What are facts for oxygen?

Answer 1

• Oxygen makes up only 21% of the air we breathe, but it is certainly the most important part, since we can't live without it.
• The Earth's atmosphere initially had very little oxygen; oxygen was added to the atmosphere by living things (anaerobic bacteria at first, plants later).
• The oxygen that we breathe now is produced by bacteria, algae and plants. It is estimated that 70 to 80% of the oxygen in the atmosphere comes from aquatic plants in the oceans.