Geologic Time Scale

Phanerozoic Eon

Eon of abundant visible life and most complex organisms.

Proterozoic Eon

Eon when early multicellular life developed.

Archean Eon

Eon when Earth’s crust cooled and first life appeared.

Hadean Eon

Earliest eon before solid crust; Earth was molten.

Cenozoic Era

"Age of Mammals"; began 66 million years ago.

Mesozoic Era

"Age of Reptiles"; includes dinosaurs; ended 66 million years ago.

Paleozoic Era

"Age of Ancient Life"; explosion of life in oceans; ended 251.9 million years ago.

Quaternary Period

Most recent period; includes humans.

Neogene Period

Includes Miocene and Pliocene epochs.

Paleogene Period

Includes Paleocene, Eocene, and Oligocene epochs.

Cretaceous Period

Last period of the Mesozoic; ended with dinosaur extinction.

Jurassic Period

Middle Mesozoic; dinosaurs flourished.

Triassic Period

First period of Mesozoic; first dinosaurs appeared.

Permian Period

Last Paleozoic period; ended with largest mass extinction.

Carboniferous Period

Includes Mississippian and Pennsylvanian; age of coal swamps.

Pennsylvanian Epoch

Subdivision of Carboniferous; rich coal-forming forests.

Mississippian Epoch

Subdivision of Carboniferous; dominated by marine life.

Devonian Period

"Age of Fishes"; first forests appear.

Silurian Period

First land plants spread widely.

Ordovician Period

Marine life diversified; ended with major extinction.

Cambrian Period

"Explosion" of complex life; first hard-shelled organisms.

Holocene Epoch

Current epoch of human civilization.

Pleistocene Epoch

Time of major ice ages and megafauna.

Pliocene Epoch

Expansion of grasslands; early hominins.

Miocene Epoch

Warm climate; mammals diversified.

Oligocene Epoch

Grasslands expand; modern mammal groups appear.

Eocene Epoch

Warm global climate; early primates.

Paleocene Epoch

Recovery of life after dinosaur extinction.

Neoproterozoic Era

Late Proterozoic; first animals appear.

Mesoproterozoic Era

Middle Proterozoic; buildup of oxygen.

Paleoproterozoic Era

Early Proterozoic; first stable continents form.

Neoarchean

Epoch with early microbial life and expanding continents.

Mesoarchean

Epoch with increased crust formation.

Paleoarchean

Epoch with earliest known bacterial fossils.

Eoarchean

Earliest part of the Archean; earliest rocks form.