Oxygen on Earth primarily comes from photosynthesis, a process where plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into sugars and oxygen. Main sources of oxygen:

• Oceans: Roughly half to about 70% of the oxygen in Earth's atmosphere is produced by marine photosynthetic organisms, mainly phytoplankton (tiny drifting plants, algae, and cyanobacteria). These organisms live in the sunlit upper layers of the ocean and release oxygen as a byproduct of photosynthesis. A notable contributor is Prochlorococcus , the smallest photosynthetic organism, which alone produces 5-10% of the oxygen we breathe

• Land plants: Trees and other terrestrial plants also produce oxygen through photosynthesis, but they contribute about 20-50% of the atmospheric oxygen, less than the oceanic sources

Historical context: Oxygen was not always abundant in Earth's atmosphere. Around 3.5 billion years ago, early microorganisms in the oceans began producing oxygen through photosynthesis. Initially, oxygen reacted with other substances and was not free in the atmosphere. Over millions of years, oxygen accumulated, reaching current levels of about 21% in the atmosphere only in the last several hundred million years

. In summary, the oxygen we breathe mostly originates from microscopic oceanic photosynthetic organisms, with land plants playing a smaller but significant role. This oxygen production is a continuous process vital for life on Earth.