Of these, only fish (especially oily fish like mackerel or sardines that are quite literally dripping with omega-3 fatty acids) seem to have the unequivocal support of nutrition science.

Signs seem to point to eating a diet rich in fresh fruit and vegetables with the occasional dose of fish and red meat will make you smarter, fitter and altogether happier. That this is the basis of practically the entire diet industry is no great surprise.

But did food make us smarter in the first place?

About 2 million years ago, our brains increased in size to about double that of the other apes. We don’t know why this occurred but it separated humans from the other apes by the finest of margins. For most of that two million years, not much happened. Toolmaking moved from Africa to Europe. But between 200,000 and 10,000 years ago (the Middle Paleolithic era for all you budding paleoanthropologists playing at home) there was an explosion in making art, harpoons and hooks for fish, tools for working animal hides, rope and needles. Something happened that unlocked potential in our species' freshly enlarged grey matter. There are an abundance of theories to suggest that a correlating factor was what humanity ate.

My favorite theories to date:

Theory 1: Cooking

Two weeks ago, Philipp Khaitovich of the Partner Institute for Computational Biology in Shanghai released a paper arguing that the changes in chemical brain processes – us getting smarter – may have been due to the development of cooking allowing humanity access to more food. From Livescience:

The finding suggests that increased access to calories spurred our cognitive advances, said Khaitovich, carefully adding that definitive claims of causation are premature.

The extra calories may not have come from more food, but rather from the emergence of pre-historic "Iron Chefs;" the first hearths also arose about 200,000 years ago.

In most animals, the gut needs a lot of energy to grind out nourishment from food sources. But cooking, by breaking down fibers and making nutrients more readily available, is a way of processing food outside the body. Eating (mostly) cooked meals would have lessened the energy needs of our digestion systems, Khaitovich explained, thereby freeing up calories for our brains.

Cooking allowed humans to eat practically anything. Grains and many plants that would otherwise be indigestible or poisonous could become viable foodstuffs. Food could be dried or cured to cope with famine.

Theory 2: Fishing

Omega 3 found in fish are one of the few uncontested foods that help the development of cognitive ability. From Popular Mechanics:

Stephen Cunnane of Sherbrooke University has a different idea of how evolution works. In a presentation at the (American Association for the Advancement of Science) in St. Louis last week, Cunnane proposed that humans evolved through expatiation rather than adaptation—in other words, luck.

Cunnane points to a "shoreline diet" as the reason for human evolution. Coastal environments provided humans with food like clams, frogs, bird eggs and fish. Because these foods were ample in amount and also rich in iodine, Cunnane believes they allowed primitive humans to have fat babies.

Baby fat is key to developing brains, Cunnane says, and thus evolution. Without those extra pounds, we would not be able to sate the need for docosahexaenoic acid (DHA)—a polyunsaturated fat key to brain development. Iodine is also a key brain nutrient (as today’s iodine-enriched salt attests)

Theory 3: The love of bitter

The ability to distinguish between different levels of bitterness may have spurred humanity to move out of Africa by lending the ability to eat plants that contain levels of cyanide especially cassava and sorghum. It was also a handy trait to stop humans from overdosing on cyanide-filled foods. An added bonus: malaria protection. From Genome Biology

"One could speculate that the reduced sensitivity towards this compound could help to increase intake of cyanide in the diet, and this in turn could have an evolutionary advantage by protecting against malaria infection," Soranzo said. The connection is "completely speculative at this point" but "worth exploring," she said.

Speculate on!