I wonder what could have caused these to evolve. After all, our ancestors spent the vast majority of their time outside in the open air. Air quality really sho

I wonder what could have caused these to evolve. After all, our ancestors spent the vast majority of their time outside in the open air. Air quality really shouldn't have been an issue almost all the time, and on the odd occasions when it was (eg you somehow get into a cave where the air is bad) this wouldn't help much.