The Archeozoic eon is also known as the Archean eon. It is the fourth eon in Earth's history and is marked by the emergence of life on this planet. The term 'Archeozoic' means 'ancient life', while 'Archean' means 'ancient'. Other synonyms that describe this eon include the 'Early Precambrian Era' and the 'Archeozoic Era'. The Archeozoic eon lasts from around 4 billion years ago to 2.5 billion years ago and is the longest eon in the history of Earth. During this time, the Earth's atmosphere was mostly made up of carbon dioxide and water vapor. It was also during this eon that the first photosynthetic organisms appeared and produced oxygen through their metabolic processes, leading to the eventual formation of the ozone layer.