The first forms of life were simple, one-cell (unicellular) microbes from chemicals that were in the environment (the Miller-Urey experiment suggests that chemical constituents of the time, combined with a spark or other high energy source created the primitive amino acids that are necessary to build a living organism). Microbes are organisms smaller than one can see with the naked eye. These early microorganisms "ate" food from existing chemicals in the environment and soon depleted these chemicals. They also existed in an oxygen-less environment, hence the name "anaerobic" lifeform.

Another form of life evolved about 2.5-3 billion years ago: organisms that could manufacture "food" from inorganic substances in the environment. A particular type of organism, the green plant (actually, the earliest forms were cyanobacteria, once known as "blue-green algae"), stored the energy of sunlight in the organic compounds it produced from carbon dioxide and water, in a process called green plant photosynthesis. The importance of this event about 2-2.5 billion years ago was that a by-product of this type of photosynthesis is oxygen (O₂) gas. This became the major source of atmospheric oxygen.

Unfortunately, anaerobes are harmed by oxygen, so a massive die-off took place. This constituted the first major (air) pollution event, where oxygen was the pollutant! It wasn't really air pollution, since life was not living out in the air yet; but if they were on dry land, they would have been killed by the oxygen now accumulating in the atmosphere.

With the build-up of atmospheric oxygen, another form of life could evolve about 2 billion years ago, the aerobe. Aerobes use up oxygen, and give off carbon dioxide as a waste product. This produces a synergistic relationship between the green plants and the aerobes, where each supplies the other an important gas for its life while giving off a waste that is used by the other organism. This cycle reached a steady-state equilibrium, which is tough to break (with such a strong synergism, it's tough for any other organism to divert significant CO₂ and/or O₂ resources out of the loop).

At the same time, the atmospheric oxygen (O₂) could absorb short wavelength ultraviolet radiation in sunlight, keeping it from reaching the ground, where it would kill lifeforms that ventured out onto dry land. The stratospheric ozone (O₃) layer also formed from the photochemical reaction involving oxygen (O₂) absorbing ultraviolet radiation. The semi-permanent ozone layer finally allowed life to leave the oceans and evolve on dry land, by blocking the ultraviolet radiation that was not being absorbed by the O₂ in the atmosphere.