The young Earth supported little multicellular life until its atmosphere acquired a healthy portion of oxygen. That change has been credited to the rise of cyanobacteria, known as blue-green algae, that produce oxygen by photosynthesis. Now, scientists argue that oxygen couldn't have built up in the atmosphere until a crucial geological mechanism kicked in and set the scene for the rise of more-complex forms of life.
By relating atmospheric composition to the chemistry of various ancient rock types, geologists have inferred that Earth went from largely oxygenfree to oxygen-rich 2.4 billion to 2.5 billion years ago (SN: 1/24/04, p. 61). But the fossil record shows that cyanobacteria existed about 2.7 billion years ago, leaving scientists to wonder why 200 million to 300 million years of oxygen production by these bacteria resulted in no accumulation of the gas.