IMO, adapting for low oxygen environment is a little different than adapting to eat grains. The need for oxygen is inescapable, and is a harsher selection criteria (for lack of a better term) than food that may not be optimal, but still somewhat digestable, providing calories, if not optimal nutrition. The deficits from a high grain diet can be made up, eventually, whereas you can't exactly breath a lot now, and only a very little bit for the next 8 weeks or so, the way you can with food (eat a lot in spring, summer and fall, and live on stored fat/hoarded food during the winter).
Being unable to survive in a low oxygen environment would likely mean immediate effects-reduced fertility, increased genetic selection, and higher rates of extinction for those that don't meet the requirements.
Please forgive the clumsy wording. I know what I'm trying to express, just having a hard time articulating it. And maybe my conjecture is way off base, but it seems logical to me.
Cattle and other ruminants CAN thrive on grains in appropriate quantities. They specifically search out seed heads in fall to eat, eating the stemmy remains ONLY after all the seeds have been stripped.
By way of example: feed lot cattle are stuffed with corn and other grains even though it makes them sick (they are ruminants and evolved to eat grass, not grains) so we pump them full of drugs to keep them alive long enough to get them to slaughter. The cows tolerate corn, sort of, but don't thrive on it.
It is not so much the fact that they are fed grain that is problematic, as it is the ratio of grain:hay (too much grain increases the acidity of the rumen, causing acidosis-which can be chronic or acute) and the concentration of animals in small areas, which causes stress, which affect rumen pH. Some quantity of grain is perfectly natural for ruminants-even corn. Cattle can do VERY well on large quantities of grain as long as the stress is kept low.
The medication used to treat acidosis caused by excess grain is - Baking soda and probiotics.
Antibiotics ARE used to treat pneumonia caused by stress. They are not used willy-nilly because they are expensive, labor intensive, and increase the risk of acidosis and bloat, AND pathogen resistance. Hormones are not used much anymore-they aren't cost effective. The statements put forth by certain authors that steers cannot finish WITHOUT because of a lack of testosterone are completely false-I've fattened more steers than I can count without hormones. And at remarkable rates of gain. They prey on the general publics lack of knowledge on the realities of finishing meat animals. Every 4H and FFA kid knows that steers (and heifers) finish just fine on hay and grain (and water, of course. And the occasional slick of ice cream. Or peppermint candy...)
Please note, however, that being able to flourish on grains is different than producing an optimally O3:O6 balanced meat.
Chief cook & bottle washer for one kid, a dog, 6 hens, 2 surprise! roosters, two horses, and a random 'herd' of quail.
~The ultimate ignorance is the rejection of something one knows nothing about and refuses to investigate~