I received a few, good responses to my question about how humans (all humans) came to be so close to the threshold of not being able to breathe while they sleep.
The runner up response came from David States:
On why we’re so close to threshold:
– no selective pressure as long as you do breath
– normal weight by modern standards is obese on an evolutionary time scale
– sleep apnea is mostly a problem of the elderly and an evolutionary time scale most of us didn’t live past 40
— David States (@statesdj) May 24, 2018
To avoid choking, human swallowing requires more neurologic precision and coordination than in lower animals. As humans sleep, flaccidity of the tongue can bring about collapse of the tongue on the soft palate or flexible, compliant walls of the oropharynx, causing obstructive sleep apnea (OSA) and/or snoring. A very positive evolutionary development resulting from the flexible, compliant adult human airway is the capability to create formant sounds, instrumental in the articulation of speech.
In adult humans the pharynx remains open for passage of air at all times except during swallowing, and regurgitation. Breathing cannot occur simultaneously with swallowing. The good news is speech. The negative consequences are choking, snoring and obstructive sleep apnea (OSA). OSA occurs when the luminal cross-sectional area of the upper airway collapses during inspiration, at or below the level of the soft palate and above the epiglottis. […]
The roof of the mouth is the floor of the nose. The lateral wall of the nose is contiguous to the medial wall of the eye. The top of the nose and eye border the floor of the brain. The chewing muscles and TMJ attach to the skull. Tongue movement, nose breathing, mouth breathing, swallowing, speaking and OSA are complexly integrated in terms of development, structure and function so that new and different function in one part may stimulate or retard others from developing as growth continues.
Speech! Speech is good! We are pro-speech! It’s definitely something for which selective pressure could have existed. But it’s freaking complicated, requiring loading up our ancestors’ internal head/neck structures with new features while they were still busy doing a lots of other useful stuff like eating and breathing (we should support those too). It got kind of complicated and crowded in there, and in a way that makes us prone to obstructive sleep apnea.
Speech is the feature. Sleep apnea is the bug. I buy that.
Finally, an answer to why those of us with sleep apnea observe variation in the number of apnea/hypopnea came from Bradley Flansbaum:
Daily ambient temperatures and environmental conditions (particulate matter in air/pollution/emissions) will vary luminal diameter. It will subtly change your nocturnal responses.
Seems totally reasonable.