The caveman lifestyle, in Mr. Durant’s interpretation, involves eating large quantities of meat and then fasting between meals to approximate the lean times that his distant ancestors faced between hunts. Vegetables and fruit are fine, but he avoids foods like bread that were unavailable before the invention of agriculture. Mr. Durant believes the human body evolved for a hunter-gatherer lifestyle, and his goal is to wean himself off what he sees as many millenniums of bad habits.
Here's an awesome video that this brother and sister team submitted to MDA as a contest entry. It's them doing their version of a primal workout, in Philly, set to the Eye of the Tiger. I think it's awesome!