Zookeepers learned long ago that in order for an animal not just to exist but to thrive, be healthy and reproduce in captivity, it was necessary to replicate as closely as possible in the zoo the animal's natural habitat.  Part of that requirement was diet.  This well-written article explains the Paleo approach to diet by Dr. Loren Cordain, a member of the faculty of the Department of Health and Exercise Science at Colorado State University.