How Humans afford their superior Brains - The Expensive Tissue Hypothesis

Basal metabolic turnover and Kleiber's law

As already stated in the beginning brain tissue is metabolically expensive, which means that it consumes a large amount of energy and, consequently, throws off an enormous amount of heat. The metabolic rate of brain tissue is nine times that of the average of the metabolic rate of the rest of the body. The following table shows the metabolic rates of various organs and their share in mass in an average human body.

Data taken from Ref. [1]

All this suggests that an organism with a pronounced brain simply needs to have a higher basal metabolic rate to cover the increased energy requirements due to the larger brain size. But there are constraints. It was already shown in 1932 by Max Kleiber, that the metabolic rate R for all organisms is proportional to the 3/4 power of the mass M of the organism.
Since the heat dissipation from an organism is proportional to its surface area, the metabolic rate can also be related to the surface of an organism, which will give also a very nice fit.
Due to the general physical and geometric principles upon which this law is based, it shows an impressive universality. Just have a look at the following graph:

Data taken from Ref. [1]

Now as we know about Kleiber's law, we know that there is a limitation for the metabolic rate and that humans are not just consuming way more energy to build and maintain their brains. Explanation: According to Kleiber's law, any animal species with roughly the same mass as a human, does also have roughly the same metabolic rate, but none of those do have such big brains as we do. Therefore it rises the question how do we pay for our larger brains?

The Expensive Tissue Hypothesis

At this point, the reader should recall again the first table: There it was shown that in terms of their relative metabolic rate the most expensive organs are the brain, the gastrointestinal tract, the liver, the kidneys and the heart.
In order to be able to afford a more pronounced brain compared to our humanoid relatives or other animals in general, humans must therefore make savings in one or all of the other expensive tissues. To shed light on this, Leslie C. Aiello and Peter Wheeler performed extensive investigations which revealed the following picture:

Data taken from Ref. [1]

Their research clearly showed, that the extended brain size was compensated by a significantly reduced GI tract size. It turned out that the GI tract of a 65 kg human is just little over half the size of the GI tract of a similar sized primate!

So now we do have an approach to find rational explanations of how human's unequalled encephalization was possible: According to Aiello and Wheeler, it was increased diet quality that allowed the gut to get smaller while still absorbing the necessary nutrients to fuel the metabolism. And how did our ancestors, the early hominoids, incresase the quality of their diets in comparison to our relatives, the primates?

To allow the gut to be smaller we need food, which is easy to digest, which delivers a lot of the nutrients that animals need to maintain their body and and these preferably in a form in which they are easily taken up. For example the essential iron uptake very much depends on the oxidation state of the iron:
Animal sources give Iron 2+, which is readily taken up.
Plant sources mainly give Iron 3+, which has a worse resorption.

So the fact that our ancestors started eating more and more meat, and even starting cooking it over fire and therefore outsourcing an energetically very costly first breakdown process, allowed them to evolve in the direction they did. Carbon-13 Isoptope Analysis starting with Australopithecus africanus and also performed on Homo confirmed the increasing consumption of meat the higher up the tree you go.

Main Conclusion

Using the here briefly summarized Expensive Tissue Hypothesis and the basic principles of Evolution Theory, we do not come to the conclusion that humans developed to be carnivores, but that we have developed because we started eating meat in the first place.
Nonetheless as already the the word "Hypothesis" implies, it is not necessarily true in all aspects. There was a lot of research work done, and very impressive, undeniable results were found, but in the end scientists working on Why- and How-questions concering historical biology are forced to establish a hypothesis, which has to fit the findings as well as possible and should be questioned for the sake of improving it.

Disclaimer

This article is not meant to attack vegetarian mentality and does also not imply in any form, that humans need a minimal meat uptake or anything like this. Our bodies are undenieable capable of processing animal products and in some aspects even in a better way than the herbal alternatives. The very fact that our digestive system is capable to process animal products and that an organism will always lose unnecessary abilities due to evolutionary pressure should speak for itself. - We are built as ominvores. - But you don't have to be one.

If you are interested to learn even more about this topic, there is a whole lot more of research results, hypotheses and rationalizations concerning the encephalization of humans. As a quick start I suggest to start with the References I stated at the very bottom.

And as always: If you have any particular question or encouragement, feel free to write them down in the comments. I will try to answer everyone.

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moutain.phil28

References:

1. Leslie C. Aiello and Peter Wheeler: The Expensive-Tissue Hypothesis,
   Current Anthropology Vol. 36, No. 2 (Apr., 1995), pp. 199-221
2. An Universe Review on the Kleiber's law
3. A Signs of the Times - Article