The food came first, then brain development. The discovery of cooking meat allowed our species to unlock and easily digest vast amounts of nutrients in relatively small meals, leaving us more time for other development.
Other mammals entire lives revolves around the search for food in the case of hunters or sitting there chewing and digesting massive amounts of vegetation to gain small amounts of nutrients, like cows and apes (huge jaws and stomachs).
One thing vegans can't seem to answer is if humans are herbivores then why is it so difficult, as in nigh on impossible, to survive purely on foraging, yet you can survive comfortably hunting and eating animals in almost cave man conditions. You see it every year on programmes likes Bear Grylls where the islanders visibly wilt away without animal proteins.
To survive on a herbivore diet you need advanced farming, sourcing of plantation, fruit and seeds from all corners of the earth and increasingly highly complex supplements.
I applaud anyone who can be healthy on a vegan diet, but we as a species are not