- Joined
- Nov 21, 2007
- Messages
- 137
Most of the oxygen came from cyaneobacterias, wich basically were almost the only organisms from 3 billion to 1.4 billions years ago. Plants appeared in an atmosphere already rich in O2.Also though as you mentioned, Plants work both ways, using both CO2 and O2, but what about when there was no O2. If plants created all the O2 from CO2, then how did they evolve to a point were they could switch the process?