Reference
Earth’s history divided into eons, eras, periods, and epochs, with boundary ages aligned to the ICS International Chronostratigraphic Chart. Ages are in millions of years ago (Mya).
The Phanerozoic, where every animal, every plant, and every human story lives, is the bottom ~12% of this chart. The first 4 billion years (the Precambrian) are the rest.
| Unit | Level | Start (Mya) | End (Mya) | Duration | Span |
|---|---|---|---|---|---|
| Hadean Formation of Earth, the Moon-forming giant impact, and a magma-ocean surface that cooled enough to host the first oceans. | Eon | 4,567 | 4,031 | 536 My | |
| Archean First continental crust, earliest microbial life, and the buildup that culminated in oxygen-producing cyanobacteria. | Eon | 4,031 | 2,500 | 1,531 My | |
| Proterozoic Great Oxidation Event, repeated Snowball Earth glaciations, and the rise of complex single-celled and early multicellular life. | Eon | 2,500 | 538.8 | 1,961 My | |
| Phanerozoic The eon of visible life: hard-bodied fossils become abundant, vertebrates appear, and ecosystems repeatedly reorganize after mass extinctions. | Eon | 538.8 | present | 539 My | |
| Paleozoic Cambrian explosion of animal body plans, conquest of land by plants and tetrapods, ending in the Permian extinction. | Era | 538.8 | 251.902 | 287 My | |
| Mesozoic Age of dinosaurs, breakup of Pangaea, first mammals, birds, and flowering plants, ending with the Chicxulub impact. | Era | 251.902 | 66.0 | 186 My | |
| Cenozoic Mammals and birds diversify, grasslands spread, the climate cools into ice ages, and humans appear. | Era | 66.0 | present | 66.0 My | |
| Cambrian Rapid appearance of nearly all modern animal phyla, including the first trilobites and early chordates. | Period | 538.8 | 485.4 | 53.4 My | |
| Ordovician Great Ordovician Biodiversification Event in the seas, first land plants, ending in a major glaciation-driven extinction. | Period | 485.4 | 443.8 | 41.6 My | |
| Silurian Jawed fishes diversify, coral reefs expand, and vascular plants establish a foothold on land. | Period | 443.8 | 419.2 | 24.6 My | |
| Devonian Age of fishes, first forests and seed plants, earliest tetrapods crawl ashore; closes with the Late Devonian extinctions. | Period | 419.2 | 358.9 | 60.3 My | |
| Carboniferous Vast coal-forming swamp forests, giant arthropods, first amniote vertebrates, and the assembly of Pangaea begins. | Period | 358.9 | 298.9 | 60.0 My | |
| Permian Pangaea fully assembled and synapsids dominate land, ending in the end-Permian extinction, the largest in Earth's record. | Period | 298.9 | 251.902 | 47.0 My | |
| Triassic Recovery from end-Permian extinction, first dinosaurs and mammals, ending with the end-Triassic extinction. | Period | 251.902 | 201.4 | 50.5 My | |
| Jurassic Dinosaurs become dominant, first birds appear, and Pangaea begins to break apart. | Period | 201.4 | 145 | 56.4 My | |
| Cretaceous Flowering plants radiate, modern groups of insects and mammals emerge, ending with the Chicxulub impact and non-avian dinosaur extinction. | Period | 145 | 66.0 | 79.0 My | |
| Paleogene Mammals and birds rapidly diversify into vacated niches; climate warms then cools as Antarctica glaciates. | Period | 66.0 | 23.0 | 43.0 My | |
| Neogene Grasslands spread, hominins emerge in Africa, and the modern continental configuration largely takes shape. | Period | 23.0 | 2.58 | 20.5 My | |
| Quaternary Repeated glacial-interglacial cycles, megafauna rise and fall, and Homo sapiens spreads across the planet. | Period | 2.58 | present | 2.58 My | |
| Pleistocene Ice ages and interglacials cycle on Milankovitch beats; megafauna flourish and many go extinct as humans expand. | Epoch | 2.58 | 0.0117 | 2.57 My | |
| Holocene Stable warm interval since the last glacial maximum, agriculture and civilization develop. | Epoch | 0.0117 | present | 12 ky |
The geological time scale is a calibrated calendar for Earth’s history. Each row above is a unit of time defined by changes in the rock and fossil record, with boundaries dated by radiometric methods and astronomical cycles. The widest rows (eons) nest into eras, periods, and epochs, the same way a year nests into months, weeks, and days. Ages are expressed in millions of years ago, abbreviated Mya, with zero meaning present.
The Phanerozoic eon, which contains every period and epoch most people recognize, covers only about the last 539 million years. The three preceding eons run from 4,567 to 538.8 Mya, roughly seven-eighths of Earth’s history. Most of that time predates abundant hard-bodied fossils, so it is divided into fewer, broader units. If you scale Earth’s history to a single year, complex animals appear in mid November and humans show up in the last few minutes of December 31.
Several period boundaries coincide with mass extinction events that reshape the biosphere. The end-Ordovician glaciation, the Late Devonian crises, the end-Permian extinction (the largest in the record, ending the Paleozoic), the end-Triassic event, and the end-Cretaceous Chicxulub impact (which closed the Mesozoic) all serve as natural punctuation marks. Boundaries are placed where the rock record changes sharply, often because so much of the previous ecosystem disappeared.
There is a subtle distinction between time itself and the rocks deposited during that time. Geochronological units (eon, era, period, epoch, age) measure time. Chronostratigraphic units (eonothem, erathem, system, series, stage) refer to the physical layers of rock that formed during those intervals. In practice the names match, so the “Jurassic Period” is the time and the “Jurassic System” is the rock. The International Commission on Stratigraphy publishes the authoritative chart and updates boundary ages as new dating evidence comes in.
For shorter timescales, the century boundary tool covers the rules for grouping calendar years into centuries and millennia. For the radiometric methods that pin most of these boundary ages, see the half-life calculator.