Michelle from nomnompaleo.com has put together a great description that captures the essence of eating Paleo:
In a nutshell, the Paleo diet (or, as I like to think of it, the Paleo template—the word “diet” tends to mislead folks into thinking this is nothing more than a temporary weight-loss program) is based on the notion that for optimal health, modern humans should go back to eating real, whole unprocessed foods that are more healthful than harmful to our bodies.
Over the past 200,000 years, humans have biologically adapted best to whole foods: plants, meat, seafood—all of them packed with the nutrients our bodies evolved to thrive on. Agriculture came on the scene a mere 10,000 years ago—a tiny fraction of our evolutionary history—and there simply hasn’t been enough time and evolutionary pressure for humans to completely adapt to eating modern foods like wheat, sugar, chemically processed vegetable and seed oils, and other “Neolithic” foods.
It’s not a coincidence that many modern diseases of civilization—including autoimmune disorders, cardiovascular disease, type 2 diabetes, and rampant obesity—have accompanied the global spread of industrialized food. That’s why the Paleo approach emphasizes returning to a more ancestral approach to eating.