Life is easy for the biosphere's first beings, fermenting a pantry filled with free organic compounds formed in the atmosphere. These anaerobic (living in the absence of oxygen) freeloaders create life's first food crisis: rapidly reproducing, they consume food faster than the atmosphere renews it.
Certain descendants of these fortunate fermenters show notable problem-solving skills: they overcome the food shortage by learning to make their own food. These planetary "primary producers" use light or chemicals to generate energy, fabricating food directly from carbon-dioxide.
Green and purple microbes evolve Earth's most important metabolic innovation: photosynthesis. These early prodigies practice specialized photosynthesis, which gives off sulfur rather than oxygen "waste." Using solar power, the microbes take hydrogen from hydrogen-sulfide gas that is spewed out of deep-sea vents and volcanoes and combine it with carbon dioxide to make their bodies.