I think our early ancestors mostly ate fruit, pip and seed, and some amount of meat. Basically like apes, which eat lots of soluble fiber that ferments in the gut to make butyric acid, lots of insoluble fiber by eating the pips which passed through (and cleaned the gut), and which eat tiny amounts of meat like mice and bugs and what ever they can scrounge).
Then there was a shift from the rainforests to the coastlines, where they could search for shellfish in shallow waters (the bipedal height adaptation) and hunt medium sized herbivores in the nearby plains by running them down in a pack (the cardio/sweat advantage). The combination of soluble fiber to butyric acid helped us first digest the animal fat and organs in excess which grew our bodies to a powerful stature, and getting lots of omega 3s from the shallow water shellfish grew our brains.
When our brains developed enough to become smart enough to use tools to hunt, this is when we started hunting actual fish and larger animals we couldn't kill before, we became even stronger and smarter!
This is also why I believe we are naturally polyphasic sleepers, because when we moved to the coast lines, the best time to catch shellfish is at night, while the best time to catch animals is at dusk. Hunter/Gatherers would catch shellfish at night, sleep around dawn, eat a big omega breakfast, then go looking for fruit in the morning, then sleep around midday after their big carb hit and insulin spike, then go hunting in the evenings, eat their fatty meaty catch then fall asleep, to wake up around midnight to go fishing again.
That's how the story works out in my head at least, of course if I read a better story than that I might be inclined to change my mind, but so far it makes sense from what I understand about our circadian rhythm.
And then of course we started eating cooked starches and drinking dairy/grain ferments, and we went downhill from there... So really, I don't think their story is too off the truth, of course we still stuck to water-sources as we adapted to starches and milk and other vegetarian cuisine , so we never actually stopped eating shellfish and fish.
