I quote Cordain from the paleodiet list: Human's living at northern latitudes preferentially hunted megafauna because these beasts contained both absolutely and relatively more fat. Hence, at northern latitudes wherein carbohydrate (plant food) sources are seasonally restricted, the fat obtained from larger mammals was sufficient to dilute the lean protein from muscle tissues. The fossil record shows that the worldwide extinction of animals that took place at the end of the Pleistocene occurred primarily in animals over 100 kg (220 lbs) (5). Using the Pitts and Bullard regression (2), a 220 lb mammal would be expected to have about 15% body fat. Applying our cubic regressions (4) to this value, a 220 lb mammal would have 60 % of its total body energy as fat and 40% as protein. The protein value then is very close to maximal protein ceiling (also 40 % of energy) -- hence it is not surprising that the "cutoff" values for megafauna extinction (100 kg) corresponds almost exactly to the value for the maximal physiological protein ceiling in humans. In animals weighing less than 100 kg, the entire carcass cannot be consumed unless there is a carbohydrate source, whereas in animals weighing more than 100 kg, the entire carcass can be eaten with no worry about protein toxicity and with no need to find a carbohydrate source.