arboreal of the great apes, and we are larger than chimps. Hunt [1994] observed that larger chimps favored shorter trees, thereby minimizing the energy expended in climbing trees. The even larger body size of humans (compared to chimps or orangutans) also reduces our efficiency in harvesting fruit growing on the outer canopy of tall trees (where a substantial part of a tree's fruit is located), as our weight restricts us to relatively large branches. (Small branches could break, and falling out of trees certainly does not promote evolutionary survival.)

Temerin et al. [1984, p. 225] note that:

Arboreal travel is precarious at best, and increasing size makes compensatory demands for stability increasingly important. One response to this is to limit travel speeds.

In larger animals, [tree] climbing can be two to three times more costly in energy terms than horizontal locomotion, a relationship that may explain why arboreality is largely confined to smaller primates. The obvious exception to this pattern is the arboreal orang-utan--the largest arboreal mammal with an approximate average body weight of 55 kilograms. It is, however, both highly specialized in its anatomy and fairly lethargic, climbing slowly and deliberately.

• Vitamins, minerals, and fats. Fruit is deficient in vitamin B-12; pure fruit diets may be deficient in zinc, and even calcium. The EFA (essential fatty acid) balance in sweet fruit, the diet promoted by fruitarian extremists, is not optimal. Needless to say, vitamin B-12 (and mineral) deficiencies do not promote evolutionary survival, and a poor balance of EFAs does not promote brain development either. (Note: B-12, zinc, and EFAs are discussed in a later section herein.)

• Evolution/survival on pure fruit diets not possible in nature given fluctuations in fruit availability. Getting enough fruit to survive on, every day of the year, is unlikely even in a pristine, primary tropical rainforest. Preceding sections have discussed that orangutans apparently cannot get enough fruit in their remote tropical rainforest homes to live on fruit all year. Instead, they are forced to eat large amounts of leaves when fruit is scarce (see Knott [1998], MacKinnon [1974, 1977] as cited in Chapman and Chapman [1990]). If orangutans--quadrupeds with superb tree-climbing skills--cannot get enough fruit to survive on a (pure) fruitarian diet, it seems highly unlikely that humans (who are larger than orangutans, require more food, and are less adapted for tree-climbing) could consistently find enough fruit in a tropical rainforest (or savanna, where fruit is less plentiful), every day of the year, to satisfy their minimum calorie requirements.

  By the way, climbing tall trees to pick fruit (orangutans and other primates do not rely on fruit that falls to the ground--apparently inadequate as a sole food source) is hard, dangerous work and greatly increases daily calorie requirements, which further increases the amount of food/fruit required. One wonders if the fruitarian extremists who promote idealistic visions of life in a jungle paradise filled with fruit trees have ever tried to pick more than minimal quantities of wild fruit--especially in a rainforest where the lowest fruit-bearing branches on a tree may be 20-30 or more meters straight up.

  Finally, no doubt some fruitarian extremists will claim that adequate fruit to support pure fruit diets was in fact available in prehistoric times. This is of course nothing but speculation, without reference to any kind of credible paleobotanical evidence. Even more relevant, where is there evidence of an adequate year-round supply of fruit (adequate to support a fruit-only diet for our prehistoric ancestors) on the African savanna of 1-3 million years ago, in the days of Australopithecus, and subsequently in the early days of the newly evolved Homo genus?

  The obvious conclusion is that a pure fruitarian diet is not feasible for hunter-gatherers or our prehistoric ancestors, as fluctuations in the fruit supply make survival on a pure fruitarian diet unlikely in the short run, and a virtual impossibility in the long run. If you cannot survive on the diet, it certainly cannot support brain evolution, or any other kind of evolution.

• Humans are the most encephalized and intelligent species in all of nature.

• The human basal metabolic rate (BMR) is normal for our body size, yet we invest far more energy in our brains than other mammals. This is a paradox: how can we "afford" it?

• The increase in encephalization in humans appears to be compensated for by a comparable decrease in gut size.

• A decrease in gut size with constant body mass and BMR is possible only via an increase in dietary quality (DQ). Prior to the development of agriculture, the only feasible/economic way to increase DQ was via consumption of animal foods.

• The available evidence supports the idea that the consumption of animal foods, including meat, was an important factor in encephalization and in the evolution of the human brain. That is, your large brain--which helps you choose your diet (and argue with others about diet)--is apparently, according to the most plausible interpretation of the evidence, the result of the evolutionary legacy of ~2.5 million years of meat consumption by your ancestors.

• Much of the material in this section comes from paleoanthropology, and is based on an analysis of the available data. The net effect of so much evidence, all supporting the same conclusion (that animal foods were a driving force in brain evolution) is both powerful and convincing. However, some readers might dissent and remind us that although the evidence is substantial, parts of it may be subject to different interpretations. That, however, is a result of the nature of parts of the available evidence, e.g., the fossil record. In this case, those who dissent are faced with the daunting task of plausibly supporting a picture of how brain evolution could have occurred on a purely plant food/fruit diet without agriculture.

• Human brain size has decreased 11% in the last 35,000 years and 8% in the last 10,000--concurrent with decreasing animal food consumption during that time brought about by the advent of agricultural subsistence. This evidence provides some grounds for suggesting that the calorically/nutritionally dense animal tissues that presumably fueled human brain evolution may not be able to be completely/adequately substituted for by the calorically dense plant-based products of agriculture such as grains.

• The claim that humans evolved as fruitarians is an example of crank science; there is no credible scientific evidence to support such theories. No doubt certain fruitarian extremists will denounce the material in this section as "pseudoscience." In light of the fact that such adherents have no credible evidence to support their views, such allegations by fruitarian extremists are simply examples of hypocrisy and, perhaps, intellectual dishonesty.

• First, the previous section regarding the fossil record and the culture/evolution feedback loop--combined with the material of this section--yields a powerful argument that animal foods were, and are, a part of the human evolutionary diet, i.e., are natural.

• Second, as this section provides evidence that the evolution of the brain may be closely tied to diet, clearly, comparative "proofs" of diet that ignore intelligence and brain size are very dubious indeed. Unfortunately, the major comparative "proofs" do ignore intelligence. We shall see in coming sections how this oversight renders major portions of those "proofs" logically invalid.

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SEE REFERENCE LIST

SEE TABLE OF CONTENTS FOR:
PART 1 PART 2 PART 3 PART 4 PART 5 PART 6 PART 7 PART 8 PART 9

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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