Oxygen atmospheres on earth have varied dramatically over the eons and eras. Scientists refer to the eons as the Hadean (4.5 to 4.0 billion years ago), Archean (4.0 to 2.5 billion years ago), Proterozoic (2.5 to 0.5 billion years ago); and Phanerozoic (0.5 billion years ago to the present). The Hadean eon is undivided; the Archean eon is subdivided into Eo-, Paleo-, Meso- and Neo- eras; the Proterozoic into Paleo-, Meso-, and Neo- eras; the Phanerozoic into Paleozoic, Mesozoic, and Cenozoic eras — this last, our current era.
Each eon, era, and subdivision marks a milestone in the remarkable story — too complex to cover here — of oxygen’s planetary equilibration amid shifting complex physical systems to accumulate and “stabilize” at the 21% atmospheric level we most naturally breathe today. The inseparable parallel story is how life forms developed and evolved to handle oxygen within the eons’ and eras’ volatile atmospheric conditions.*
As remarkable as the story of oxygen’s past is, its present and future point in a different direction. The isolation of oxygen at various times between the 15th and the 18th centuries moved us to where we are today: capable of creating oxygen atmospheres containing levels far in excess of 21%. Attainment of this feat preceded awareness of oxygen’s biophysical significance and development of a nomenclature suitable to its medicinal properties. In brief, we used oxygen medically before we understood its importance, risks, benefits, and limitations. Interestingly, the 20th century saw humanity asking how to control “the atom” — radiation — in multiple settings. Perhaps the 21st century will see us determining how to rationally use and wisely control oxygen in medical settings.
* The technically minded reader might enjoy: Hsia CWC, Schmitz A, Lambertz M, Perry SF, Maina JN. Evolution of air breathing: oxygen homeostasis and the transition from water to land and sky. Compr Physiol. 2013; 3:849-915. doi:10.1002/cphy.c120003
