The Paleo diet, for those of you without a meathead cousin who always wants to discuss coconut oil and body-fat percentage, is based on how Homo sapiens in the Paleolithic era supposedly ate. Grains like wheat are out because what we know as agriculture came late to the human game, and our bodies are -- again, supposedly -- less well-adapted to eating it. Instead, meals are mostly nuts, fruit, veggies, and meat. A typical Paleo meal might be a sweet potato, an avocado, and chicken breast, just like our prehistoric brethren ate.
No animal is off-limits for consumption, either -- the thinking is that early humans were hunter-gatherers, so they probably stayed up late grilling, eating nuts, and talking about whether the paintings of antelopes on cave walls are really "art" or just a superficial, masturbatory attempt to get attention.