I find the 'expensive tissue hypothesis' (based on brain/gut ratio) quite persuasive. See https://en.wikipedia.org/wiki/Expensive_tissue_hypothesis https://en.

I find the 'expensive tissue hypothesis' (based on brain/gut ratio) quite persuasive. See https://en.wikipedia.org/wiki/Expensive_tissue_hypothesis https://en.wikipedia.org/wiki/Expensive_tissue_hypothesis