Highly active organisms like ourselves need oxygen in copious quantities. Surprisingly, an adult human consumes about 250 ml of oxygen each minute at rest and 5,000 ml per minute during vigorous exercise. Such high consumption rates of oxygen can only be satisfied by directly absorbing oxygen from an oxygen-rich atmosphere, at concentrations high enough to allow its rapid and efficient uptake within our lungs… Our current atmosphere contains about 21% oxygen, generating a partial pressure of about 150 mm Hg. Oxygen is needed at this level in order to sustain active metabolism… On the other hand, if atmospheric levels of oxygen rise much above 21%, spontaneous combustion of carbon compounds becomes an increasing danger. The fact that oxygen levels sufficient to support high levels of metabolism by air-breathing organisms do not at the same time support spontaneous conflagrations is clearly a coincidence of great relevance for terrestrial life.Michael Denton, “The Place of Life and Man in Nature” (BIO-Complexity)
For more information about the unique properties of oxygen, and how they make human life and the life of other organisms possible, view The Privileged Species or read Chapter 6, “The Vital Gases” in Michael Denton’s book Nature’s Destiny.