(This is a repost of content that I first posted on the original Running Primate blog in October 2016. I haven't updated anything but I *think* I stand by what I wrote. I'm resposting this because it goes hand-in-hand with another old article I reposted- "An evolutionary perspective on cross training for runners". Together I think these articles present a relatively-coherent hypothesis explaining the evolutionary factors that have influenced the exercise response.
(11/10/18 edit: the main idea here seems to have come from Dan Lieberman and is not my own (see this paper), and is otherwise inspired by the work of Herman Pontzer.)
Have you ever wondered why fitness is so hard to get? Why you have to run so many miles and lift so many things to gain endurance, speed or strength? And why these fitness gains diminish so quickly after even just a few days away from training? To extend the question to all animals: Why aren't creatures simply endowed, genetically, with the physical abilities they'll need in life?
Exercise capacity in animals is not simply programmed by genes. Like all other genetically influenced traits, the physiological components of fitness-- things like muscle size, mitochondrial density, stroke volume of the heart, etc etc etc-- are encoded by genes but these genes are only expressed in the right environment. Without an exercise stimulus, these genes are not expressed. To a biologist, this is a classic illustration of the principle that genes don't work in a vacuum. (Genes evolve as part of an entire organism, and the organism is part of a complex ecosystem.)
Evolutionary biology has a pretty good explanation for the Use It Or Lose It nature of fitness: it's a matter of energetic economics. Remember that natural selection is blind to everything other than reproductive success. In other words, only genes and traits that contribute to increased reproduction, directly or indirectly, should evolve to become more common. This is evolutionary biology dogma and I'm not suggesting anything new here (or at any point yet in this post). It makes sense that animals have genes that can, in the right conditions, make them fast, strong, and have great endurance-- being strong and fast can help get food or avoid being food, and this will help you have more offspring. But because the physiological components of fitness require energy to produce and maintain, it also makes sense that animals have evolved highly-sensitive mechanisms for producing these adaptations, and that they are no longer maintained when you become sedentary again. An antelope's aerobic capacity will develop as it runs more, much as yours will too; but if the environment doesn't require much running, it makes sense to spend that metabolic currency elsewhere, on things that will more directly lead to increased reproduction, such as a stronger immune system (and indeed intense exercise does compromise the human immune system). In short: There's only so much energy to go around and your body will only spend it on exercise adaptations if it's apparent that they're necessary.
Some adaptations to endurance exercise, off the top of my head, that all incur a metabolic cost to produce and maintain: Increased muscle mitochondrial number and sizeIncreased muscle oxidative enzymesNew blood capillaries to feed muscleBone formation- thicker cortical bone, increased thickness of trabecular boneIncreased thickness of heart muscle, heart overall grows in sizeIncrease in lung capacityIncreased production of red blood cellsAdaptations to shorter, more intense exercise, such as strength and sprint training: Increased muscle size (muscle is a very expensive tissue!)Increased muscle recruitment by the nervous system (ability to engage more muscle tissue)...and, endurance and strength adaptations will often diminish each other. Endurance training limits the muscle growth response triggered by strength training. Muscle fibers change which proteins they express and take on characteristics useful to only strength or endurance, not both. This seems to be true for mammals in general and not unique to humans.
To me, the observation that humans have the capacity to be great endurance athletes, but that our bodies will only realize this capacity with a sustained exercise stimulus, is evidence for two profound truths about our evolution. First, our ancestors lived on the margins of energy balance, at least at key points in our evolution; that is, they barely scraped by, energetically speaking. Second, endurance was part of our ecological niche, our survival strategy. Natural selection favored those who could develop endurance when they needed it and conserve every pithy calorie otherwise. Further evidence for this lies in our body fat: we can stores prodigious amounts, more than any other primate, and it's both a great starvation buffer (energy saved for lean times) and great endurance fuel. Of course, being energetically thrifty and only producing exercise adaptations when necessary is not unique to humans- these genetic traits evolved in earlier animal groups-- but humans seem to have elaborated on this trait. And, I'd like to say that non-running animals don't respond as robustly to exercise as running animals do (we do know they have lower aerobic capacity relative to body mass), but I'm not sure this research has been done. If you see any, please send that along!
The fairly recent discovery that some people respond more vigorously to exercise than others can inform this topic. (NYT article here , though the Wellness/Phys Ed section of the Time really chafes me.) About 35% of people fall outside of the normal curve for exercise adaptation-- 15-20% are "non responders", getting little or no gain in aerobic capacity, and 15-20% are "super-responders", reaping greater aerobic awards than others. As with other genetic traits that have a bell-curve like distribution, this is evidence that natural selection has worked against both extremes-- the non-responders and the super-responders alike. In evolutionary biology this is called "stabilizing selection"-- the middle phenotype (physical trait) is most common as the extremes are selected against. So even though being a super-responder might be great now, as a person trying to get fit or as a competitive athlete (and I suspect that many elite athletes are super-responders), it clearly wasn't so great in our past, or this phenotype would be way more common.
This makes sense given what we discussed above. Early humans who thanks to the genetic lottery were unable to increase their aerobic capacity would have had less reproductive success-- why, exactly, we're not sure, because this brings us back to the big unanswered question of "what exactly did humans need endurance for?", a question that drives me and will for many years. (The answer surely has to do with covering ground to find food.) Genes influencing super-responding, too, have been selected against and are now fairly uncommon because adapting too vigorously, or producing adaptations at too low a stimulus threshold, would expend more metabolic energy than is necessary, pushing these folks closer to the business-end of energy balance and therefore compromising their immune systems and generally putting them at all sorts of disadvantages.
Evolution does not have the benefit of limitless traits upon which to act and select. Instead, it is a ceaseless process of selection between only a handful of available genes; and then the next handful; and then the next. But this process does favor traits that are even 1% more advantageous than the alternatives, until at some point you end up with a pretty good solution, with continued selection against the other alternatives (above, the super- and non-responders). But of course, given our modern lifestyle, selection for the ideal exercise response is surely relaxed, and who knows what this nice bell curve graph would look like in 10,000 years?....