Initially thought of as a ‘good idea’ as most ideas are, the Paleo diet comes from our hunter gatherer lifestyles a very, long time ago (substantially pre-iPhone days). Pro-Paleos argued that we as humans weren’t designed for agri-business and genetically modified foods that give us disease. This apparently includes the humble sweet potato and whole wheat bread. They also declined to realize the human lifespan was about 20 some years old. The article from Scientific American “Proponents of the Paleo diet follow a nutritional plan based on the eating habits of our ancestors in the Paleolithic period, between 2.5 million and 10,000 years ago. Before agriculture and industry, humans presumably lived as hunter–gatherers: picking berry after berry off of bushes; digging up tumescent tubers; chasing mammals to the point of exhaustion; scavenging meat, fat and organs from animals that larger predators had killed; and eventually learning to fish with lines and hooks and hunt with spears, nets, bows and arrows.”—Ferris Jabr
Currently, paleo diets aren’t really paleo. They’re described as basically not eating: processed foods, dairy, lentils, peas, beans, peanuts. Nuts are acceptable because they were growing to some extent 2.5 million years ago. Basically, humans have evolved since our neanderthal days. We all have different ethnic backgrounds and can eat or digest different things. Your Italian grandmother from Sicily might not have consumed much dairy (because there are few resources for it in southern Italy) but your Danish grandfather might have no problems with whole milk as it had been a part of their society longer.
Vegetables have evolved in several million years, check out the book Eating On The Wild Side. Tomatoes, for instance, were more of a sad little berry than a large juicy fruit (thanks modern agriculture). Keep in mind that this diet wasn’t meant for getting every nutrient in. You’ll have to eat a lot of greens to get calcium (if the oxaloacetic acid doesn’t suck it out first). We also didn’t live very long 2.5 million years ago, and current hunter-gather societies today aren’t the picture of health either.