Prehistoric landscape. Computer artwork of the rocky remains of two volcanic calderas during the Eoarchean era (about 4000-3600 million years ago) where a variety of single-celled organisms add colour to an otherwise monochromatic landscape. 770 million years after the formation of the Earth (approx. 3.8 billion years ago) the first life may have appeared in the form of simple, single-celled organisms, bacteria and archaea. The atmosphere probably consisted of gases vented by volcanoes with the lack of oxygen unable to support complex life forms.