"With some evolutionary irony, the past 10,000 years of human existence actually shrank our brains. Limited nutrition in agricultural populations may have been an important driver of this trend. Industrial societies in the past 100 years, however, have seen brain size rebound, as childhood nutrition increased and disease declined. Although the past does not predict future evolution, a greater integration with technology and genetic engineering may catapult the human brain into the unknown."
https://www.scientificamerican.com/article/how-has-human-brain-evolved/
Now with that said, it's important to know we can't fully understand how ancient humans brains actually operated. You know? There's only so much we can find out from the size of skulls. Furthermore, some species of humans had really small brains but apparently made stone tools. So some of the things science tells us are debatable.
We do know that the consumption of meat is what lead to our enlarged brains though. And all the evidence suggests that bigger brains are way better at developing advanced strategies. Therefore we should continue to have a mostly protein diet. Also, this is supported by all the latest diet trends. Protein good, carbs bad.