Scientists have been pursuing the answer to a question for a long time, that is: what gives the earth enough oxygen to enable it to breed animals and humans. Oxygen appeared in the earth's atmosphere 2.4 billion years ago. About 470 million years ago, bryophytes spread rapidly on the earth. It was not until 400 million years ago that the oxygen content in the earth's atmosphere reached today's level. 
The first terrestrial plants to appear on the earth were unexpectedly prolific, and they greatly increased the oxygen content in the earth's atmosphere.
 some scientists believe that the large increase in oxygen on the earth originated from bryophytes.