Recent brain-size decreases in humans as further evidence of the brain/diet connection

Why has brain size decreased 11% in the last 35,000 years and 8% in the last 10,000? Another interesting observation about the brain/diet connection comes from recently updated and more rigorous analysis of changes in brain size in humans over the last 1.8 million years. Ruff, Trinkaus, and Holliday [1997] found that encephalization quotient (EQ) began reaching its peak with the first anatomically modern humans of approximately 90,000 years ago and has since remained fairly constant [see p. 174, Table 1]. Most surprisingly, however, absolute brain size--on the other hand--has decreased by 11% since 35,000 years ago, with most of this decrease (8%) coming in just the last 10,000 years. (The decrease in absolute brain size has been paralleled by roughly similar decreases in body size during the same period, resulting in EQ values that have remained roughly the same as before.)
The significance of constant EQ vs. shrinking brain size in context. This data suggests two points. The first point--relating to EQ--is subject to two possible interpretations, at least on the face of it. One interpretation (characterized by somewhat wishful thinking) might be that, if we disregard the absolute decrease in brain and body size, and focus only on EQ, we can observe that EQ has remained constant over the last 10,000-35,000 years. One could then further conjecture that this implies humans have in some sense been successful in maintaining dietary quality during this time period, even considering the significant dietary changes that came with the advent of the agricultural revolution (roughly the last 10,000 years). However, the problem with such an interpretation is exactly that it depends on disregarding the information that overall body size diminished along with brain size--a most important point which needs to be taken into account.

The alternate, and more plausible and genetically consistent interpretation begins by noting that EQ represents a genetically governed trait determined by our evolutionary heritage. Hence one would not expect EQ itself to have changed materially in just 10,000 years, as it would be unlikely such a brief period of evolutionary time could have been long enough for the actual genetics governing EQ (that is, relative brain size compared to body size) to have changed significantly regardless of dietary or other conditions.

Dietary/physiological mechanism may be responsible. This brings up the second point, which is that the specific question here concerns a slightly different issue: the absolute decrease in brain size rather than the issue of EQ. Since the greatest majority of this decrease took place in just the last 10,000 years , a genetic mutation is no more likely as an explanation for the decrease in absolute brain size than it is for relative brain size, or EQ. This leaves us once again with a physiological/biochemical mechanism as the responsible factor, which of course puts diet squarely into the picture . (Not to mention that it is difficult to imagine plausible evolutionary selective pressures for brain size--primarily cultural/social/behavioral--that could conceivably be responsible for the reversal in brain size, since human cultural evolution has accelerated considerably during this period.)

Far-reaching dietary changes over the last 10,000 years. This leaves us with the indication that there has likely been some kind of recent historical shortfall in some aspect of overall human nutrition--one that presents a limiting factor preventing the body/brain from reaching their complete genetic potential in terms of absolute physical development. The most obvious and far-reaching dietary change during the last 10,000 years has, of course, been the precipitous drop in animal food consumption (from perhaps 50% of diet to 10% in some cases) with the advent of agriculture, accompanied by a large rise in grain consumption--a pattern that persists today. This provides suggestive evidence that the considerable changes in human diet from the previous hunter-gatherer way of life have likely had--and continue to have--substantial consequences.

Brain growth dependent on preformed long-chain fatty acids such as DHA. The most plausible current hypothesis for the biological mechanism(s) responsible for the absolute decrease in brain size is that the shortfall in consumption of animal foods since the late Paleolithic has brought with it a consequent shortfall in consumption of preformed long-chain fatty acids [Eaton and Eaton 1998]. Specifically, for optimal growth, the brain is dependent on the fatty acids DHA (docosahexaenoic acid), DTA (docosatetraenoic acid), and AA (arachidonic acid) during development to support its growth during the formative years, particularly infancy. These are far more plentiful in animal foods than plant.

Eaton et al. [1998] analyze the likely levels of intake of EFAs involved in brain metabolism (DHA, DTA, AA) in prehistoric times, under a wide range of assumptions regarding possible diets and EFA contents. Their model suggests that the levels of EFAs provided in the prehistoric diets was sufficient to support the brain expansion and evolution from prehistoric times to the present, and their analysis also suggests that the current low levels of EFA intake (provided by agricultural diets) may explain the recent smaller human brain size.

Rate of synthesis of DHA from plant-food precursors does not equal amounts available in animal foods. Although the human body will synthesize long-chain fatty acids from precursors in the diet when not directly available, the rates of synthesis generally do not support the levels obtained when they are gotten directly in the diet. This is particularly critical in infancy, as human milk contains preformed DHA and other long-chain essential fatty acids, while plant-food based formulas do not (unless they have been supplemented).

Animal studies indicate that synthesis of DHA from plant-source precursor fatty acids does not equal the levels of DHA observed when those are included in the diet: Anderson et al. [1990] as cited in Farquharson et al. [1992], Anderson and Connor [1994], Woods et al. [1996]. Similar results are reported from studies using human infants as subjects: Carlson et al. [1986], Farquharson et al. [1992], Salem et al. [1996]. For a discussion of the above studies, plus additional studies showing low levels of EFAs in body tissues of vegans, see Key Nutrients vis-a-vis Omnivorous Adaptation and Vegetarianism: Essential Fatty Acids.

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To summarize

The data that human brain size has fallen 11% in the last 35,000 years--with the bulk of that decrease (8%) coming in the last 10,000 years--furnishes, by extension, suggestive, potential corroborative support for the hypotheses explored earlier in this section that increasing brain development earlier in human evolution is correlated positively with the level of animal food in the diet. It also indicates that animal food may be a key component of dietary quality (DQ) that cannot be fully substituted for by increasing other components in the diet in its absence (such as grains).
This indication is important to consider, because evidence available on the changes in food practices of more recent prehistoric humans (and of course, humans today) can be assessed in more depth and with a higher degree of resolution than dietary inferences about earlier humans. In conjunction with data about DHA synthesis in the body vs. obtaining it directly from diet, this provides a potentially important point of comparison for assessing hypotheses about the brain/diet connection.

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