I would like to try to spell out a couple of key concepts behind Paleo diets as I understand and use them.
The idea is not to duplicate the way that our Paleolithic ancestors ate. For example, there is no reason to reject technology and return to the stone age or to become hunter-gatherers. Instead, the idea is to duplicate the metabolic milieu that we evolved under, while also staying consistent with your individual needs. This means diets may be somewhat different for those with Northern European heritage than those with ancestors from the tropics or Africa, for example. It also means that it won’t be exactly the same for everyone who has the same ancestors: there is plenty of room for person-to-person differences for things like allergies, individual taste, etc.
Duplicating the metabolic milieu leads to things like eliminating grains, since they weren’t a big factor in the diet of humans until the advent of agriculture only 10,000 years ago. Grains are associated with gluten sensitivity, which may be much more widespread than commonly thought. This approach also leads to things like eating grass-fed animal products, maintaining food variety, eliminating processed or artificial foods and drinks, eating foods that are available locally in-season, avoiding those that are imported from far away (such as tropical fruits for those living in North America or Europe), and eating mostly cooked foods and comparatively few raw ones.
Another concept is minimizing insulin levels. This is because insulin is now known to be a pro-aging hormone, and is responsible for a large constellation of modern health problems, ranging from high blood pressure to heart disease to diabetes and even cancer. Although it partly follows from the first concept, it needs to be called out on its own, since several derivative principles aren’t clear otherwise, such as minimizing carb intake and adding support to the benefits of exercise. How strict one needs to be to achieve this goal seems to vary tremendously from one person to another. You can get some idea how you’re doing by measuring your blood glucose level an hour or so after you eat (using an inexpensive meter), and by using KetoStix to watch for ketones in your urine (small amounts of ketones means you’re in mild ketosis, which also tends to imply that your insulin levels are low).
I personally follow a third concept, similar to the “defenseless when dead” approach advocated by Kurt Harris. My spin on that probably isn’t part of “traditional” Paleo, if there even is such a thing. That is, to minimize toxins in my food and drinks, and with things that I smell or put on my skin. Through my research, I’ve learned about the tremendous negative impact that even micro amounts of toxins can have on the body (the brain and the immune system seem to be especially sensitive), particularly over long periods or if you’re starting from a nutritionally deficient state. In addition to eating organic, nutrient-dense foods, as supported by the first concept, this also leads to avoiding certain foods, such as seeds, which contain chemical defenses against being eaten.
Other than those high-level concepts and their derivatives, many of the specific food choices and preparation methods are open to individual choice.