Summary:
- The first organisms were made of only one cell (Figure below).
- The earliest cells were prokaryotes.
- Photosynthesis allows organisms to produce food energy with oxygen as a by-product.
- After photosynthesis evolved, iron oxidized and then oxygen entered the atmosphere.
- Ozone was created in the upper atmosphere.
- The ozone layer formed to protect Earth from harmful ultraviolet radiation.
- This made the environment able to support more complex life forms.
- These organisms may have been around as far back as 3.5 billion years.
- Cyanobacteria are still alive today.