Cordain, More

Robert Crayhon: Did we move from a hunter-gatherer lifestyle by choice, or were we forced into the shift due to animal extinction?
Loren Cordain: If we examine the fossil record, it suggests that a number of environmental pressures may have forced humans to adopt agriculture, including increases in human population densities and the depletion of easily hunted game. The extinction of large mammals all over Northern Europe, Asia, and North America coincide with the adoption of agriculture.
It is quite likely that pre-agricultural man had sufficient knowledge of his environment to know the life cycle of plants, to be able to sow seeds and grow plants. However, ecologically, it was not necessary, nor energetically efficient to do so when human population numbers were low and game was plentiful. Although agriculture is a vast science and can encompass numerous disciplines, early agriculture essentially involved the domestication, growing and harvesting of cereal grains.
Robert Crayhon: Is there enough evidence to suggest that a diet that includes a large amount of grains is a step down nutritionally, and one that is far from optimal for humans? And how much of the prehistoric diet was animal, and how much was vegetable?
Loren Cordain: The fossil evidence as well as the ethnographic evidence from groups of hunter-gatherers studied in historical times suggests that the diet of pre-agricultural humans was derived primarily from animal based foods.
It is difficult to quantitatively determine from the fossil record the proportion of plant to animal food that was included in the diet of prehistoric humans. However, we do know that hunting of game was an important part of all pre-agricultural societies. Most prehistoric humans followed large game herds, and manufactured tools and weapons which were used to regularly kill and butcher these animals.
Ethnographic studies of living hunter-gatherer societies represent the best surrogate we have for estimating quantitatively the plant to animal subsistence ratios of stone-age humans. We have recently compiled ethnographic data from 181 worldwide societies of hunter-gatherers showing that the mean plant to animal subsistence ratio in terms of energy was 35% plant and 65% animal.
Thus, the fossil and ethnographic data suggests that humans evolved on a diet that was primarily animal based and consequently low to moderate in carbohydrate, high in protein and low to moderate in fat. This is in contrast to the low fat, high carbohydrate, plant based diet which is almost universally recommended by modern day nutritionists.
Clearly, humans can adapt to many types of diets involving multiple macronutrient combinations with varying amounts of fat, protein and carbohydrate. However, our genetic constitutions, including our nutritional requirements were established in the remote past over eons of evolutionary experience.
Human health and well being can be optimized when we use the evolutionary paradigm as the starting point for present day nutrition. 
Obviously, humans have had little evolutionary experience with the modern high carbohydrate, high fat, cereal based diet which is omnipresent in western, industrialized countries, and there is considerable evidence to suggest that these types of diets have the potential for creating health problems in some, but not all people.

http://www.mercola.com/article/carbohydrates/paleolithic_diet.htm