Archaean (Archaeozoic, Archean) and Proterozoic

Archaean

The Archean (also spelled Archaean, formerly called the Archaeozoic, also spelled Archeozoic or Archæozoic) is a geologic eon before the Proterozoic and Paleoproterozoic, before 2.5 Ga (billion years ago, or 2,500 Ma). Instead of being based on stratigraphy, this date is defined chronometrically. The lower boundary (starting point) has not been officially recognized by the International Commission on Stratigraphy, but it is usually set to 3.8 Ga, at the end of the Hadean eon. In older literature, the Hadean is included as part of the Archean.The name comes from the ancient Greek "Αρχή" (Arkhē), meaning "beginning, origin" (ref).

Proterozoic

The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life." The Proterozoic Eon extended from 2500 Ma to 542.0 ± 1.0 Ma (million years ago), and is the most recent part of the old, informally named ‘Precambrian’ time.

The Proterozoic (ref) consists of 3 geologic eras, from oldest to youngest:

• Paleoproterozoic
• Mesoproterozoic
• Neoproterozoic

The well-identified events were:

• The transition to an oxygenated atmosphere during the Mesoproterozoic.
• Several glaciations, including the hypothesized Snowball Earth during the Cryogenian period in the late Neoproterozoic.
• The Ediacaran Period (635 to 542 Ma) which is characterized by the evolution of abundant soft-bodied multicellular organisms.

Precambrian

The Precambrian (ref) (Pre-Cambrian) is an informal name for the supereon comprising the eons of the geologic timescale that came before the current Phanerozoic eon. It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant macroscopic hard-shelled animals, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 Ma. It spans the Proterozoic, Archean and Hadean eons. It is named after the Roman name for Wales, Cambria, where rocks from this age were first studied.

The Precambrian has sometimes been referred to erroneously as an "Era", which is a subdivision of an eon.

Return to “Time and Evolution” Chapter.