But, realistically speaking, the whole argument falls apart when one considers mamalian carnivores who routinely eat and digest their prey whole along with the brains, as they should be as intelligent as humans therefore, what with all the DHA-intake. And thee is no proof that ancient hominids only ate brains and marrow, it is extremely unlikely that they would have focused on just 2 organ-meats for their survival.
None of this argument falls apart by what you've written. Thanks for bringing up these points, it gives me a chance to further clarify my position.
I believe you are assuming that a big brain is the ultimate key to a species success. This is far from reality as dinosaurs, being a great example, ruled the earth with walnut-sized brains for 160 million years. There are clearly much more important factors to consider when evolving than a big brain. Mammalian carnivores likewise had other more pressing matters to do with their energy intake than use it to develop a large brain. Given their body-type, they had to focus on staying strong and mobile. If they used their energy intake to build large brains they would have had to have sacrificed something else - probably muscle tissue. Animals cannot simultaneously evolve top-notch specialized systems at the same time. There is a reason they don't have big brains. They would have failed as a species.
Humans seem to have sacrificed muscle strength and mobility for larger brains as well as and more importantly a larger gut. But, fortunately we are blessed with something that carnivores don't have and that is extraordinarily good limb dexterity and opposable thumbs which enable us to more easily exploit our environment and utilize our intelligence. Its very hard to imagine an intelligent lion trying to figure out how to make a spear or set a trap. They can easily kill their prey with their extremely coordinated attack system built within them. Humans are very weak compared to other apes. Similarly sized chimps are much stronger than the strongest human and would destroy even our best much heavier fighters no problem.
Also, the paper does not state that humans only ate brains and marrow. I never implied that. A species can probably develop a larger brain without eating brain but the addition of brain probably did help. The main point I was trying to make was that it seems that eating foods with only trace amounts of DHA would likely not be optimal for the human diet. We might not need much of it but why not add it in?
There is a reason no "super-creature" has evolved that is both intelligent and a non-tool using ferocious predator in the 4 billion years or so that life has been on earth.