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.

Monday, June 5, 2017

Sherpa metabolic adaptations

The Sherpa ethnic group of the high Himalaya have long interested physiologists.  Along with being better at just generally living at altitude, with all that requires (try keeping up a good immune system and successfully gestating a fetus at 14,000 feet!), they have superior endurance abilities at altitude.  Some are counterintuitive- for example, they don't respond to increased altitude by producing way more red blood cells as flatlanders do- and some of these adaptations are understood to be evolutionary*.  That is, over the thousands of years that Sherpa have lived on the Tibetan plateau, natural selection has favored genes (well, phenotypes) that better support endurance activity at high altitude.  A recent study has identified several biological, evolutionary adaptations at the metabolic level, meaning the chemistry that provides energy for muscles.  Muscle biopsies were collected from Sherpa and lowlander mountaineers during a high-altitude trek and the Sherpa muscles showed some interesting differences: they utilize fat differently as fuel; their mitochondria are more efficient in their use of oxygen (oxidation is more "tightly coupled" to energy production); and there are differences in anaerobic energy production (anaerobic meaning without oxygen).

NPR story here: http://www.npr.org/sections/goatsandsoda/2017/05/28/530204187/the-science-behind-the-super-abilities-of-sherpas
research paper here: http://www.pnas.org/content/early/2017/05/16/1700527114.full

*note that there are three other levels of adaptation:  1) Acclimation, the physiological responses that happen as soon as the body's homeostasis is distrupted. For example, increased breathing and heart rate at altitude, to compensate for less oxygen per breath. 2) Acclimatization, a longer-term (days/weeks/months) response to disrupted homeostasis.  For example, making more red blood cells to compensate for less oxygen at altitude. 3) Cultural/behavioral adaptation, meaning all the ways that people cope with the environment other than biologically.  For example, wearing lots of warm clothes in cold climates. Acclimation and acclimatization are often used interchangeably, which is confusing, but regardless the two adaptation types are distinct.

Tuesday, April 25, 2017

quick share: Homo naledi has been dated

This story doesn't (at least yet) inform the evolution of human energetics, running, or the like, but I feel remiss if I don't share some of the big human evolution stories here, and this one's big.

A few years ago, the Naledi hominin fossils joined the ranks of the Dmanisi and Flores specimens in confusing the hell out of us.  Small brained, primitive looking humans were in places we didn't expect, at times we really didn't expect.  The Naledi hominins are the strangest of all, but we didn't know how old they were.  They've finally been dated to 200,000 to 300,000 years.  That is... very recent.  If this date is verified, there are some big implications for our understanding of the human timeline.