The early atmosphere was composed mainly of water vapor, carbon dioxide and monoxide, nitrogen, hydrogen, and other gases released by volcanoes. By about 4.3 billion years ago, the atmosphere contained no oxygen and about 54 percent carbon dioxide. About 2.2 billion years ago, plants in the oceans began to produce oxygen by photosynthesis, which involved taking in carbon dioxide. By two billion years ago, there was one percent oxygen in the atmosphere, and plants and carbonate rocks caused carbon dioxide levels to decline to only four percent. By about 600 million years ago, atmospheric oxygen continued to increase as volcanoes and climate changes buried a great deal of plant material—plants that would have absorbed oxygen from the atmosphere if they had decomposed in the open. Today, our planet’s atmosphere levels measure 21 percent oxygen, 78 percent nitrogen, and only 0.036 percent carbon dioxide.