Supereon: Precambrian
 Eon: Archean
  Era: Eoarchean
 

Eoarchean Earth

770 million years after the formation of the Earth--3.8 billion years ago--the first life may have appeared in the form of simple, single-celled organisms. Bacteria and archaea may have even found a way to populate the otherwise sterile and calamitous surface. The atmosphere would likely have consisted of gases vented by volcanoes: carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen, methane, hydrogen, methane, ammonia, and water vapor. There was probably very little oxygen and no protective ozone layer to filter the shorter wavelengths of ultraviolet light from the Sun that are harmful to most forms of life.

In this image, a variety of single-celled organisms have ventured out of the ocean and onto the rocky remains of two volcanic calderas, adding color to an otherwise monochromatic landscape. While the lack of breathable oxygen and shortwave ultraviolet light from the Sun would be anathema to complex life forms like ourselves, this primitive environment was a rich source of sustenance for these terrestrial vanguards.