A paleo diet is an arbitrary thing. Which paleo diet should we eat? The one from twelve thousand years ago? A hundred thousand years ago? Forty million years ago? I would argue that, IF we want to return to our ancestral diets, we might reasonably eat what our ancestors spent the most time eating during the largest periods of the evolution of our guts. If that is the case, we need to be eating fruits, nuts, and vegetables—especially fungus-covered tropical leaves.
We need to understand the diet of our ancestors during the time when the main features of our guts, and their magical abilities to turn food into life, evolved. We need, in other words, to look at apes, monkeys and other non-human primates.
The diets of nearly all monkeys and apes (except the leaf-eaters) are composed of fruits, nuts, leaves, insects, and sometimes the odd snack of a bird or a lizard. They have the capacity for eating sugary fruit, the capacity for eating leaves and the capacity for eating meat, although that capacity tends to rarely be invoked. Sure, chimpanzees sometimes kill and devour a monkey, but the proportion of the diet of the average chimpanzee composed of meat is small, less than 3% by mass. The majority of the food consumed by primates today–and every indication is for the last thirty million years–is vegetable, not animal. Plants are what our apey and even earlier ancestors ate; they were our paleo diet for most of the last thirty million years during which our bodies, and our guts in particular, were evolving.