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Patrick Guilfoile: Do our brains make us weak?

One of the big questions in evolutionary biology is why we are so different compared to our closest living relative, the chimpanzee. Even though we are genetically very similar, there are great differences in outward appearance, behavior, culture and other characteristics.

Consequently, scientists have explored the role of non-genetic differences in distinguishing us from our other primate relatives. In a recent report, scientists from universities and institutes in China, Japan, the United States and Europe teamed up to better understand some of these non-genetic differences. Specifically, they looked at metabolites, small chemicals that are products of the breakdown of foods, in different tissues in humans, chimpanzees, macaque monkeys, and mice.

The scientists determined the presence of metabolites using several different tools that gave them confidence that what they were seeing wasn't simply an artifact of a particular analytical method. In the case of some of these tissues (for example the kidney) the differences between humans and the other species was modest and expected, based on the evolutionary distance between us and them.

However, for two of the tissues, there were dramatic changes in the metabolic profile. In the pre-frontal cortex, a part of the brain associated with cognition, there was a dramatic difference in the type and quantity of metabolites, when comparing humans and the other animals. More specifically, the pattern of metabolites for this region of the brain had changed about four times faster than expected, based on the time since we shared a common ancestor with Chimpanzees. While this is impressive, even more striking was the discovery that the metabolites in our muscle tissue changed about eight times more than expected.

The researchers did two other experiments to try to put these results in context. One experiment involved having macaque monkeys live a "couch potato" lifestyle of limited exercises and a fat and sugar-rich diet. This didn't lead to much change in the metabolite profile in the muscle or brain, suggesting that environmental factors couldn't explain most of the differences between humans and the other animals. The other experiment involved testing the strength of humans and the other primates, to see if those metabolic differences mattered. Although this testing had a number of caveats, it appeared that the chimps and monkeys were about twice as strong as even highly trained human athletes.

Although this is preliminary work that requires further testing, it does lead to an intriguing idea. Our large and active brains use a lot of energy. Therefore, it may be that our muscles have gotten weaker to allow the brain to consume a larger portion of the energy available in our bodies. Or, to put it into a philosophical context "I think, therefore I am weak".

More information is available in the article by Bozek K, Wei Y, Yan Z, Liu X, Xiong J, et al. "Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes Parallels Human Cognitive and Physical Uniqueness." PLOS Biology doi:10.1371/journal.pbio.1001871, May 27, 2014