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A Complete Overview of the Geologic Time Scale

Explore Earth's 4.6-billion-year history divided into geologic eras based on fossils and rock layers.

Precambrian (~4.6 billion years ago – ~540 million years ago)

The longest span, making up about 90% of Earth's history, divided into the Hadean, Archean, and Proterozoic, during which single-celled organisms and early multicellular life appeared. This spans the hot period right after Earth formed (the Hadean), through the appearance of the first single-celled life forms (the Archean), to the period when photosynthetic organisms increased, oxygen accumulated in the atmosphere, and the first multicellular organisms appeared (the Proterozoic). Because fossil records are extremely rare from this period, it was long a difficult span to study.

Paleozoic Era (~540 million years ago – ~250 million years ago)

An era in which the Cambrian Explosion caused a diverse array of animal phyla to appear explosively, fish and amphibians emerged, and life moved from the sea onto land. The Paleozoic is divided into six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. During the Cambrian, most of the animal phyla that exist today appeared within a short span of time, in what is known as the "Cambrian Explosion," and during the Devonian, fish flourished so extensively that it is also called the "Age of Fishes." Plants, insects, and amphibians subsequently moved onto land, and the Paleozoic came to a close with the Permian mass extinction, known as the largest in Earth's history.

Mesozoic Era (~250 million years ago – ~66 million years ago)

Commonly known as the "Age of Dinosaurs," during which dinosaurs and reptiles dominated the land, sky, and sea across the Triassic, Jurassic, and Cretaceous periods. During the Mesozoic, reptiles flourished greatly, with a variety of large reptiles including dinosaurs dominating the terrestrial ecosystem, while pterosaurs ruled the skies and marine reptiles like ichthyosaurs and plesiosaurs lived in the oceans. Around the Jurassic, the first birds and early mammals also appeared. The Mesozoic ended with the K-Pg mass extinction, whose leading cause is believed to be a massive asteroid impact, wiping out most of the dinosaurs.

Cenozoic Era (~66 million years ago – present)

The "Age of Mammals," in which mammals rapidly filled the niche left behind by the dinosaurs, and the era in which the ancestors of humans also appeared. The Cenozoic is broadly divided into the Tertiary and Quaternary periods, during which a variety of mammals flourished, filling the ecological gap left by the disappearance of large reptiles. The climate gradually cooled, and in the latter part of the Cenozoic, early human ancestors appeared and evolved in Africa. The present era we live in also belongs to the Cenozoic.

How is geologic time divided?

Geologic time is divided based on the sequence of rock layers deposited over time and the fossil record preserved within them. From largest to smallest unit, it is subdivided into eons, eras, periods, and epochs, and era-level divisions in particular are often drawn at points when a major change occurred in Earth's ecosystem, such as a mass extinction.

Why did the dinosaurs disappear?

The dinosaurs that dominated the Mesozoic largely disappeared in a mass extinction that occurred about 66 million years ago. A massive asteroid impact is considered the leading cause, but various other hypotheses exist, suggesting other factors such as volcanic activity also played a role. For more detail, see our complete overview of theories on dinosaur extinction.

How are fossils and rock used to reveal geologic time?

Index fossils, which lived only during a specific era, serve as an important clue for gauging the age of a rock layer, and the process by which rock itself accumulates and transforms is also a key piece of evidence for understanding geologic time. If you're curious about the principles behind how rock forms and cycles, see our guide to the rock cycle; if you're curious about how fossils form, see our guide to the principles behind how fossils are formed.

Frequently Asked Questions

Could the precise dates of geologic eras change in the future?

Yes, they could. The boundary dates of geologic eras continue to be refined with greater precision as scientific techniques such as radioactive isotope dating advance, and the dates summarized in this article are approximate figures that are widely used today.

Which geologic era are we in right now?

We are currently in the Cenozoic Era, and geologically classified within one of several epochs of the Cenozoic's Quaternary period. As humanity's impact on Earth's environment grows, there is also ongoing academic discussion about whether this period should be classified as a distinct era of its own.