• 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.
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