Evidence for the unique function of docosahexaenoic acid during the evolution of the modern hominid brain

The African savanna ecosystem of the large mammals and primates was associa ted with a dramatic decline in relative brain capacity associated with litt le docosahexaenoic acid (DHA), which is required for brain structures and g rowth. The biochemistry implies that the expansion of the human brain requi red a plentiful source of preformed DHA. The richest source of DHA is the m arine food chain, while the savanna environment offers very little of it. C onsequently Homo sapiens could not have evolved on the savannas. Recent fos sil evidence indicates that the lacustrine and marine food chain was being extensively exploited at the time cerebral expansion took place and suggest s the alternative that the transition from the archaic to modern humans too k place at the land/water interface. Contemporary data on tropical lakeshor e dwellers reaffirm the above View with nutritional support for the vascula r system, the development of which would have been a prerequisite for cereb ral expansion. Both arachidonic acid and DHA would have been freely availab le from such habitats providing the double stimulus of preformed acyl compo nents for the developing blood vessels and brain. The n-3 docosapentaenoic acid precursor (n-3 DPA) was the major n-3-metabolite in the savanna mammal s. Despite this abundance, neither it nor the corresponding n-6 DPA was use d for the photoreceptor nor the synapse. A substantial difference between D HA and other fatty acids is required to explain this high specificity. Stud ies on fluidity and other mechanical features of cell membranes did not rev eal a difference of such magnitude between even alpha-linolenic acid and DH A sufficient to explain the exclusive use of DHA. We suggest that the evolu tion of the large human brain depended on a rich source of DHA from the lan d/water interface. We review a number of proposals for the possible influen ce of DHA on physical properties of the brain that are essential for its fu nction.