Friday, June 30, 2017

"Chimpanzee super strength and human skeletal muscle evolution"

We know that chimps are super strong-- commonly they're thought of as being six times stronger, pound for pound, than a human.

Fantastic new research (paper here) puts this number at more like 1.5x.  Still, there are clearly differences in human and chimpanzee muscle, and the main contribution of this paper is that they've figured out what this difference is: muscle fiber type.  Chimpanzees, probably like most other primates, have a preponderance of type II (fast twitch) muscle fibers, which generate more force (but fatigue easily) than type I (slow twitch) fibers, which are good for endurance.  (These fiber types differ in protein content, mitochondrial number, stored glycogen and fats, enzymes, capillarization and all sorts of other things that aid in either anaerobic force production of aerobic, oxidative energy production).  Through computer modeling, the researchers rule out other candidate causes for chimpanzees' relative super strength, leaving only fiber type composition and fiber length.  A simple explanation that really makes sense...why would evolution bend over backwards to produce increased strength when something as simple as fiber type ratios will suffice?

But this isn't a muscle physiology paper, nor a primatology paper-- it's really about human evolution.  As our closest living relative, chimps are a great comparison: differences between us reflect the different evolutionary pressures each lineage (the one leading to chimps and another leading to humans) faced over the last 7 million years.  The authors interpret their results as further evidence that hominins (early human ancestors) shifted from the ancestral condition common to all apes-- lots of fast twitch muscle fiber, helpful when you haul yourself around in trees-- to a higher ratio of slow twitch fibers, to complement the multitude of other adaptations that arose for 2-legged locomotion.  Brief mention is made to expanded daily travel distance, and (predictably) I'd again say that running should be included here.  The derived human condition of mostly-slow-twitch muscle, along with all of our other endurance-type adaptations, scream long-distance locomotion.  All in all, this is further evidence telling the story of evolved human endurance.

More on the evolution of human vs. ape muscle here:

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