I arrived at this conclusion both from personal experience (I've tried all the apparently reasonable diets) and the scientific data. Nuts are barely digestible. Our digestive system doesn't look like a herbivore / fruit eater at all. Surely we've had some, but that's not what we're build for. I think we're like dogs, we can survive on a diversity of foods (and get sick from some of them) but we thrive on meat. That's my n=1 intimate conviction, not one I sought to arrive at, but once I had experience it there's absolutely no doubt in my mind. Naturally I sought the people that had the same experience and they do have the data and the explanations for why, but it's too lengthy to provide here, although I can give hints if you wish.

Oh and australopithecus ate meat, this goes far beyond modern humans. This is why we got smart, because intelligence pays off far more for hunting than for foraging, particularly for an otherwise frail creature.