In 1663, Otto Guericke reported, in resect to an experiment done with a lit candle inside of a water filled vessel (see: Illustration VIII), that 10% of the volume of the air was consumed by the flame: 
“But then, indeed, I saw all the water rise into the glass f and at the same time to draw up many bubbles. This was a clear indication that some of the air contained in the receiver had been consumed. Indeed, at least a tenth part of the air was consumed by the flame of the candle and perhaps all might have been consumed had not the flame gone out.”— Otto Guericke (1663), Magdeburg Experiments on the Vacuum (pg. 140)
Other work done on the discovery of oxygen, include: Michael Sendivogius (1604), Carl Scheele (1771), Joseph Priestley (1774), all of which was assimilated and explained correctly by Antoine Lavoisier (1777).
In the human molecule, comprising 61 percent by mass of its structure, oxygen is the most abundant element.
Roughly 60% of the human body is H2O, which is essential for almost all chemical reactions within the body. 
The following are related quotes:
“Why should a group of simple, stable compounds of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N), 'struggle' for billions of years to organize themselves into a professor of chemistry? What's the motive?”– Robert Pirsig (1991), Lila: an Inquiry into Morals
● Animal heat
1. Thims, Libb. (2008). The Human Molecule (issuu) (preview) (Google Books) (docstoc) (pgs. 52-55). LuLu.
● Oxygen – Wikipedia.