We have all heard the argument that, due to intestinal lengths, humans were not designed and/or physically evolved to consume meat. The argument usually goes something like this:
Carnivores have intestinal tracts that are only 3 times the length of their body. Herbivores have intestinal tracts 10-12 times the length of their body. Humans have intestinal tracts that are 10-12 times the length of their body therefore humans are Herbivores.
To further this argument, often the antagonists suggest that the fact that humans lack teeth and claws is empirical evidence that humans are unfit for meat consumption. Some arguments go further and also claim that Herbivores perspire through skin pores, humans have skin pores, therefore humans are herbivores.
Professor of Human Evolutionary Sciences at Harvard University, Daniel Lieberman’s excellent book The Story of the Human Body: Evolution, Health and Disease delves right in and tackles the subject matter very thoroughly and scientifically in a sub-chapter entitled “Guts and Brains.”
Examining the evolution of the Guts and Brains of genus Homo, there is a clear relationship with hunting and gathering. Lieberman demonstrates that both the intestines and the brains are very calorie costly tissues, each requiring about 15% of the body’s total metabolic toll and each requiring similar amounts of blood and oxygen. A unique characteristic of humans is that both our guts and our brains are very large. If you compare humans to other animals of similar body mass, the animal brains are much smaller while their guts are much larger -usually twice as large as the intestines of humans. Basically, humans have comparatively small guts and comparatively large brains.
Referencing the Expensive Tissue Hypothesis (Current Anthology vol 36/ Aiello, Wheeler), the conclusion is that hunter gatherers traded off large guts for even larger brains by gradually shifting to higher quality diets that included meat as a cornerstone. Conveniently, arguments that counter this conclusion tend to omit the importance of tools and weapons. Humans were able to digest a wide variety of food that included meat, fruits, tubers, nuts and seeds because they had developed the ability to process these foods through means of slicing, grinding, tenderizing and, of course, cooking. Based upon this, “the energetic benefits of hunting and gathering appear to have made possible the evolution of bigger brains in part by allowing the first humans to make do with smaller guts” (Lieberman, 92). Carving out an existence by hunting and gathering required more cognition (i.e. a larger brains): Bands of humans had to cooperate and further develop technology in order to survive as hunter gatherers. Likewise, our bipedal frame, unique ability to perspire and lack of fur allowed us to run great distances without overheating.
Known as persistence hunting, the method allowed hunter gatherers to run game animals (including Herbivores who will overheat at a gallop because of their inability to perspire through thousands of skin pores) into exhaustion and then dispatch of the animal with handheld weapons such as rocks and spears. Eventually, human cognition crafted the bow and arrow which replaced the need for persistence hunting, but the human endurance abilities remain with us to this day (see Christopher McDougall’s Born to Run for more information)