Remember that the human body is adapted to frequent physical exercise. So the story would really be more accurate to say that lack of exercise impairs brain function due to less release of irisin.
Perhaps a small difference in wording, but I think the change in perspective is important.
Nearly everything about human physiology can be thought about what would have worked for hunter-gathers. We're specialised in some areas, most of which doesn't require a constant practise (e.g. a 60 year old can start training for a marathon). But the one of the basics of that life was daily exercise, it's a shame it appears avoidable to us now.
Of course, "daily exercise" may have simply included what amounts to a long walk, no? Not everything was heavily labor based. Certainly not year long.
More, if you go far enough back (not actually that far, frighteningly) and things like "respect for the life of others" drops out rather sharply.
So.... what, exactly, is the lesson here? My takeaway is that rose colored glasses can make even some of the most horrid of conditions (human history), somehow look good. Me? I'll take the present where I have a reasonable expectation that my kids will not die before me. By a long shot.
I would find that a little tough to believe. I could see long walks occasionally, but sustained walks of ~8 hours a day just doesn't make sense. Where would we have been walking? Just giant circles?
"... the Hadza, a population of traditional hunter-gatherers living in the open savannah of northern Tanzania. Despite spending their days trekking long distances to forage for wild plants and game, the Hadza burned no more calories each day than adults in the U.S. and Europe. The team ran several analyses accounting for the effects of body weight, body fat percentage, age, and gender. In all analyses, daily energy expenditure among the Hadza hunter-gatherers was indistinguishable from that of Westerners."
From what I've read, I think I'll gladly take the physiology forced upon me by what hunter-gatherers were used to, over what subsistence farmers were used to.
Perhaps a small difference in wording, but I think the change in perspective is important.