Geologic Time Scales

Paleozoic Era
Cambrian Period
Ordovician Period
Silurian Period
Devonian Period
Carboniferous Period
Mesozoic Era
Triassic Period
Jurassic Period
Cretaceous Period
Cenozoic Era
Tertiary Period
Quaternary Period
Geologic Timeline

Web Sites

Precambrian. UC Berkeley
Paleo May Project. Indiana State University

Return SAS Home
e-mail Kevin C. Hartzog

Geologic Time Scales


The Precambrian, dating 4.5-544 million years ago (mya), marks the period of Earth's history from its initial formation to the beginning of life. During this time range, our Earth went through vast changes. The Earth had solidified from the solar disk left over from the Sun's own formation. The Moon formed, setting the stage for later tidal fluctuations. As the Earth cooled, it developed its initial atmosphere that contained no oxygen, yet setting the stage for the development of the first life forms, prokaryotes. From their interaction with the then prevalently carbon dioxide and sulfur rich atmosphere, they slowly altered, developing into the first eukaryotes, and creating yet another profound change in Earth, the production of oxygen. As the atmosphere slowly became enriched in oxygen, complex multicellular organisms evolved, and the curtain had fallen upon one era of Earth's history, to arise in a new era, the Paleozoic. With one of the greatest events in life's theater on Earth, the Precambrian Explosion marked a moment when an abundance of fossils showed a vast increase in the numbers and diversity of life. Later shown to be a more gradual increase in Earth's diversity, this was still the most significant increase in life forms on our new, young planet.