[suneilp]

Since oxygen plays a rather vital role in our functioning, I'm curious why we can't detect low oxygen levels. Or perhaps we do detect them but only when it's dangerously low. How has it been confirmed that we detect CO2 levels instead of low oxygen?

[autokad]

perhaps its because we and our ancestors do not have the environmental stress to have that as one of our optimizations

[hwillis]

Directly: https://en.m.wikipedia.org/wiki/Inert_gas_asphyxiation

You are totally unaware of your brain shutting down as the oxygen level decreases. People lose the ability to do arithmetic or simple puzzles including blocks in holes, but by the time that happens youre too far gone to recognize what that means.

[sn9]

As CO2 concentrations rise, it raises levels of carbonic acid in the blood (something CO2 does in aqueous environments; same thing that's bleaching corals in the sea). Our body tracks blood pH and when it dips too low from high CO2, that's when the body knows it needs to breathe.

(The above is just going off my memory of my physiology class from years ago. Apologies if I misremembered something.)

[aquaticor]

Just a guess, but CO2 levels in our body are much lower which would mean small changes are easier to detect. Also just a guess, when we were all just fish making a first few steps into land, atmosphere O2 concentration was lower, CO2 higher, so maybe we evolved to detect what was more abundant then.