Appearance of the first vertebrates (about 550 millions years...

Appearance of the first vertebrates (about 550 millions years ago) and the development of prototypes of human lungs took place when air was made up of only about 1% O2, while having much higher percentage of CO2 (Maina, 1998), likely over 7%. Normal air today has many times more O2 (about 20%) and only a fraction of the CO2 (0.03%). However, our cells now still live in the air that existed hundred millions years ago: “But the cells of animals and humans need about 7 % CO2 and only 2% O2 in the surrounding environment. This is the way how our cells live: cells of the heart, brain, and kidneys” (Buteyko, 1977).

http://www.normalbreathing.com/causes-air-evolution.php

A Russian perspective backed by the Russian Ministry of Health.