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(Comparative Anatomy and Physiology Brought Up to Date--continued, Part 4A)

Intelligence, Evolution of the
Human Brain, and Diet

Introduction: Claims of the Comparative "Proofs"

Human intelligence ignored or rationalized. One of the key systematic restraints on the alleged comparative anatomy/physiology "proofs" that promote particular diets is such proofs generally do not consider the many important features that make humans unique in nature. In particular, human intelligence is usually ignored or dismissed (via rationalizations) in such "proofs." For example, Fit Food for Humanity asserts (p. 14):

But merely having a superior brain does not alter our anatomy and physiology which, according to natural law, remain characteristic of a total-vegetarian mammal, meant to eat a variety of vegetables, nuts and seeds.

Brain size discounted. Le Gros Clark is sometimes quoted by those advocating comparative "proofs" of vegetarian diets. He also appears to minimize the importance of large brains in humans (the term is "encephalization," discussed later herein). From Le Gros Clark [1964, pp. 4-5]:

In Homo the large size of the brain relative to the body weight is certainly a feature which distinguishes this genus from all other Hominoidea, but it actually represents no more than an extension of the trend toward a progressive elaboration of the brain shown in the evolution of related primates.

The attitudes stated in the quote from Fit Food for Humanity reflect an underlying denial of the importance of human intelligence, in particular its impact on behavior (and, ultimately on morphology via evolution). The attitude one finds in some raw/veg*n circles is that human intelligence is suspect because it allows us to "make errors," i.e., to eat foods different from those promoted by dietary advocates (who often behave as if they are absolutely 100% certain that they know better than you what foods you should eat).

Hidden, contradictory views on the value of intelligence. An irony here is that there is a contradiction in the logic of the attitudes of certain dietary advocates regarding intelligence. Some fruitarian extremists promote the alleged naturalness of fruitarian diets via the "humans are naked apes, without tools" myth discussed in the last section. This falsehood is often presented as actual science (needless to say, it is crank science) by those who promote it. Inasmuch as the advanced use of tools is an evolutionary characteristic of human intelligence, we can observe that those promoting the myth are saying that you should reject tool use in seeking your "natural" diet (this nonsense may even be presented as being scientific or logical). However, the preceding is equivalent to telling you to reject your intelligence, and even reject your status as a human being, in order to select the (allegedly) optimal diet.

The argument made by fruitarian extremists is thus contradictory; the argument can be stated as: Use your intelligence to agree with the extremist that humans are "naked apes, without tools," and thus reject, in the future, your use of intelligence in food choices. Another irony here is that some of the extremists promoting this false myth present themselves as "scientific." Crank science (or science fiction) is a more accurate description for such myths, however. The contradictory logic of the "naked ape" myth is a good example of the ambivalent, confused attitude toward intelligence displayed by some dietary advocates.

Recent evolutionary research now emphasizes the interaction of diet and brain development. Further, recent research has rendered the quotations above outdated. The remarks of Milton [1993] on the interaction between brain evolution and diet provide a brief introduction to a more modern perspective (p. 92):

Specialized carnivores and herbivores that abound in the African savannas were evolving at the same time as early humans, perhaps forcing them [humans] to become a new type of omnivore, one ultimately dependent on social and technological innovation, and thus, to a great extent, on brain power.

This section will review some of the research on the human brain, specifically:
We begin our review with the topic of encephalization, or brain size.



The most significant features that make humans unique in all of nature are our high intelligence and "large" brains. Here "large" means the brain is large relative to body size. Encephalization, or the relative size of the brain, is analyzed using a measure known as the encephalization quotient.

"Expected" vs. actual brain size. In order to measure encephalization, statistical models have been developed that compare body size with brain size across species, thereby enabling the estimation of the "expected" brain mass for a given species based on its body mass. The actual brain mass of a species compared to (divided by) its "expected" brain mass gives the encephalization quotient. Higher quotients indicate species with larger-than-expected brain sizes. Thus, a quotient greater than 1 indicates an actual brain mass greater than predicted, while quotients less than 1 indicate less-than-expected brain mass.

The encephalization quotient is important because it allows the quantitative study and comparison of brain sizes between different species by automatically adjusting for body size. For example, elephants, which are folivores, and certain carnivorous marine mammals have larger brains (actual physical mass) than humans. However, after adjusting for body size, humans have much "larger" brains than elephants or marine mammals. Additionally, the complexity of the brain is significant as well (and, of course, encephalization does not directly measure complexity--it only measures size).

Kleiber's Law. Kleiber's Law expresses the relationship between body size--specifically body mass--and body metabolic energy requirements, i.e., RMR (resting metabolic energy requirements), also known as BMR (basal metabolic energy requirements). The form of the equation is:

RMR = 70 * (W0.75)

where RMR is measured in kcal/day, and W = weight in kg. (The above is adapted from Leonard and Robertson [1994].) An understanding of Kleiber's Law is important to several of the discussions in this paper.

Brain and digestive system compete for limited share of metabolic energy budget. A key observation to note about relative brain size when averaged across species is that the equation for how brain size varies in proportion to body size uses an exponential scaling factor almost identical to the one used in the equation for how an organism's basal metabolic rate (BMR) varies with body size, i.e. Kleiber's Law. (The exponential scaling coefficient used in the equation for how brain mass varies in relation to body mass is 0.76 [Foley and Lee 1991]; the analogous scaling coefficient for BMR is 0.75; Kleiber [1961] as cited in Foley and Lee [1991].) This is important because it directly implies that brain size is closely linked to the amount of metabolic energy available to sustain it [Milton 1988, Parker 1990].

This point will become central as we proceed. For now it is enough to observe that the amount of energy available to the brain is dependent on how the body's total energy budget has to be allocated between the brain and other energy-intensive organs and systems, particularly the digestive system. Further, how much energy the digestive system requires (and thus how much is left over for the brain and other "expensive" organs) is a function of the kind of diet that a species has developed to handle during its evolution. As we proceed, we will return to the ramifications of this for human diet as it relates to the evolution of the large human brain.

For more information on the derivation of encephalization quotients, Kleiber's Law, and the statistical fitting procedures used, consult Appendix 2.

A comparative anatomy analysis of primate brains

Stephan [1972] provides a comparative anatomy analysis of primate brains, including modern humans, non-human primates, and our prehistoric ancestors. Below is a summary of the important points made in Stephan [1972]. Note here that the Stephan paper was done before the Martin research cited above; thus Stephan uses a slightly different measure of encephalization.

Also of interest here is the additional remark in Stephan [1972], regarding comparative anatomy in this context (p. 174): "It must be stressed, however, that because our scientific approach is indirect, it can provide only inferences, not proofs."

Factors in Encephalization: Energy (Metabolism) and Diet

The reality of encephalization--the relatively large human brain--with its correspondingly high intelligence, is readily apparent. The object of current research and debate, however, is the examination of what evolutionary factors have driven the development of increased human encephalization. Such research provides insight into our evolutionary diet, and also reveals why any comparative "proof" that ignores intelligence and the significant impact of brain size on metabolic requirements is logically dubious.

Life cycle and energy requirements

Parker [1990] analyzes intelligence and encephalization from the perspective of life history strategy (LHS) theory, a branch of behavioral ecology. LHS is based on the premise that evolutionary selection determines the timing of major life-cycle events--especially those related to reproduction--as the solution to energy optimization problems.

Extensive energy required for brain growth. Parker discusses the life history variables in non-human primates, and then examines how life history events relate to large brain size, gestation period, maturity at birth, growth rates and milk consumption, weaning and birth intervals, age of puberty, and other events. The motivation for studying such events is that the brain is the "pacemaker of the human life cycle" [Parker 1990, p. 144], and the slow pace of most human life history events reflects the extensive energy required for brain growth and maintenance.

Foley and Lee [1991] analyze the evolutionary pattern of encephalization with respect to foraging and dietary strategies. They clearly state the difficulty of separating cause and effect in this regard; from Foley and Lee [1991, p. 223]

In considering, for example, the development of human foraging strategies, increased returns for foraging effort and food processing may be an important prerequisite for encephalization, and in turn a large brain is necessary to organize human foraging behaviour.

Dietary quality is correlated with brain size. Foley and Lee first consider brain size vs. primate feeding strategies, and note that folivorous diets (leaves) are correlated with smaller brains, while fruit and animal foods (insects, meat) are correlated with larger brains. The energetic costs, both daily and cumulative, of brains in humans and chimps, over the first 1-5 years of life are then compared. They note [Foley and Lee 1991, p. 226]:

Overall the energetic costs of brain maintenance for modern humans are about three times those of a chimpanzee. Growth costs will also be commensurately larger.

Then they consider encephalization and delayed maturation in humans (compared to apes), and conclude, based on an analysis of brain growth, that the high energy costs of brain development are responsible for the delay in maturation.

Dietary shift beginning with Homo. Finally, they consider the dietary shifts that are found in the fossil record with the advent of humans (genus Homo), remarking that [Foley and Lee 1991, p. 229]:

The recent debate over the importance of meat-eating in human evolution has focused closely on the means of acquirement... but rather less on the quantities involved...

In considering the evolution of human carnivory it may be that a level of 10-20% of nutritional intake may be sufficient to have major evolutionary consequences...

Meat-eating, it may be argued, represents an expansion of resource breadth beyond that found in non-human primates...

Homo, with its associated encephalization, may have been the product of the selection for individuals capable of exploiting these energy- and protein-rich resources as the habitats expanded (Foley 1987a).

The last sentence in the preceding quote is provocative indeed--it suggests that we, and our large brains, may be the evolutionary result of selection that specifically favored meat-eating and a high-protein diet, i.e., a faunivorous diet.

How dietary quality relates to the brain's share of total metabolic budget

The research of Leonard and Robertson [1992, 1994] provides an in-depth analysis of brain and body metabolism energy requirements. Relevant points from their research:

In a followup paper, Leonard and Robertson [1994] expanded the analysis of their 1992 paper by looking at the relationship of dietary quality to body size and metabolic rates. Important points from their 1994 paper:

The paradox: Where does the energy for the large human brain come from?

In any event, as we have seen, what begs explanation is that humans "spend" far more energy on the brain than other primates: 20-25% of RMR vs. roughly 8% in the great apes. Yet the total human RMR remains in line with predictions based purely on body size. This presents a paradox: where do humans get the extra energy to "spend" on our large brains? As we will see later in the research of Aiello and Wheeler, the most feasible hypothesis is that the answer lies in considerations of dietary efficiency and quality. Leonard and Robertson [1994, p. 83] conclude:

These results imply that changes in diet quality during hominid evolution were linked with the evolution of brain size. The shift to a more calorically dense diet was probably needed in order to substantially increase the amount of metabolic energy being used by the hominid brain. Thus, while nutritional factors alone are not sufficient to explain the evolution of our large brains, it seems clear that certain dietary changes were necessary for substantial brain evolution to take place.

In other words, while the evolutionary causes of the enlarging human brain themselves are thought to have been due to factors that go beyond diet alone (increasing social organization being prime among the proposed factors usually cited), a diet of sufficient quality would nevertheless have been an important prerequisite. That is, diet would have been an important hurdle--or limiting factor--to surmount in providing the necessary physiological basis for brain enlargement to occur within the context of whatever those other primary selective pressures might have been.

To summarize:

The significance of Leonard and Robertson's research [1992, 1994] lies in their analysis of energy metabolism, which reveals the paradox: How do humans meet the dramatically higher energy needs of our brains, without a corresponding increase in RMR (which is related to our body size)? They argue that the factor that allows us to overcome the paradox is our higher-quality diet compared to other primates. Of course, prior to the advent of agriculture and the availability of grains, the primary source of such increased dietary quality was the consumption of fauna--animal foods, including insects.


(Relationship of Dietary Quality/Gut Efficiency to Brain Size)

Return to beginning of article



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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