It's good to question conventional wisdom, indeed most of us here came here precisely because we did that.
The evidence? Well have two primary points:
1. It seems common sense, to follow the eating and lifestyle of our ancestors. This is especially pertinent today, when we see so many people overweight, undernourished and generally sick.
2. We now have a luxury that many in the beginning didn't have, which is the scads and scads of glowing testimonials and success stories.
I've said it before but it's worth repeating, if anything I think it's BECAUSE we don't really have many studies that this appeals to me so much.
It's not practical and it's not moral to experiment on humans. There are very few long-term studies that really prove anything one way or another. For almost every study you find saying something is good there's another study saying it's bad. We act as though we've figured out everything about the human body except the brain, yet 'death by doctor' is one of the leading causes of death in the Western world? We've tried the 'food pyramid' for decades, watching our populations get fatter and sicker.
No 2 people are the same. Even identical twins will have confounding variables, such as how much sleep, stress or joy in their lives, how much sun exposure, the quality of the multivitamins they might or might not take. Throw in injuries, allergies, environment...
Just too many variables and it's not just future job security, it's actually true that "more studies are needed".
If you're not getting paid for the full-time job of trying to figure this stuff out, it gets daunting real fast! Information overload, and that's information from the same people killing us at about the same rate as traffic accidents?
Or you can just say "To hell with all that" and just follow the 10 basic guidelines of living like what we're generally adapted for as a species. To me that makes sense.
Even if, in the meantime, I'm eating nothing but potatoes...