HadeanThe first eon, from about 4.6 to 4 billion years ago.
ArcheanThe second eon, from about 4 to 2.5 billion years ago.
ProterozoicThe third eon, from 2.5 billion to about 540 million years ago.
PhanerozoicThe eon we live in: about the last 540 million years.
PaleozoicThe first era, from about 540 to 252 million years ago.
MesozoicThe middle era, from 252 to 66 million years ago.
CenozoicThe newest era: the last 66 million years.
Key events 11
Earth formsAbout 4.6 billion years ago. The day on this clock starts here.
The Moon formsAbout 4.5 billion years ago, soon after Earth formed.
Oldest known rocksAbout 4 billion years old. They are in northwestern Canada.
First traces of lifeFossil traces of life are found from about 3.5 billion years ago.
Oxygen builds upOxygen made by microbes began to fill the air. About 2.4 to 2.2 billion years ago.
First cells with a nucleusSome 1.8 billion years ago. Such cells are called eukaryotes.
First animalsNear the end of the Proterozoic, in its last hundred million years.
Cambrian explosionAbout 540 million years ago. Many kinds of animals first appear as fossils.
First dinosaursIn the Triassic Period, about 230 million years ago.
Dinosaurs die out66 million years ago, in the mass extinction that ends the Mesozoic.
Our species appearsAbout 300,000 years ago. On this dial that is almost midnight.
How this list was compiled: Names, order and ages checked on 2026-10-03 in two independent sources each. The eons and eras: (1) National Park Service, Geologic Time Scale, https://www.nps.gov/subjects/geology/time-scale.htm (Hadean 4600 to 4000, Archean 4000 to 2500, Proterozoic 2500 to 541, Paleozoic 541 to 252.2, Mesozoic 252.2 to 66.0, Cenozoic 66.0 to now, in millions of years) and (2) Wikipedia, Geologic time scale, https://en.wikipedia.org/wiki/Geologic_time_scale , which gives the ages of the International Chronostratigraphic Chart (538.8, 251.9, 66.0). Where the two differ the page says about. The events: Earth forms, (1) 4,600 million years; (2) puts the start of the Hadean at 4,567 million years and (3) US Geological Survey, Age of the Earth, https://pubs.usgs.gov/gip/geotime/age.html , gives 4.54 billion years as the best age, so the page says about 4.6 billion, the round figure of (1); the Moon, (4) NASA, Moon Formation, https://science.nasa.gov/moon/formation/ (about 4.5 billion years ago) and (2) Timeline of life; the oldest known rocks, (3) the Acasta Gneisses of northwestern Canada, 4.03 billion years, and (2); first traces of life, oxygen and cells with a nucleus, (5) University of California Museum of Paleontology, The Proterozoic Eon, https://ucmp.berkeley.edu/precambrian/proterozoic.php (traces of life nearly 3.5 billion years ago; a rapid rise of oxygen about 2.2 billion years ago; eukaryotic cells as fossils by about 1.8 billion years ago; the first animals toward the end of the Proterozoic) and (2) (fossils of 3.48 billion years; the Great Oxidation Event 2.4 billion years ago; eukaryotic cells 1,850 million years ago; the Ediacaran biota from 580 million years ago, which falls in the Proterozoic's last hundred million years, where (5) puts the oldest well-studied animal fossils); the Cambrian, the first dinosaurs in the Triassic and the extinction at the end of the Cretaceous, (1), (2) and (6) British Geological Survey, Geological timechart, https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/geological-timechart/ ; the name Cambrian explosion, (2) and (9) University of California Museum of Paleontology, The Cambrian Period, https://ucmp.berkeley.edu/cambrian/cambrian.php ; the age of the first dinosaurs, (8) US Geological Survey, Dinosaurs: Facts and Fiction, https://pubs.usgs.gov/gip/dinosaurs/when.html (the oldest dinosaur types are about 230 million years old) and (2) Dinosaur (the oldest dinosaur fossils known from substantial remains come from rocks dated at 233 to 230 million years); our species, (7) Smithsonian National Museum of Natural History, Homo sapiens, https://humanorigins.si.edu/evidence/human-fossils/species/homo-sapiens (about 300,000 years ago) and (2) Human (Homo sapiens emerged around 300,000 years ago). Licence: own work
The drawing: Drawn for this site on 2026-10-03 by script from the ages in this file. Own original drawing; nothing is traced or copied. Licence: own work
How it works
Earth's history as a clock squeezes about 4.6 billion years into one day. The rim of the dial is the four eons. Marks show key events, and a second drawing opens up the last three hours.
Read the full guide
Reading the dial
The day starts at the top, when Earth formed, and runs clockwise to now, back at the top. The arcs of the rim are the eons, drawn to scale. The Proterozoic is the longest.
The last three hours
Shelled animals, dinosaurs and people all belong to the Phanerozoic, the short last arc. The second drawing opens it up to show its three eras: the Paleozoic, the Mesozoic and the Cenozoic.
Each mark sits where its age falls on the dial.
Our own species appears so late that its mark touches the 24.
Questions and answers
How do I label the eons, eras and key events on a clock of Earth's history?
Start at the top and go clockwise: Hadean, Archean, Proterozoic, Phanerozoic. The Paleozoic, Mesozoic and Cenozoic eras divide the Phanerozoic. Print the dial blank and write them in, or press Quiz me and tap each one.
What is the clock analogy of Earth's history?
It shrinks Earth's whole history, about 4.6 billion years, into a single 24-hour day, so that the lengths of the eons and the gaps between events can be compared at a glance.
Which eon is the longest?
The Proterozoic. It runs from 2.5 billion to about 540 million years ago, longer than the Hadean, the Archean or the Phanerozoic.
Where are the dinosaurs on the clock of Earth's history?
Late in the day. The first dinosaurs appear in the Triassic Period, about 230 million years ago, and they die out 66 million years ago. Both marks lie in the last three hours of the dial.