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Department of Environmental Science, Policy and Management Division of Insect Biology University of California Berkeley, CA 94720-3112 E-mail: kmilton{at}socrates.berkeley.edu
Modern humans are noted for their large brain but factors related to the brain's evolution are imperfectly understood. Cunnane states that a "shore-based ecologic niche was uniquely able to stimulate expansion of the primate brain. . . ." In a previous article, Cunnane and others (1) described "the African savanna ecosystem of large mammals and primates [as] associated with a dramatic decrease in relative brain capacity associated with little docosahexaenoic acid" (DHA; 22:6n-3). Abundant long-chain polyunsaturated fatty acids (LCPUFAs), particularly DHA and arachidonic acid (AA; 20:4n-6) are regarded as absolute requirements for advanced neural growth in humans and other mammals (1–3).
Because tropical freshwater fish and shellfish and many marine fish offer plentiful preformed DHA, Cunnane et al propose that the lacustrine and marine food chain was being extensively exploited at the time cerebral expansion took place in the ancestral line leading to modern humans (1, 2). I found little evidence to support this.
Members of the genus Homo have always been distinguished by a large brain relative to body size (4, 5). Data suggest that the major increase in encephalization in Homo occurred during the Middle Pleistocene, 600–150 thousand years before present (BP) (4). Well before this, overall body size and degree of sexual dimorphism in Homo had arrived at essentially the modern level (5). By 150–100 thousand years BP, absolute brain size in Homo appears to have been within the modern range, whether Homo is viewed as a single or multiple species (4, 5).
The first evidence supporting the systematic use of coastal resources is dated between 127 and 57 thousand years BP (5). If consumption of coastal resources underlies expansion of the modern human brain, what factors explain the precipitous increase in brain size on 3 different continents by members of the genus Homo well before evidence for the exploitation of shore-based resources? If humans in the African Rift Valley consistently utilized lacustrine resources (2), why the long period of stasis in human encephalization between 1800 and 600 thousand years BP (4)?
Another puzzle concerns the technologic explosion (5)—a burst of creativity in anatomically modern humans that appears to have begun fairly abruptly in the Late Paleolithic period some 40 thousand years BP and involved the dramatic acceleration of cultural evolution. This technologic explosion was not accompanied by any increase in human brain size and thus some other factor, possibly the development of fully modern language, has been suggested to underlie it (5).
Taken together, the fossil and archaeologic records suggest that the modern physical form of our species evolved before the modern capacity for culture (5). Although the question of where and when anatomically modern humans originated remains unresolved (5), data do not suggest any causal association between the exploitation of aquatic foods and human brain expansion.
The idea that the African savanna could not support large-brained species (1, 2) seems inaccurate. No evidence suggests that primates in this environment have brains smaller for their body mass or a lower encephalization quotient (6) than do their counterparts in tropical forests. In fact, the savanna baboon (Papio spp.) and savanna-woodland vervet (Cercopithecus aethiops) have relatively large brains and high encephalization quotients compared with most African forest primates (6). Nor is it the case that all large savanna species have small brains relative to their body mass. Elephants, for example, have brains that, over the course of their evolution, "were enlarged even beyond the extent expected for their large bodies"(6).
Where do humans get the LCPUFAs that are so critical in brain development? Preformed LCPUFAs can be obtained from foods, or LCPUFAs can be synthesized in the mammalian liver from dietary precursors, ie, the essential fatty acids linoleic acid and -linolenic acid. Tissues of the eye and brain can also synthesize DHA if the appropriate precursors are available (7). In humans, a progressive increase in fatty acid length and degree of unsaturation from maternal liver to placenta, fetal liver, and fetal brain has been documented (3). The direct incorporation of dietary LCPUFAs in the developing brain was also shown (3). Full-term infants can synthesize DHA, and human breast milk contains both linoleic acid and -linolenic acid as well as LCPUFAs, including DHA (8, 9).
Although the conversion of -linolenic acid to DHA in humans is stated to be weak (2), "elongation and desaturation of 3 fatty acids in the human liver is very active and capable of providing the high levels of long 3 PUFA required by the developing brain" during the crucial stage of brain development (3). Although n-3 deficiency can be induced in humans by a very poor supply of -linolenic acid or an excessive supply of linoleic acid relative to -linolenic acid, the possibility of n-3 fatty acid deficiency in the wild-food diets of evolving humans seems unlikely "because of the abundance of these fatty acids in nature, their small minimum requirements and the enzyme preference for the linolenate family" (3).
The "aquatic foods argument" also offers no real explanation for why these foods stimulated human brain expansion. In this Lamarckian scenario, the quiescent brain appears to be waiting patiently for humans to discover aquatic foods and then, eureka, the brain is free to enlarge and modern humans result. Not only are the selective pressures involved in this scenario unspecified, no information is provided as to how these large-brained humans were then able to provide DHA and other brain-specific nutrients for themselves or their developing offspring once they moved away from lacustrine or shore-based environments.
Dietary pressures appear to have been a major stimulus in human evolution (10). The association of stone tools with the earliest evidence for hominid exploitation of meat and marrow from large terrestrial ungulates strongly suggests that even the earliest humans used extrasomatic (cultural) innovations to help them solve immediate dietary problems (11). The brains, flesh, liver, tongue, marrow, and other parts of wild terrestrial mammals would have served as a concentrated source of many essential nutrients required by early humans, including LCPUFAs (12, 13). Because wild animals consume diets with very low ratios of n-6 to n-3 fatty acids, their tissues have relatively high proportions of n-3 fatty acids, including eicosapentaenoic acid (a precursor of DHA) and DHA (12, 13) and wild-plant foods would provide -linolenic acid and linoleic acid.
Archaeologic evidence testifies to the increasing technologic proficiency and continuous exploitation of terrestrial mammals by members of the genus Homo over the course of their evolution (5). Calculations indicate that a diet composed of 35% terrestrial animal matter and 65% terrestrial plant matter would have provided more than adequate raw material for brain-building purposes, not only sufficient amounts of DHA but also of AA and docosatetraenoic acid (14). As highly opportunistic foragers, ancestral humans likely would have exploited aquatic foods whenever possible, but such foods seem unnecessary for brain expansion in the human lineage.
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