Humans evolved with a staple source of the omega-3 fat docosahexaenoic acid (DHA) in their diets. The introduction of high-quality, easily digested nutrients from seafood into the human diet coincided with the rapid expansion of grey matter in the cerebral cortex -- a defining characteristic of the modern human brain.
The DHA molecule has unique structural properties that provide optimal conditions for a wide range of cell membrane functions, and grey matter is a particularly membrane-rich tissue.
According to a study in the journal Nutrients, reprinted on the website Green Med Info:
“The excessive consumption of omega-6 fatty acids in the modern Western diet further displaces DHA from membrane phospholipids. An emerging body of research is exploring a unique role for DHA in neurodevelopment and the prevention of neuropsychiatric and neurodegenerative disorders. DHA is increasingly being added back into the food supply as fish oil or algal oil supplementation.”