Proterozoic

The Proterozoic is the geological eon from about 2.5 billion to 539 million years ago, between the Archean and Phanerozoic. It includes major increases in atmospheric oxygen and the emergence of complex eukaryotic life.

Great Oxidation Event: A major rise in atmospheric oxygen beginning around 2.4 billion years ago. It marks the first major atmospheric transformation within the Proterozoic.
Paleoproterozoic: The earliest Proterozoic era, spanning about 2.5 to 1.6 billion years ago. Its record includes the Great Oxidation Event and major early glaciations.
Geologic time scale: A system that divides Earth's history into ranked units based on rocks and geological events. The Proterozoic is one eon within this hierarchy of geological time.
Eukaryote: An organism whose cells have a nucleus and other membrane-bound compartments. Eukaryotes diversified during the eon and became the basis of complex multicellular life.
Oxygenation of Earth's atmosphere: The long-term accumulation of oxygen in Earth's atmosphere through biological and geological processes. Researchers continue to debate the timing, pace, and causes of Proterozoic oxygen increases.
Snowball Earth: The hypothesis that ice periodically covered nearly the entire Earth. Cryogenian glaciations radically altered Proterozoic climate and ocean chemistry.
Mesoproterozoic: The middle Proterozoic era, spanning about 1.6 to 1.0 billion years ago. It captures a long interval of relative environmental stability and evolving eukaryotic life.
Archean: The eon preceding the Proterozoic, extending from about 4.0 to 2.5 billion years ago. Its older rocks provide the direct geological baseline for the Proterozoic.
Multicellularity: The condition of having bodies composed of multiple interacting cells. Its early forms emerged before the end of the Proterozoic.
Eukaryote fossil record: The geological evidence used to identify and date ancient eukaryotic organisms. Fossil interpretation shapes estimates of when eukaryotes originated and diversified.

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