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It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis. The rest is produced via photosynthesis on land by plants. Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO 2 / O 2 balance since the early Precambrian Eon.
Today oxygen makes up about 20 percent of Earth's atmosphere, but for the first two billion years after Earth formed, its atmosphere was anoxic (oxygen-free). About 2.3 billion years ago, oxygen increased from a trace gas to perhaps one percent of Earth's atmosphere.Another jump took place about 600 million years ago, paving the way for multi-cellular life forms to expand during the Cambrian Explosion.ndeed, the jumps in atmospheric oxygen at 2.3 billion years ago and 600 million years ago do seem to be associated with Snowball Earth episodes (Fig. 14). However, scientists are still unsure exactly what the connection might be between the extreme ice ages and changes in the oxygen content of the atmosphere.