A Walk Through Time

Metabolic Modes

3000 Million Years Ago

Earth life forms require energy and carbon for growth and reproduction. They satisfy these requirements through a variety of processes. Some skilled individuals produce their own food; many rely on others to make it for them.

By this time, microbes have evolved every metabolic mode known to 20th-century scientists. Tiny gas-eating microbes, without using light, refine methods of making food and energy from sulfide, methane, ammonia, oxygen, and carbon dioxide. Cyanobacteria and their kin, using sunlight as a source of energy, create food from atmospheric carbon dioxide. Dependent on these primary producers, many microbes mix and match modes.

Stromatolite reefs, monuments of bacterial life, continue expanding across the planet.

Top: This awesome blue-green bacterium is a great adapter, a switch hitter. Usually, it grabs hydrogen electrons from water, releasing oxygen. Under high hydrogen sulfide conditions, it stores gobs of yellow sulfur instead. (Photograph by Lynn Margulis)

Bottom: This tall Canadian has stepped back 1,800 million years, in the Canadian Northwest Territories, to show us just how grand bacterial building can be. (Photograph by Fred Campbell)

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