Eon in planet Earth's geological record lasting from ~2.5 billion years ago to ~540 million years ago.
The Proterozoic eon begins with the accumulation of molecular oxygen within the atmosphere to approximately modern levels. Relatively early in this eon eukaryotic cells make their first appearance in the fossil record.
This is followed by the origin, at some uncertain time, of metazoans and later there is a divergence of invertebrate animals. This is particularly ones lacking in readily fossilized hard tissues since hard tissues, i.e., shells, bone, and exoskeletons, for the most part had not yet made an appearance in the fossil record.
The Proterozoic Eon is preceded by the Archaean eon followed by the Cenozoic eon. Together the Archaean and Proterozoic eons make up what less formally has been known as precambrian times.
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