WATER, EARTH, AIR, FIRE - AND LIFE

     Life can probably not exist without liquid water, but the other three Aristotelian "elements" are also essential.  This is well illustrated by the evolution of life on our planet.  Earth and air have always supplied essential nutrients.  The fire of the sun and of the Earth's interior have supplied the needed energy and the required volatiles.

     The Earth and its biosphere have coevolved.  The composition of volatiles added to the atmosphere and oceans have probably changed with time, and with it the composition of the atmosphere, the oceans, and the biosphere.  Prior to ca. 2.3 Ga these volatiles were sufficiently reducing, so that 20% of the added CO₂ could be reduced to organic matter, all of the sulfur removed as a constituent of pyrite, and excess H₂ lost to interplanetary space.  O₂ was absent from the atmosphere, CH₄ was probably present at a level on the order of 0.1%, and the concentration of sulfate in the oceans was probably minimal.  Since ca. 2.3 Ga volatiles added to the atmosphere and oceans have been somewhat more oxidized.  On average only ca. half of their sulfur has been removed as a constituent of pyrite, the remainder as a constituent of gypsum and anhydrite.  Atmospheric CH₄ has been displaced almost completely by O₂.

     The presence of O₂ in the atmosphere is due to the evolution of cyanobacteria and other organisms that use photosystem II.  The level of atmospheric O₂ has been determined by the required balance of O₂ production and O₂ use, and by the mechanisms which establish this balance.  The very complex interactions of the biosphere with the four “elements” on Earth are probably a measure of their complexity on other planets that harbor life.