The reasoning is that human genes evolved over millions of years to reflect the hunter-gatherer lifestyle.  The Paleo Diet tries to avoid foods such as dairy, grains, legumes, processed oils and refined sugar that were unavailable to cave men.  Anthropologists Ken Sayers and Owen Lovejoy concluded that the elements of the Paleo Diet are almost impossible to determine.  Cave people ate whatever was available, and no specific food group was universally important.  The average life span of Palaeolithic people was 25 years, but some lived to be older.  Nutrition was based on survival and reproduction, so genetic adaptations were not necessarily aimed at extreme longevity. Also, modern fruits and vegetables have been selected over time to promote desirable properties that are much different from those available during Palaeolithic times. Many diseases linked to poor nutrition occur because we are living so much longer.  It’s difficult to say whether consuming a simulated Palaeolithic diet – whatever that is – is any healthier.