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The Proterozoic Eon extended from 2.5 billion to 541 million years ago and is often divided into the Paleoproterozoic (2.5 billion to 1.6 billion years ago), the Mesoproterozoic (1.6 billion to 1 billion years ago), and the Neoproterozoic (1 billion to 541 million years ago) eras.

Megascopic eukaryotes first appeared about 2.3 billion years ago and became widespread by about 1.8 billion years ago.

Eukaryotes employed a form of respiration and oxidative metabolism; they had a central nucleus that could split into separate sex cells, and so for the first time a mixed and variable genetic code could be passed to younger generations.