Obviously, one of the most important things that separate man from all other animals is his intellectual ability. The rapid increase in the size of our brains over just a few thousand generations is the combined result of a high-fat, high protein diet (see rule #1) and a continued reliance on complex thought – working the brain out just like a muscle. Hunter gatherers all around the world have developed language, tools and superior hunting methods independently. The fact that some haven’t entered the industrial age doesn’t mean they don’t possess the same ability to process information rapidly and effectively (try living in a jungle where you need to catalog thousands of different plant and animal species, knowing which can kill you and which can sustain you).
Exercise your brain daily as our ancestors did. Be inventive, creative, and aware. If your work is not stimulating (or even if it is), find time to read, write, play an instrument and interact socially.