Origins

Try to picture the world without photosynthesis. Obviously, you’d have to strip away the greenery. Not just the redwoods and sunflowers, but the humble algae and the light-capturing bacteria that nourish many of the world’s ecosystems. Gone, too, would be everything that depends on photosynthetic organisms, directly or indirectly, for sustenance—from leaf-munching beetles to meat-eating lions. Even corals, which play host to algal partners, would lose their main food source.

Given its importance in making and keeping Earth lush, photosynthesis ranks high on the top-10 list of evolutionary milestones. In Science's Origins essay this week, author Mitch Leslie describes how scientists are delving into ancient rocks and poring over DNA sequences to try to piece together how and when organisms first began to harness light’s energy.

Although most modern photosynthesizers make oxygen from water, the earliest solar-powered bacteria relied on different ingredients, perhaps hydrogen sulfide. Over time, the photosynthetic machinery became more sophisticated, eventually leading to the green, well-oxygenated world that surrounds us today. In the lab, some biochemists are recapitulating the chemical steps that led to this increased complexity. Other researchers are locked in debates over just when this transition happened, 2.4 billion years ago or much earlier.