Take a deep breath. Feel better now? You should, because you just inhaled gases that include about 20% oxygen—oxygen that you require for life. We have the understandable, if unconscious, belief that things have always been pretty much the same. However, if you went back a few billion years, that deep breath wouldn't have done you a bit of good. In fact, you would have been brain dead within about 4 minutes. Why? Because the latest indications are that oxygen levels didn't surpass about one part per million until around 2.4 billion years ago.
Furthermore, it looks like oxygen levels similar to those of today
didn't evolve until perhaps 600 to 700 million years ago. Interestingly enough,
this is when multicellular animals began to appear, joining the single-celled organisms
quite happy to do without oxygen. The farther back in time we go, the more difficulty
we have in determining details, but the approximately 2.4-billion-year figure for the
appearance of discernible oxygen appears firm; only then does talk of the breath of
life have meaning.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.