Wednesday, May 25, 2011

Did Humans Evolve to Eat Meat or to be Vegan?

The Curious Vegan has only eight days left to complete his school project and his period of temporary veganism.  My current reading has taken me to some psychology and some anthropology.  The anthropology is interesting but descriptive rather than normative.  It is inevitable that people who feel strongly about eating meat or not eating meat will grasp for evolutionary arguments in their favor.  However, The Curious Vegan has been trained to be positive.  To take an extreme example, humans might "naturally" kill competitor humans or even step-children, but that hardly teaches us to be killers when our turn comes.  Indeed, our legal system has good reason to fight and discourage those "natural" inclinations.

In the case of meat, the anthropological question is whether humans are "naturally" meat eaters, or vegetarians, or even vegans or raw vegans.  Humans have less impressive teeth than lions; we are slower than cheetahs; we cannot swim in pursuit of smaller fish; we cannot fish well with our "bear" hands; our "paws" do not fell gazelle; and our thumbs are arguably excellent for gathering nuts and fruits.  It is easy to see why many people jump to the conclusion that we are natural vegetarians.  On the other hand, it is easy to get carried away with this kind of thinking.  Gorillas have very impressive canines and yet they are mostly herbivores.  One cannot look at an animal and draw quick conclusions about its natural diet, if there is such a thing.

Then I read an interesting article that convinced me that there is a strong case for the idea that, in fact, humans evolved because they (we) ate meat.  We are what we ate, I guess.  Here is the argument.  Larger animals have higher metabolic rates.  Max Kleiber, between 1947 and 1961, showed that the metabolic rate of virtually any animal could be determined by knowing its body size.  We produce more heat than does a rat, and a horse produces more heat than do we.  On the other hand, there is a negative correlation between body size and metabolic rate per kilogram.  These two correlations form the basis of Kleiber's Law. Kleiber's Law thus connects body weight and metabolic rate among many observed species.  The law can also be used to estimate the metabolic rate of extinct creatures.  The problem with this is that it says nothing about brain size and we know that the brain uses an enormous amount of energy.  If the total energy is the energy used or required by all the organs, and the human brain is much larger than the brains of earlier hominids, then it stands to reason that either the energy produced of humans is higher than for other hominids or another organ has shrunk so that it uses less energy.  Assuming Kleiber's Law is correct, only the second option remains.  Only a few of the human organs use significant energy.  The brain, the heart, the kidneys, the gastro-intestinal tract, and the liver use the vast majority of the energy.  In fact, when humans are compared to similar-sized primates, it turns out that the GI tract in humans is smaller by an amount almost identical to the increase in mass of the human brain.  The human brain grew, and the human guts shrunk to compensate for the increased demand of the brain.  This could only happen if a change in the diet allowed a smaller gut to process food sufficiently.  This, in turn, would require a "higher quality diet."  In short, if we had remained on a diet high in vegetation, as gorillas do, we would not have been able to expand our brains.   It is common to attribute the larger brain to the advantages of co-operation and other complex group activities, but of course gorillas would also benefit from larger brains.

There were at least two big steps in brain size.  One can be associated with the introduction of meat into the human diet and the other with the introduction of cooking.  Cooked meat is yet easier to digest than raw meat.  Cooked vegetables are also easier to digest than raw vegetables, but the first step focuses attention on meat, because the introduction of cooked vegetables would not explain both steps.

The Curious Vegan refuses to make a final normative jump.  It is tempting to say that if we evolved because we ate meat, we should not stop now.  However, I have already cautioned against legislating our present and future behavior on the basis of the evolutionary past.

1 comment:

  1. I'm not sure why you think that it is even "tempting" to say that if we evolved to eat meat, we shouldn't stop now. Doesn't it all depend on whether meat is necessary for our bodies in the present day given our current form of life? Many things that had some payoff in evolutionary prehistory are not important now, and may even be harmful: the whole field of "behavioral economics" exists to study behavioral patterns that may lead us to make choices that are often inefficient. And it's pretty likely that many of the worst things about us (the narrowness of our sympathy, our tendency toward aggressive putdowns of others, male domination of women) had some evolutionary basis. Still, if we can figure out ways to live better by reining these tendencies in, we ought to do it! Competitive sports, for example, gives an outlet for aggression that is pretty harmless, and certainly preferable to rampages of killing. Evolution shows us only what equipment we have, which may either help or impede us; it doesn't tell us what to value.