Harvesting light to produce energy and oxygen (photosynthesis) is the signature of all land plants. This ability was co-opted from a precocious and ancient form of life known as cyanobacteria. Today these bacteria, as well as microscopic algae, supply oxygen to the atmosphere and churn out fixed nitrogen in Earth's vast oceans. Microorganisms may also have played a major role in atmosphere evolution before the rise of oxygen. Under the more dim light of a young sun cooler than today's, certain groups of anaerobic bacteria may have been pumping out large amounts of methane, thereby keeping the early climate warm and inviting. The evolution of Earth's atmosphere is linked tightly to the evolution of its biota.