Food webs were expected to depend ultimately on photosynthetic primary production powered by sunlight.

The familiar route into a food web begins with sunlight. Plants and algae use photosynthesis to turn light energy into organic matter. Herbivores eat them, predators eat the herbivores, and even much of the deep sea depends on organic material that eventually sinks down from sunlit waters.

In 1977, scientists exploring the Galápagos Rift found something unexpected. Around deep-sea hydrothermal vents they found dense communities of giant tube worms, clams, and other animals. No sunlight reaches those depths.

The animals were not simply living on an unusually large supply of food falling from above. Microorganisms around the vents were using energy from chemical reactions involving substances such as hydrogen sulfide to make organic matter from carbon dioxide. That process is called chemosynthesis.

Some vent animals feed on these microorganisms; others house them inside their bodies and depend on them directly. In either case, chemical energy from the vent can form the base of the food web.

Chemosynthesis itself was known before 1977. What the vents revealed was how large and complex an ecosystem could be without local photosynthesis. Most ecosystems still depend ultimately on sunlight, but not all of them do.