- as the Earth formed, in addition to rocky silicates and metallic iron, impacting bodies emplaced volatile materials such as carbon dioxide, hydrogen and hydrogen compounds (methane and ammonia), and nitrogen in the mantle.

- the Earth's atmosphere was formed by outgassing; the volatile gases were released from the mantle via volcanic activity

- the early atmosphere was probably rich in carbon dioxide, nitrogen, methane, ammonia, hydrogen, and water vapor; this reducing atmosphere was devoid of free oxygen (O2)

- the oceans condensed from the water vapor that outgased from the mantle

- the carbon dioxide rich oceans were very acidic (carbon dioxide + water = carbonic acid)

- the photodissociation of water vapor may have produced some free oxygen

- hydrogen escaped into space; carbon dioxide was used by photosynthetic organisms; oxygen was added to the atmosphere by photosynthesis; nitrogen gradually increased by steady outgassing to eventually become the most abundant gas in the atmosphere

- photosynthesis produced large quantities of oxygen once life evolved by 3.5 b.y.; this eventually led to the oxidizing atmosphere with free oxygen that we have today

- photosynthesis produced large quantities of oxygen and removed much carbon dioxide from the atmosphere once life evolved by 3.5 b.y.

- by about 3 b.y. banded iron formations (BIFs) show that free oxygen was available periodically, perhaps seasonally, to oxidize and precipitate the abundant iron cations that were dissolved in ocean water (remember these iron cations were the product of carbonic acid weathering silicate rocks)

- by about 2 - 1.7 b.y. redbeds (hematite-cemented sandstones and shales) started to form, indicating a continuous supply of free oxygen

- by about 2 - 1.7 b.y. carbonate rocks/limestones and other carbonate rocks became common indicating that oxygen had increased and carbon dioxide reduced through photosynthesis so that ocean acidity had decreased enough to allow calcium carbonate to precipitate