Earth is believed to have formed about 5 billion years ago.

In the first 500 million years a dense atmosphere emerged from the vapor and gases that were expelled during degassing of the planet's interior. These gases may have consisted of hydrogen (H₂), water vapor, methane (CH₄) , and carbon oxides.

Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide (CO₂), carbon monoxide (CO), water (H₂O), nitrogen (N₂), and hydrogen.

The hydrosphere was formed 4 billion years ago from the condensation of water vapor, resulting in oceans of water in which sedimentation occurred. [The source of the water vapor is still debated.]

The most important feature of the ancient environment was the absence of free oxygen. Evidence of such an anaerobic reducing atmosphere is hidden in early rock formations (Banded Iron Formations or BIFs) that contain many elements, such as iron and uranium, in their reduced states. Elements in this state are not found in the rocks of mid-Precambrian and younger ages, less than 3 billion years old.

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