My understanding is that the research shows that a highly active hunter gathering and an inactive office worker burn roughly the same number of calories per day. But I find that hard to believe. What is the office worker's body doing to make up the difference for all the movement and muscle contraction?

Human bodies are highly efficient at certain movements, e.g. walking. There's also a complicated part of hunter gatherer life style where you don't know if you'll find something if you walk another mile, so often the response is to just... not do that.

Being able to have a life style where you're constantly expending energy, like training athletes do, really only works if you're guaranteed food, which is generally not how hunter gathering works.

Lots and lots (and lots) of small subconscious movements throughout the day. Many thousands of them. Moving your heard around, tilting it a bit, wiggling your toes, rolling your shoulders, scratching an itch, etc.

Anecdotally, I believe I have a personal example of the sort of thing that changes. I went from being ultra-sedentary to exercising 5+ days a week over the past year. My body had enough of these tics that they had long since passed over the subconscious and unnoticed level into "why the fuck do I keep bouncing my legs around all the time when sitting or in bed." Now, I never do. After years of trying, I stopped making any sort of intentional effort to stop - and even things like weighted blankets, etc., in the past did nothing.

Now, I just don't do it. I fidget less at my desk while working. I make a whole lot less 'random' movements for no discernible reason.

Add up enough of all these tiny things over the course of a day, things you probably don't even realize you're doing, and each tiny fraction of a calorie expended eventually pushes you to the levels you see with more regular exercise.

(Now, of course, exercise is hugely important for a variety of other health factors)