Tuesday, June 20, 2017

Pontzer: Economy and Endurance in Human Evolution

Herman Pontzer, whose interests for years have orbited around biological, evolutionary and ecological issues related to human endurance, has finally jumped headfirst into the question of whether/how/why/when human endurance running evolved.  The paper (here) in Current Biology -titled "Economy and Endurance in Human Evolution"- is a review and doesn't present original research.  There is, however, some novel (or at least more explicit) synthesis here.  A few things jump out:

1) He begins by distinguishing the terms economy, endurance, and efficiency.  (I've been saying that greater speed and endurance, which together would permit larger travel distances, need not have been super economical in terms of energy cost if they permitted access to novel food resources.  This isn't a point he makes here but he's hinted at it before and the distinction of terms reminds us that these are different things.)  In short: economy is calories spent to move a given distance, per unit of body mass. Endurance: how far you can go at a given speed.  Efficiency: how well the energy-consuming parts of the locomotor chain are translated into forward motion.

2) The paper outlines the biological determinants of endurance and describes which of these features are observable in the fossil record and which can be inferred through other means.  Leg length and joint surface areas tell us alot, and they fossilize.  Other things don't fossilize but we can make inferences:  Daily ranging distance probably increased in early Homo because there's good evidence for increased hunting and scavenging, and carnivores are known to have larger home ranges than herbivores.  Another intersting point: he chalks human endurance gains up to increased leg muscle and mitochondria, which permit greater VO2 max, which he says is the most important advancement.  I wonder if muscle and mitochondria are really more important than other parts of the cardiorespiratory chain-- hematocrit, capillarization, heart size/stroke volume, pulmonary diffusing capacity, etc.

3) As early hominins adopted part-time bipedalism, they would have lost some locomotor speed and endurance, as their still-long forelimbs were taken out of the locmotor equation.  No longer able to use their considerable upper-body muscle mass for walking, their VO2 max would drop.  A chimp suffers a 22% VO2 max detriment when walking bipedally.  This reinforces the idea that the move to bipedalism was not driven by economy, but rather it evolved despite its costs because it had other benefits (and there are lots of ideas about what those could be).

4) We're reminded that traits enabling modern humans' running economy and endurance didn't show up all at once, but followed mosaic and piecemeal patterns of evolution.  Long legs seem to be present before H. erectus, with Australopithecus (and thus the former didn't immediately represent a big change in walking/running capabilities).  As for the emergence of increased running economy (energy cost to run a given distance), he pegs that at around 1 million years ago, a million years into the era of H. erectus.  (The modern human foot shows up then.)  H. erectus specimens found within the last 20 years demonstrate that this species was highly variable, with some populations retaining primitive traits while others looked more modern in their endurance anatomy (think Dmanisi erectus vs. the lanky Turkana Boy).  Truly modern running economy shows up perhaps as recently as 200,00 years ago, only with our own species, as hips didn't narrow fully until then.  But, even early erectus was enough of a runner that this behavior was almost certainly important.

It's this last bit- discussing when and where the most recent advances in economy and endurance occured- that I find most interesting.  It's also the hardest to pin down.  We are still a long way from understanding when running was used, how important it was to survival, and therefore how strongly natural selection favored running economy and endurance.  Every week it seems like a new find adds complexity to the story of this time period-- evolution tinkered with unique combinations of traits; some populations became isolated while others mixed with their neighbors.  I wonder how close we'll ever actually get to understanding the origins of modern endurance.

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