Photosynthesizers

The details of life’s origins are ineffable, but soon after the first forms of life appeared on Earth, organisms must have learned to use the premium energy of sunlight to grow and to sustain themselves. The first photosynthesizing bacteria used sunlight in a complex series of steps to take carbon from carbon dioxide from the air and ocean and use it to build their bodies. The present system of photosynthesis using chlorophyll was an early step. Oxygen was excreted as a by-product. The early Earth environment was reactive toward oxygen and there was no accumulation of this gas like now.

Methanogens

A world with photosynthesizers only is unstable. They would soon have locked up in their bodies most of the available carbon. Their removal of carbon dioxide would have so weakened the greenhouse that the world would have frozen, and life ceased. This never happened. There coexisted with the photosynthesizers simple fermenters, the methanogens. These organisms processed the organic matter made by the photosynthesizers and returned carbon to the air as a mixture of methane and carbon dioxide, restoring the greenhouse. A bonus from the presence of methane was the creation of an ‘organic smog" layer in the upper atmosphere, shielding the surface from ultraviolet radiation, much as the ozone layer does now. (see variation 19)

Consumers

Limited to isolated pockets near the surface, where enough oxygen would have been produced by the photosynthesizers to support them, early consumers would have lived on the organic products of the photosynthesizers.