Imagine eating like cavemen. That's what people who follow the Paleo Diet do.
The theory behind it is that our bodies haven't changed a lot since the Palaeolithic Age, and we should still be eating the same way our ancestors did back then.
Basically, it involves eating anything that can be hunted, fished or gathered: game, fish, vegetables, fruit... or food that is as close as possible to that ideal, such as grass-fed beef and lamb. Dairy and grains are almost completely excluded.
Truth is no one really knows if this is a healthy diet. Some experts say it is a fad diet, while others see it as the future of nutrition.