co2 makes you fat, page-17

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.