A look at the shepherd's digestive system shows that it is remarkably similar to that of the dog in form, digestive enzymes and length. The only significant difference is that our saliva contains amylase, an enzyme that is used to digest starch. However, in common with all carnivores, we have no digestive enzyme that will break down a plant's cell walls to release that starch. Unlike the sheep we also do not possess in our guts bacteria or other micro-organisms to do the job.

If we eat a largely plant-based diet, the bacteria in our colons will change the environment in our colons from alkaline to acid, which favours the herbivore-type fermentative bacteria. These will break down plant material but, as no absorption of nutrients these contain takes place in the human colon, this is of no nutritional value. All it does is upset our guts and cause flatulence!

The totality of evidence demonstrates that the human digestive tract is extremely inefficient when coping with foods of vegetable origin. With no bacteria and no enzymes capable of breaking down the cell walls to release the small amounts of nutrients inside, we can only eat many of these foods after they have been cooked. As Nature must have intended that all foods should be eaten raw, they cannot have formed a significant part of our diet during our evolution.

During our evolution, therefore, when we lived well, our diet must have been high in animal protein and fat, supplemented with wild fruit, but only during lean times would it include other foods of vegetable origin. As more than 99.9% of our genetic makeup evolved and was determined before we, as a species, started to heat and cook foods, that must still be the correct diet for us today.

In The Naive vegetarian I compared the size of the gorilla to that of a man. The gorilla is a herbivorous animal with a simple stomach. Robert Yerkes found that, in the wild, this vegetarian animal had many protozoa in its stomach which digested plant proteins and synthesised animal proteins. (3) In captivity, however, Yerkes noted that these protozoa gradually disappeared from the gorilla's stomach. In this state, the gorilla was unable to synthesise its own animal protein, and had to be fed meat, milk, or other animal proteins to remain healthy. This was because, although we think of the gorilla as a herbivore, it actually consumes significant amounts of 'animal' protein and fats in the form of insects and other small creatures that live on or in the leaves the gorilla eats. Like all other primates, therefore, the gorilla is not a strict herbivore.