Thank you for your reply, Dr. Luisi! That's an informative list of potential causes, including one I hadn't thought of before - braces (which I had)! As I mentioned in my original post, I'm most interested in what about our modern lifestyle causes OSA in non-obese people. For the purpose of this thought exercise, this assumes that our bodies evolved over hundreds of thousands of years, weeding out maladaptations to arrive at its current form, fairly well-adapted to live a long, healthy life on this planet, assuming conditions similar to most of human history, and not the environment and lifestyle we have been living for past couple/few centuries. Comments/questions, based on your list:
1. Born with large tongues and soft palates: Can't think of why this trait would have emerged and why it stays with us. Maybe because this trait doesn't become a cause of OSA as much until combined with aging, at which point humans have already reproduced, so this trait never had a chance to succumb to natural selection.
2. Enlarged tonsils and adenoids: It's possible that the amount of foreign substances in our modern air, both outdoors (pollution) and indoors (product off-gassing), would cause inflammation of these parts of our immune system. We also now migrate to completely different biomes than where our genetic ancestors evolved, meaning our immune system may have evolved for different natural airborne particles.
3. Allergies: Same as #2: an immune system reaction.
4. Deviated septum: Mostly caused by injury, and no reason for that to be higher with modern lifestyle. Possibly like #1, didn't have an impact until combined with aging.
5. Orthodontics: Now you're talking. There's an unnatural intervention of modern lifestyle that many of us have had, that had never been around while humans evolved. Which leads to me add a new theory, related to one I originally suggested:
6. Change to agricultural diet causing smaller jaws, weaker muscles: The advent of farming ~10,000 years ago meant that humans began eating softer foods that required less vigorous chewing than their hunter-gatherer ancestors. This has been confirmed in anthropology studies comparing jaw sizes of many agricultural vs. hunter/gatherer societies. Continuing this trend, impacted wisdom teeth became 10 times more common after the Industrial Revolution. This is why we now usually remove wisdom teeth - they don't fit in our small jaws. While jaw size has been the main focus of these studies, I'm thinking that tongue, jaw, and throat muscle tone would also suffer due to this dietary shift. Maybe I should give those anti-snoring throat/tongue exercises a try?
7. Aging: With our modern understanding of germ theory, vaccines, and life-saving trauma medicine and technology, the average human lifetime has increased since the Industrial Revolution. Maybe more of us are reaching an age in which sagging muscle tone, on top of any or all of the above reasons, is becoming an issue.
Thank you again, for listing so many causes and inspiring some new thoughts for me, and to anyone out there reading my thinking out loud. I'm no expert, but I think I have advanced my understanding of this through this exercise. Still interested in anyone else who has other theories or any success stories in reversing OSA so that you no longer need to use a CPAP machine. Thanks!