The following is an excerpt from Human Evolution, Diet and Health.  This title is currently available to buy online at Amazon and Waterstone's.

Brain Development
The brain contains approximately 600 grams of fat per kilogram of total weight. Animal fats have been an essential source of a number of polyunsaturated fatty acids, including omega-six arachidonic acid (AA) and omega-three docosahexaenoic acid (DHA). These fatty acids make up over 90% of the polyunsaturated fats found in the grey matter of all mammalian brains. The availability of these fats in the appropriate ratios, combined with increasing social complexity, allowed our cranial capacity to become three times that of our australopithecine ancestors. 
It might not be best to take our larger brains for granted. Since our brain capacity peaked in the Late Palaeolithic, we have seen a reduction in capacity of approximately 11%. This diminution has paralleled our diminished intake of the preformed, long-chain polyunsaturated fats, which we used to obtain in sufficient quantities from animal fat and freshwater fish and shellfish. An abundant and balanced intake of these fatty acids has been an absolute requirement for sustaining the very rapid expansion of the human cerebral cortex over the last one to two million years. Whilst we can manufacture these required fats internally, the process appears to be too slow to supply the quantities and types of fats needed for optimal brain growth, especially during foetal development and infancy. Deficiencies during infant development can result in an irreversible failure to accomplish certain specific components of brain growth.
Our current deficiency of DHA, brought about by dietary changes in the last century is particularly important because our intake of omega-six fatty acids inhibits the formation of DHA. It is clearly tempting to speculate about a relationship between our inappropriate ratios of these fats, together with our decreased consumption of animal fats, and our decreasing cranial capacity. Furthermore, the imbalance between these fats could be related to the prevalence of depression, which is a debilitating disorder responsible for more disability-related 'lost years' than heart attacks, lung cancer or AIDS. Our sub-clinical omega-three deficiency has also been linked to cardiovascular disease, inflammatory disorders, other mental and psychiatric disorders, and sub-optimal neural development.
Studies have found a relationship between high ratios of omega-six to omega-three fatty acids and depression. There is a direct association between higher ratios and more frequent and more severe depressive episodes. Since the industrial revolution, omega-six intake has soared relative to omega-three intake, due primarily to increased vegetable oil consumption and the use of corn-feeding livestock. The fatty acid profile of wild game animals is far higher in preferable polyunsaturated fats than commercial livestock, and our ancestors could have had up to ten times more of these fats than we have in our contemporary diets. Fish, on the other hand, is a good sources of omega-threes and national fish consumption is inversely related to national rates of depression. Increased omega-three intake is associated with a reduced risk of mortality from cardiovascular diseases, improved neonatal neurodevelopment, and lower blood pressure later in life.
Bone marrow and brain tissue are rich sources of both AA and DHA, whereas liver and muscle tissues are good sources of AA and moderate sources of DHA. Freshwater fish and seafood are also good sources of AA and DHA. We have evidence that both meat and fish consumption increased approximately two million years ago, with the earliest evidence of our own species and tool use being associated with aquatic resource bases. Many of our ancestral populations lived along waterways and near coastal regions, and their migration routes may have kept them close to water throughout their lives. The archaeological evidence suggests that our ancestors exploited fish and other aquatic foods more than we had previously realised. Tropical freshwater fish and shellfish have polyunsaturated lipid profiles more similar to those of the human brain than any other food source known. Birds, small mammals, reptiles and amphibians that ate aquatic foods may have also provided us with adequate DHA when we, in turn, ate them.
Human Evolution, Diet and Health is available to buy online at Amazon and Waterstone's.