We all tend to think of oxygen as being essential to life. Oxygen is a molecule that we rely on and without which we could not exist. However, scientists believe there was little molecular oxygen in the early atmosphere of Earth. Life first evolved in its absence. With the evolution of photosynthesis in blue-green algae, oxygen was produced in large quantities as a by-product of the algae's photosynthetic metabolism. Oxygen began to enter the atmosphere and our seas.
For a long time, most of the oxygen produced on Earth was largely captured through oxidation of elemental iron to iron oxide or ``rust.'' The evidence for that process can be found in the beautiful ``banded-iron'' formations of rock. Eventually, the oxygen also began to enter the atmosphere. As it did, certain life forms evolved to make good use of the highly reactive molecular oxygen through the process of respiration. The level of oxygen in our present atmosphere is well-suited for the life of respiratory organisms that rely on oxygen as an essential element of metabolism.
Oxygen in the atmosphere is also an essential ingredient to the The Evolution of Darwin. The rusting of the iron filings, to create the ``rust heads,'' is the chemistry that led to our great deposits of iron oxides in the Earth and the banded iron formations. In that process, elemental iron encounters molecular oxygen. The iron ``donates'' electrons and is oxidized. The oxygen ``accepts'' electrons and is reduced. Some of the reduced oxygen combines with iron to make iron oxides, which we recognize by the characteristic ``rust'' color.
The nature of oxidation and reduction is that electrons are transferred from one species to another. If they can be transferred from elemental iron to oxygen, they can also be transfered back from the reduced oxygen to the oxidized iron. So the process of rusting is reversible. We can rust iron, but we can also mine rust and from it make elemental iron. The general study of oxidation and reduction is known as ``electrochemistry.'' It is a field of study that is at the heart of many industrial processes, like the smelting of iron ores, and also at the center of our study of life on Earth. For it is not only humans that have discovered the utility of oxidation and reduction, life as we know it, whether bacterial or human, would not be possible without the cyclic process of reversible oxidation and reduction.
John Straub,
BU Dept of Chemistry
