Although we don't know the precise foods the very first humans lived on, most people will try to find something in the literature to back up the way they currently eat, be it a meat-based diet, or one of mostly fruits, grains and vegetables. We argue that if our ancient ancestors ate a particular way, than that is how we should eat too, not taking into consideration that early humans no doubt ate whatever they could get their hands on, whether it was rooted in the ground, running through a forest, or swimming in a stream. Food most likely did not show up reliably and regularly, and when it did, the presence or lack of long term health benefits probably wasn't a consideration. I would imagine there was great joy in finding anything at all that seemed safe and edible at that moment.
Animals were certainly part of man's diet in the past, and people will use this information to justify eating animals today. We affirm that it must be right since we've evolved for thousands and thousands of years eating meat. But really, the only criteria necessary to perpetuate any species is to live to the age of maturity, reproduce, and nurture our off-spring to the age of independence. For humans this can be accomplished easily before reaching the age of 30. If we don't die of some other cause before then, most likely we will live at least a few more years before the complications of an animal based diet begin.
But today we know that the long term effects of eating an animal based diet is devastating to ourselves, the animals, and the Earth. Our focus isn't, or shouldn't be, staying alive just long enough to reproduce. We no longer need to eat animals to survive, and in fact our survival most certainly depends on humans evolving to a completely plant based diet. We need to let go of the harmful attachment to our past, the sentimentality that lures us into justifying eating practices that are no longer valid in today's world.