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Department of Nutritional Sciences University of Toronto Toronto M5S 3E2 Canada E-mail: s.cunnane{at}utoronto.ca
Dear Sir:
Cordain et al (1) estimated that Paleolithic hunter-gatherers would have consumed as much animal food as possible. I support the inclusion of fish and shellfish in Cordain et al's estimate of animal food intake because I believe that fish, shellfish, and other shore-based foods were crucial for human brain evolution (2–4). I have 2 comments about the reference values Cordain et al used for plant and animal macronutrient composition.
First, their reference macronutrient values for plant foods were 62% carbohydrate, 24% fat, and 14% protein. The list of food types for which this reference macronutrient profile was obtained did not include vegetables (see Table 3 of reference 1). In commonly available databases of the macronutrient contents of plant foods other than nuts and seeds, the fat content rarely seems to exceed 1% by weight. Does this imply that Cordain et al's macronutrient database does not really represent most plant foods or, alternatively, that nuts and seeds are interpreted to represent most plant foods consumed? Even if plant foods in the Paleolithic period did contain an average of 24% of energy as fat by proximate analysis, this value needs to be corrected downward by 30% to yield the content of actual fatty acids that are available for energy from plant material other than nuts. Was this correction made?
Second, Cordain et al emphasized the risk of protein toxicity by referring extensively to the outcome of the consumption of large amounts of meat containing <5% fat by weight. Lean muscle tissue is 2–3% fat, but animal organs other than muscle, which tend to be 5–10% fat, would also have been consumed. Body fat itself would also have been eaten. Furthermore, hominids would have faced fierce competition from carnivores for the copious amounts of meat needed to be eaten to induce protein toxicity; therefore, protein toxicity probably did not occur often, and certainly not for extended periods.
As Milton's (5) editorial points out, it makes empirical sense that foods of relatively high nutrient and energy densities would be consumed when available. However, Milton says that "Hunter-gatherers were not free to determine their diets, rather it was their predetermined biological requirements for particular nutrients that constrained their evolution. At the same time, these dietary needs apparently allowed for selection to favor increased brain size in the human lineage and the concomitant development of technologic, social, and other abilities directed at securing these nutrients" (5). Modern humans in a totally free-choice situation ultimately choose a diet that is complete in energy and nutrients. Those who cannot choose freely often develop malnutrition or specific nutrient deficiencies. Many factors, including climate, competition, and food availability would have been constraints affecting the daily or seasonal diet of hunter-gatherers; in that sense they may not have been totally free to determine their diets. However, in my view, it was the discovery of and adaptation to a high-quality shore-based diet that was a major determinant of the rate and extent of human brain evolution, not the other way around as implied by Milton (5).
We argue that the shore-based ecologic niche was uniquely able to stimulate expansion of the primate brain because, in addition to being a plentiful supply of dietary energy and protein, it provided certain brain-selective nutrients, such as docosahexaenoate, iodine, zinc, copper, and iron (2–4). The basis for this hypothesis is that terrestrial foods are deficient in iodine and contain little docosahexaenoate (only in animal tissue). Zinc, copper, and iron are more abundant and available from seafood than from plants. Dietary or genetically imposed deficiencies of all of these brain-selective nutrients leaves the modern human brain extremely vulnerable to subnormal development. Equally important is the issue of access to reliable sources of foods rich in brain-selective nutrients that required minimal effort to locate and consume. Such foods would have to have been available for thousands of years before intelligence had risen sufficiently to conceive of and experiment successfully with true fishing or hunting and trapping of wild animals. The hominid fossil record shows that at least fish and shellfish—but probably also eggs, amphibians, and plants on lakeshores and seacoasts—provided an abundance of this important dietary stimulus for human brain evolution without special effort or substantial competition from predators (5).
If the nutrient and energy supplies were consistently inadequate in some geographic areas over thousands of years, human brain evolution would have faltered and long-term colonization of those areas would have ceased until the appropriate foods were found or supplements were invented. This is what happened most clearly with iodine deficiency, which affects more than a billion mostly vegetarian people in inland areas of all continents. Iodine is essential for energy metabolism, normal brain development, and fertility (6). People can survive even severe iodine deficiency but they cannot thrive or reproduce. In contrast, coastal peoples experience no known nutrient deficiencies affecting brain function. Hence, as we argued (2–4), marine, estuarine, and lacustrine locations probably favored human brain evolution by providing abundant energy and protein but, equally importantly, brain-selective nutrients.
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