evidence for the unique function of dha during the evolution of the modern hominid brain

The African savanna ecosystem of the large mammals and primates was associated with a dramatic decline in relative brain capacity. This reduction happened to be associated with a decline in docosahexaenoic acid (DHA) from the food chain. DHA is required for brain structures and growth. The biochemistry implies that the expansion of the human brain required a plentiful source of preformed DHA. The richest source of DHA is the marine food chain while the savannah environment offers very little of it. Consequently H. sapiens could not have evolved on the savannahs. Recent fossil evidence indicates that the lacustrine and marine food chain was being extensively exploited at the time cerebral expansion took place and suggests the alternative that the transition from the archaic to modern humans took place at the land\\water interface. Contemporary data on tropical lake shore dwellers reaffirms the above view. Lacustrine habitats provide nutritional support for the vascular system, the development of which would have been a prerequisite for cerebral expansion. Both arachidonic acid (AA) and DHA would have been freely available from such habitats providing the double stimulus of preformed acyl components for the developing blood vessels and brain. The w3 docosapentaenoic acid precursor (w3DPA) was the major w3 metabolite in the savanna mammals. Despite this abundance, neither it or the corresponding w6DPA were used for the photoreceptor nor the synapse. A substantial difference between DHA and other fatty acids is required to explain this high specificity. Studies on fluidity and other mechanical features of cell membranes have not revealed a difference of such magnitude between even a-linolenic acid (LNA) and DHA sufficient to explain the exclusive use of DHA. We suggest that the evolution of the large human brain depended on a rich source of DHA from the land\\water interface. We review a number of proposals for the possible influence of DHA on physical properties of the brain that are essential for its function.