intake profile would have been in terms of saturated vs. polyunsaturated and monounsaturated fats.
Organ meats favored in preference to muscle meats in hunter-gatherer diets. Observations of modern hunter-gatherers have shown that muscle meats (the leanest part of the animal) are least preferred, sometimes even being thrown away in times of plenty, in preference to the fattier portions. Eaten first are the organs such as brains, eyeballs, tongue, kidneys, bone marrow (high in monounsaturated fat), and storage fat areas such as mesenteric (gut) fat. (Even this gut fat is much less saturated in composition, however, than the kind of marbled fat found in the muscle meat of modern feedlot animals.) There is no reason to believe earlier hunter-gatherers would have been any different in these preferences, since other species of animals who eat other animals for food also follow the same general order of consumption.
Type of fat may be more important than amount of fat. As a related point, while it is likely to be controversial for some time to come, an increasing amount of recent research on fats in the diet suggests there may actually be little if any correlation between the overall level of fat consumption and heart disease, atherosclerosis, cancer, etc., contrary to previous conclusions. (See The World's Biggest Fad Diet, offsite, for an overview of the recent studies that have called into question the conventional wisdom's low-fat gospel.) Instead, the evidence seems to point more specifically to the trans fats (hydrogenated vegetable oils used to emulsify and preserve foods) which permeate much of the modern food supply in supermarkets, and highly saturated fats (including dairy products and probably saturated commercial meats, but not lean game meats) along with, perhaps, a high consumption of starches and refined carbohydrates driving the hyperinsulinism syndrome. (See the postscript to Part 2 for a brief discussion of the recent findings on hyperinsulinism.)
Cardiovascular disease and cancer are rare in hunter-gatherers, despite their fat/protein intake. Given that atherosclerosis, heart disease, and cancer are almost non-existent even in longer-lived members of hunter-gatherer tribes eating large amounts of meat, and whose diets much better approximate what the evolutionary diet is thought to have been like than the diets of modern cultures, there is good reason to believe the earlier mainstream research on fats was faulty. (For details on health and disease in hunter-gatherers, see Hunter-Gatherers: Examples of Healthy Omnivores, on this site. The next page of Part 1 here will also discuss corrected anthropological survey data showing the average level of meat consumption of hunter-gatherers to be in excess of 50% of the diet.)
Review papers and publications by fat and cholesterol researcher Mary Enig, Ph.D., and by Russell Smith and others have revealed a pattern of misinterpretation and misrepresentation of results of many earlier studies on dietary fat and cholesterol. (Enig recommends Smith's comprehensive Diet, Blood Cholesterol and Coronary Heart Disease: A Critical Review of the Literature [Vector Enterprises, Santa Monica, CA (1988)] in particular, for examples. Also see Enig's three-part online interview, "Health Risks of Processed Foods and Trans Fats," [Part 1] [Part 2] [Part 3], for her views, as well as her own website on trans fats.)
Co-evolution of increased human brain
size with decreased size of digestive system |
Data points to increasing dependence on denser foods, processed by a less energy-intensive gut to free up energy for the evolving brain.
Also, left completely out of Part 1 of the interview above due to my initial passing familiarity with further evidence are recent findings pointing to a correlation between increasing levels of animal flesh in the diet over the eons at the same time the human brain was in the process of near-tripling in size--from 375-550cc at the time of Australopithecus, to 500-800cc in Homo habilis, 775-1225cc in Homo erectus, and 1350cc in modern humans (Homo sapiens).
Sufficient amounts of long-chain fatty acids essential to support brain growth. While the specific evolutionary factor(s) that drove the increase in human brain size are still being speculated about, one recent paper suggests that--whatever the causes--the evolutionary increase in brain size would not have been able to be supported physiologically without an increased intake of preformed long-chain fatty acids, which are an essential component in the formation of brain tissue. [Crawford 1992]
Animal prey likeliest source for required amounts of long-chain fatty acids during human brain evolution. Lack of sufficient intake of long-chain fatty acids in the diet would therefore be a limiting factor on brain growth, and these are much richer in animal foods than plant. (Relative brain size development in herbivorous mammals was apparently limited by the amount of these fatty acids in plant food that was available to them.) Given the foods available in humanity's habitat during evolution, the necessary level of long-chain fatty acids to support the increasing size of the human brain would therefore presumably only have been available through increased intake of flesh.
Human brain size since the late Paleolithic has decreased in tandem with decreasing contribution of animal food to diet. In addition, a recent analysis updating the picture of encephalization (relative brain size) changes in humans during our evolutionary history has revealed that human cranial capacity has decreased by 11% in the last 35,000 years, the bulk of it (8%) in the last 10,000 [Ruff, Trinkaus, and Holliday 1997]. Eaton [1998] notes that this correlates well with decreasing amounts of animal food in the human diet during this timeframe. (Of particular relevance here is that most of this decrease in animal foods correlates with the dawn of agriculture 10,000 years ago.)
The central role of DHA in brain growth. Eaton [1988] also notes the obvious hypothesis here would be that shortfalls in the preformed long-chain fatty acids important to brain development are logical candidates as the potentially responsible factors, most particularly docosahexaenoic acid (DHA), which is the long-chain fatty acid in most abundance in brain tissue, as well as docosatetraenoic acid (DTA), and arachidonic acid (AA). (The human body can synthesize these from their 18-carbon precursors linoleic acid (LA) and a-linolenic acid (ALA)--obtainable from plant foods--but the rate of synthesis does not match the amounts that can be gotten directly from animal foods. Additionally, an excessive amount of LA compared to ALA, which is likely when plant foods predominate in the diet, inhibits the body's ability to synthesize DHA endogenously, compounding the problem.)
This evidence of decreasing brain size in the last 35,000 years, and particularly the last 10,000, represents important potentially corroborative evidence for the continuing role of animal foods in human brain development, since dietary changes in this most recent period of human prehistory can be estimated with more precision than dietary composition earlier in human evolution. While it should be clearly noted here that correlation alone is not causation, at the same time it should be acknowledged that there seem to be no other worthy hypotheses as yet to explain the dietary basis that could have supported the dramatic increase in brain size during human evolution.
Recent tuber-based hypothesis for evolutionary brain expansion fails to address key issues such as DHA and the recent fossil record. As a case in point, there has been one tentative alternative hypothesis put forward recently by primatologist Richard Wrangham et al. [1999] suggesting that perhaps cooked tubers (primarily a starch-based food) provided additional calories/energy that might have supported brain expansion during human evolution.
However, this idea suffers from some serious, apparently fatal flaws, in that the paper failed to mention or address critical pieces of key evidence regarding brain expansion that contradict the thesis. For instance, it overlooks the crucial DHA and/or DHA-substrate adequacy issue just discussed above, which is central to brain development and perhaps the most gaping of the holes. It's further contradicted by the evidence of 8% decrease in human brain size during the last 10,000 years, despite massive increases in starch consumption since the Neolithic revolution which began at about that time. (Whether the starch is from grain or tubers does not essentially matter in this context.) Meat and therefore presumed DHA consumption levels, both positive *and* negative-trending over human evolution, track relatively well not simply with the observed brain size increases during human evolution, but with the Neolithic-era decrease as well, on the other hand. [Eaton 1998]
These holes, among others in the hypothesis, will undoubtedly be drawing comment from paleo researchers in future papers, and hopefully there will be a writeup on Beyond Veg as more is published in the peer-review journals in response to the idea. At this point, however, it does not appear to be a serious contender in plausibly accounting for all the known evidence.
Co-evolution of increased brain size with concurrent reduction in size of the human gut. Recent work is showing that the brain (20-25% of the human metabolic budget) and the intestinal system are both so metabolically energy-expensive that in mammals generally (and this holds particularly in primates), an increase in the size of one comes at the expense of the size of the other in order not to exceed the organism's limited "energy budget" that is dictated by its basal metabolic rate. The suggestion here is not that the shrinkage in gut size caused the increase in brain size, but rather that it was a necessary accompaniment. In other words, gut size is a constraining factor on potential brain size, and vice versa. [Aiello and Wheeler 1995]
Human gut has evolved to be more dependent on nutrient- and energy-dense foods than other primates. The relationship of all this to animal flesh intake is that compared to the other primates, the design of the more compact human gut is less efficient at extracting sufficient energy and nutrition from fibrous foods and considerably more dependent on higher-density, higher-bioavailable foods, which require less energy for their digestion per unit of energy/nutrition released. Again, while it is not clear that the increasing levels of animal flesh in the human diet were a directly causative factor in the growth of the evolving human brain, their absence would have been a limiting factor regardless, without which the change likely could not have occurred. Other supporting data suggest that in other animals there is a pattern whereby those with larger brain-to-body-size ratios are carnivores and omnivores, with smaller, less complex guts, and dependent on diets of denser nutrients of higher bioavailability.
Vegetarian philosophy has traditionally relied on observing that the ratio of intestinal length to body trunk length parallels that of the other primates as an indication the human diet should also parallel their more frugivorous/vegetarian diet. However, this observation is based on the oversimplification that gut length is the relevant factor, when in fact both cell types and intestinal surface area are the more important operative factors, the latter of which can vary greatly depending on the density of villi lining the intestinal walls. In these respects, the human gut shares characteristics common to both omnivores and carnivores. [McArdle 1996, p. 174] Also, intestinal length does not necessarily accurately predict total gut mass (i.e., weight), which is the operative criterion where brain size/gut size relationships are at issue. The human pattern of an overall smaller gut with a proportionately longer small intestine dedicated more to absorptive functions, combined with a simple stomach, fits the same pattern seen in carnivores. [Aiello and Wheeler 1995, p. 206]
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(Corrected Anthropological Survey Data Shows Meat Averages Over 50% of Hunter-Gatherer Diets)
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SEE BIBLIOGRAPHY
SEE TABLE OF CONTENTS FOR: PART 1 PART 2 PART 3
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