Hungry brains and human endurance

A May 2014 study was widely reported in the news as demonstrating a co-evolution between brain size and weakening muscles.  Headlines were all along the lines of, "Humans traded brawn for brains", etc-- see a list of articles here.  (We've known for a while that hominin brains expanded tremendously over the past 5 million years, and this roughly coincided with a trend of thinner, lighter bones and probably reduced musculature.)  At the time I gave the paper a cursory read and didn't think much of it.  But lately I've been on a bent looking for comparisons of exercise biochemistry between humans and other animals, and in rereading this paper, I think there's a big idea in here that the news overlooked.

Human skeletal muscle.

The paper (here) analyzed several body tissues of humans, chimpanzees, macaques, and mice, looking at over 10,000 "metabolites"-- small molecules involved in, and mostly products of, energy metabolism.  The idea is that any difference in the types and amounts of these molecules represents a difference in how each species' tissues use energy.  Most telling would be any differences between humans and chimps-- these differences must have evolved in the time since we shared a common ancestor, 6-7 million years ago.  The big story is that the researchers found very big differences in metabolites between species.  Metabolism in the human brain--and most interesting to me, skeletal muscle (the vastus lateralis, a quadrucep muscle)--has changed way more in humans than in chimps in these 6-7 million years: Human brain metabolism has changed 4x more than that of chimps, and muscle metabolism 8x more.  In other words, there was bigtime evolution happening in hominin brains and especially muscles in the way these tissues use energy.  

We know how the human brain is different from other primates-- it's bigger and more complex.  To assess how the observed differences in metabolites have translated into actual muscular differences between species, the researchers conducted a simple test confirming what we already knew, that macaques (a stand-in for all primates) have greater muscular strength than humans.  The researchers concluded that evolution was reducing muscular strength in human ancestors at the same time that brain size was increasing, and that these processes were interdependent: muscle energy consumption was reduced to accommodate the energy needs of a bigger brain.  Thus, the authors were arguing a modification of the expensive tissue hypothesis...a bigger brain could only be supported by reducing energy consumption in other tissues. 

(Recent research questions this "zero sum" model of human energy budgets.  A more "expensive" brain could have been fueled by increasing overall energy intake, without necessitating that some other body tissue reduce its energy consumption.  But I digress.)

This conclusion-- that the difference in human vs. primate muscle metabolites points to an adaptive reduction in energy use by human muscles, in order to feed a hungry brain-- is what the news seized upon.  But the authors suggested another idea too: human muscle metabolism may be different from other primates not because we had to get weaker in terms of muscular strength, but because human muscle had to develop more endurance.  If natural selection shaped our muscles for endurance pursuits-- long distance walking, even running--this could explain the profound differences in metabolites seen in this study, and it jives with the observation that humans are built (metabolically, muscularly, anatomically) for distance in a way that no other primate is.  And rather than reducing its energy need, human muscle would actually be requiring more energy than ever. 

To me, the story here is that this study has identified selection for endurance-specific muscle metabolism in humans.  Further investigation into some of these specific metabolites could tell us a lot more.  I expect that such analysis would reveal that many of the uniquely human metabolites are involved in aerobic metabolism of type 1 muscle fibers.  Many of these may be derived traits of human metabolism, though some I suspect will be similar to independently-derived metabolic traits in other cursorial species, like wolves. (Gotta love convergent evolution.)  This could be some of the best evidence yet for the evolution of endurance.