Prehistoric humans likely exercised as a matter of course. They weren't sedentary. The brain evolved assuming the condition of exercise. naturally occurring byproducts of exercise were just always there for the brain to use.
This is my favorite explanation. It may be that the deficiencies of these chemicals are harmful and the release of them returns us to "regular" function, but since we are so sedentary it appears that we are getting a benefit.
Physical activity levels were also within a standard deviation of one another (between western and h/g populations).
The brain evolved in similar conditions to modern man. I would argue current conditions are far better for the brain in terms of good nutrition and low disease.
The effect in the OP was not selected for, it is likely a serendipitous biochemical interaction. Repair enzymes from one part of they also effecting another.
Within a standard deviation (just) of the hunter gatherers, but not for Western males.
PAL is 2.26 +- 0.48 for Hadza male and 1.81 +- 0.21 for Western Male. That's an average 25% difference in activity. I'm not a biologist but it seems like that could surely be significant.
Especially if the protein being talked about is generated by strenuous activity above some level. 25% total difference in activity could be a very large difference in activity above a threshold, since both groups are going to spend a lot of the day sitting around, sleeping or walking slowly.
Selection pressure doesn't care about standard deviations, believe it or not. Even a small difference can exert evolutionary pressure over a long period of time.
Minute changes are actually a huge part of evolution. Evolution is much more about minute changes than it is about huge (standard-deviation-level) changes.
As far as calories go, exercise doesn't have much impact. And maybe the Hunter/Gatherers are more efficient in their metabolisms.
As far as exercise goes, maybe one standard deviation is enough to produce a brain effect. And maybe there is a lot of variation within modern populations, with many far enough below the average to have a significant effect.
I find it really hard to believe that any healthy Hunter/Gatherers sit around all day like so many moderns do.
> Repair enzymes from one part of they also effecting another
That's an even more fascinating idea; so what other things could create the conditions for other repair systems to be activated. Maybe meditation, maybe different drugs, plants, ketosis etc...
Additionally, the lymphatic system doesn't have a pump like the circulatory system. So not only does exercise improve blood circulation thereby improving oxygen and nutrient delivery body wide, exercise improves lymph flow thereby assisting in removing metabolic waste products from the interstitial fluid. Also, the lymphatic system can be considered an immune organ, so exercise improves the immune response.
And, of course, we know nutrients like Zinc and Vitamin C are essential for tissue repair via being essential for DNA replication.
This does not imply that, for example, a daily or weekly peak of energy expenditure was the same. As training once per week for 15 minutes with weights is enough to grow muscles, it could be that hunter/gatheres needed to release energy at the max level like running away from a lion much more often than we do nowdays and during those releases the brain also had to work hardest.
>during those releases the brain also had to work hardest
That is speculation, and is unintuitive. Why would the brain "work hardest" when running? Why would modern humans not exert the same forces during sport? Why would humans, the top group-oriented predator, run from a lion?
The data linked above shows that observed Physical Activity Levels and caloric use are comparable to modern day. Stop inserting your own assumptions into the data.
The data do not indicate that we experience the same need to work at max energy for brief periods of time as hunter/gatheres. Besides running away from predators in most efficient way, consider throwing оf javelins or stones during hunting. It requires a very precise muscle work at max energy after intense brain activity of target selection. Modern day life does not involve that, yet evolutionary this combination of brain/muscle work at peak could be advantageous.
Any study that shows present-day humans (at least those living in Western societies) to get the same amounts of exercise as any other culture is bound to be deeply flawed.
