With this article, I'll give some insights into a highly interesting anthropological work, which deals with the *extraordinary intellectual competencies* of humans: ____ **The Expensive Tissue Hypothesis** ____

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First of all, the problem to be discussed: **Brain tissue** is **metabolically expensive**, and anthropologists have been speculating about the **outstanding fast development** and the **striking relative size** of human brains for a long time. From a softball sized brain roughly **2 million years ago** to what we have nowadays: In relation to our body mass humans have the biggest brains! So ***how*** could we afford this? _____ [Image Source](http://jonlieffmd.com/blog/brain-evolution) _____ A lot of work was done on the topic of *encephalization in humans and other primates* asking the ***Why-question***: Why different primate taxa show different relative brain sizes or why humans did show this rapid brain development. Hypotheses tackling these questions primiarily involve socio-ecological factors, but none of them answers the ***How-question***. _____ > 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. _____ ![2.PNG](https://steemitimages.com/DQmVTNE3RcpqxMR29FNXoBGhf5iZ2T8bHE2XD2rsiPS1F5K/2.PNG) 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: ____ ![3.jpg](https://steemitimages.com/DQmSPiFFBhMdYDcMnNEDjRRaiPEBjyJnD9PTkwZcmYNuq6z/3.jpg) 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: ____ ![4.PNG](https://steemitimages.com/DQmfRx8ygEaMaDA3bhFXrYsfynkE82E6UieTUTsaPApLvhz/4.PNG) 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. For more scientific insights follow me! Best, moutain.phil28 ____ References: 1. Leslie C. Aiello and Peter Wheeler: [The Expensive-Tissue Hypothesis](http://www.jstor.org/stable/2744104?seq=1#page_scan_tab_contents), *Current Anthropology* Vol. 36, No. 2 (Apr., **1995**), pp. 199-221 2. An [Universe Review](https://universe-review.ca/R10-35-metabolic.htm) on the Kleiber's law 3. A [Signs of the Times - Article](https://www.sott.net/article/311231-We-became-human-BECAUSE-we-ate-meat-Kleibers-law-and-how-humans-got-their-big-brains)

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