Hippie environmentalists are at the root of many fads. They promote the idyllic image of early man singing dancing about in fields of daisies with nothing else to do but eat, sleep and have sex, being super healthy and happy. Nothing could be further from the truth. Early men and women lived in huts or under tree branches. They probably had all sorts of bodily parasites and skin afflictions, and ate all sorts of unsanitary food, not to mention drinking water straight from whatever puddles were on the ground at the time. Early humans probably ate the dead carcasses that were left by other predators. They were very likely a food source themselves, a part of the big cats' paleo diet consisting of rodents, bovines and early humans.
Personally, I would not trade my unhealthy life of so far almost sixty years, with the roast turkey, grilled steak, broiled chicken or fried bacon and eggs that I have been eating for most of that time for a diet consisting of partially-eaten carcasses and entrails. My worst health problems are caused by being overweight, but I find that much better than near-starvation.
And yes, hippies are the ones that are coming up with this paleo diet crap as a way to make us think that this is not the Golden Age, which it is, and that we are not actually living in the Promised Land, which we are.
It’s ain’t about what they ate, it’s about the fact that they couldn’t sit on their fat arses while the food came to them. They had to get out there and work for it, so they were fit.
If you need an explanation (and you're over 12), no explanation is possible.