In keeping with society and the media's traditional fixation on fad diets, some proponents of the Paleo diet don't give due credit to, or neglect entirely, the impact that physical activity plays in maintaining a healthy weight. This is somewhat ironic, because the most important reason that their idolized caveman ancestors were so "lean and mean" was probably due to how they lived instead of how they ate.
In searching for nuts and berries and hunting for wild game, our ancestors were forced to exert themselves physically. Attaining nourishment was not as simple as a stop at the supermarket; it required time and sometimes stressful exertions. An entire day's duration was, occasionally, spent searching for food. For the modern-day American living a mostly sedentary lifestyle, simply eating a carbohydrate-restricted Paleo diet will not make up for the deficit in caloric expenditure.
The comforting truth is that humans can thrive on a host of different diets, including ones featuring lots of carbohydrates. Our species really is a physiological wonder. Throughout history, humans have subsisted as hunter-gatherers, pastoralists, and agriculturalists and have managed to not become excessively rotund or unhealthy. This is because they balanced their eating with physical exertions.
The hunter-gatherer !Kung of Botswana attain about 67% of their energy from plants and consume about 2,100 calories a day. Their average BMI is 19 (normal). The pastoral Quencha of Peru get about 95% of their food from plants and consume about 2,000 calories a day. Their BMI is 21 (normal) . In 2002, Americans received about 77% of their food from plant-based sources and consumed 2,250 calories on average. Our average BMI was 26 (overweight).
To the question of how to be healthy, equilibrium is the unequivocal answer. There is no dietary silver bullet. Severely restricting essential nutrients from your diet -- be they proteins, fats, or carbohydrates -- probably isn't a good idea. You must learn to balance energy intake with energy expenditure.
A good way to achieve this energy stability may not be by eating like a caveman, but instead by living like one. Americans don't have to gather mushrooms or hunt squirrels with spears, but they can try walking to the grocery store. Or they could go "primal" by pretending they're hunting mammoth while on the treadmill.
Heck, standing instead of sitting burns about fifty more calories per hour. Considering that some Americans spend as many as twelve hours per day on their bottoms, that might be a good place to start.