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A Complete Overview of the Geologic Time Scale

Explore Earth's history, about 4.54 billion years long, as scientists divide it: by the rock layers and fossils that record it. The dates below are rounded, because the boundaries are periodically refined.

Precambrian (about 4.54 billion to 539 million years ago)

The Precambrian is not an official unit but a handy name for everything before the Cambrian, and it covers roughly 88 percent of Earth's history. It includes the Hadean (an informal name for the hot, early Earth), the Archean, and the Proterozoic. Early single-celled life appeared during the Archean, and photosynthesizing microbes began releasing oxygen. The Great Oxidation Event, around 2.4 billion years ago, raised atmospheric oxygen and reshaped ocean chemistry. Complex multicellular life, including the soft-bodied Ediacaran organisms, shows up only near the end. Fossils are scarce because most organisms had no hard parts, and older rocks have been heavily altered or destroyed, which is why this vast span remains the hardest to study.

Paleozoic Era (about 539 to 252 million years ago)

The era of ancient life saw animals diversify, fish evolve, and life move from sea to land. It is divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. At the start of the Cambrian, most of the major animal groups appear in the fossil record within a geologically short interval, known as the Cambrian explosion. Land plants spread from the Ordovician and Silurian onward, the Devonian is called the Age of Fishes, and four-limbed vertebrates emerged late in the Devonian. Huge swamp forests of the Carboniferous became coal. The era ended with the end-Permian extinction, the most severe in the fossil record.

Mesozoic Era (about 252 to 66 million years ago)

Often called the Age of Dinosaurs, the Mesozoic spans the Triassic, Jurassic, and Cretaceous. Dinosaurs appeared in the Triassic and dominated land ecosystems for well over 100 million years, while pterosaurs flew and marine reptiles such as ichthyosaurs and plesiosaurs hunted in the seas. The earliest mammals date to the Late Triassic, and the first known birds, such as Archaeopteryx, appear in the Late Jurassic. Flowering plants spread during the Cretaceous. The supercontinent Pangaea began breaking apart during this era. It closed with the end-Cretaceous extinction about 66 million years ago, in which non-avian dinosaurs disappeared.

Cenozoic Era (about 66 million years ago to the present)

The Age of Mammals, in which mammals diversified to fill the ecological space left by the dinosaurs. The Cenozoic is formally divided into the Paleogene, Neogene, and Quaternary periods; the older term Tertiary is no longer an official unit. Climate gradually cooled, with ice sheets building on Antarctica and, in the last couple of million years, repeated glacial cycles. Grasslands spread, primates diversified, and the human lineage evolved in Africa. Our own species, Homo sapiens, appears in the fossil record only a few hundred thousand years ago, a tiny fraction of the era. The present epoch, the Holocene, began about 11,700 years ago.

The hierarchy: eon, era, period, epoch, age

Geologic time is organized from the largest unit to the smallest as eon, era, period, epoch, and age. Earth's history is divided into the Hadean, Archean, Proterozoic, and Phanerozoic eons, and the Phanerozoic, which began with the Cambrian about 539 million years ago, is subdivided into the Paleozoic, Mesozoic, and Cenozoic eras. Names often reflect where a rock was first described: Jurassic comes from the Jura Mountains, and Devonian from Devon in England. The matching term for the rocks themselves is a chronostratigraphic unit (erathem, system, series, stage), which describes layers rather than time.

The five big mass extinctions

Paleontologists commonly highlight five major extinction events: the end of the Ordovician, the late Devonian, the end of the Permian, the end of the Triassic, and the end of the Cretaceous. The end-Permian event, roughly 252 million years ago, was the worst, with an estimated 90 percent or more of marine species lost; large volcanic eruptions in what is now Siberia are the leading suspect. The end-Cretaceous event, 66 million years ago, wiped out the non-avian dinosaurs and many other groups, and is linked to an asteroid impact at Chicxulub in the Yucatan region plus volcanic activity in India. Estimates of how many species were lost vary by method.

How the dates are determined

Relative ordering comes from superposition, the principle that layers lower in an undisturbed sequence are older, together with index fossils that lived for a limited time over a wide area. Numerical ages come mainly from radiometric dating, which measures how much of a radioactive isotope has decayed into its product, for example uranium into lead in zircon crystals. Because suitable minerals are found mostly in volcanic rocks, ages of sedimentary layers are often bracketed by dated ash beds. Boundaries between periods are usually defined by a reference point in a specific rock section, then dated, so the figures are refined as methods improve.

The Anthropocene debate

Some scientists proposed naming a new epoch, the Anthropocene, to mark humanity's large effect on the planet. In March 2024 the International Union of Geological Sciences approved the rejection of this proposal as a formal unit of the geologic time scale, so the present epoch remains the Holocene. The term is still widely used informally in science and public discussion to describe the period of strong human impact.

How is geologic time divided?

The scale is built from the sequence of rock layers and the fossils preserved in them. Because the layers cannot be traced continuously across the whole planet, geologists match them using fossils, chemical signals, and radiometric dates. Boundaries are often placed at points of major change in the fossil record, which is why many of them line up with mass extinctions. The International Commission on Stratigraphy maintains the official chart and publishes updated versions, and the chart gives ages with small uncertainty ranges, so the dates in textbooks can differ slightly from edition to edition.

Why did the dinosaurs disappear?

Non-avian dinosaurs disappeared at the end of the Cretaceous, about 66 million years ago. The best-supported explanation is a large asteroid impact at Chicxulub, which is supported by a thin worldwide layer of iridium-rich clay and a buried crater. Intense volcanic activity in India (the Deccan Traps) was also occurring and probably added stress to ecosystems, and scientists still discuss how much each factor contributed. Birds are living descendants of a dinosaur lineage, so in that sense dinosaurs did not entirely vanish.

How are fossils and rocks used to read geologic time?

An index fossil is the remains of an organism that was widespread and short-lived, so finding it dates the rock layer containing it. Sedimentary rocks record changing environments, so a sequence of layers can show a shallow sea turning into a swamp or desert. Reading the sequence correctly requires watching for folding, faulting, and erosion gaps, called unconformities, where part of the record is missing. Most of the time represented by a rock sequence is often missing in the gaps, not preserved in the layers.

A way to remember the scale

It helps to treat Earth's history as a single 24-hour day. On that scale, the Cambrian explosion happens close to 9 pm, dinosaurs rise a little before 11 pm, they disappear at roughly 11:40 pm, and all of recorded human history fits into the last fraction of a second. The analogy only conveys proportion, and exact clock times depend on the dates chosen, but it shows how recent complex life is compared with the long Precambrian.

Frequently Asked Questions

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

Yes. Boundary ages are refined as dating techniques and fossil finds improve, and the figures in this article are rounded values widely used today. The official chart is updated by the International Commission on Stratigraphy.

Which geologic era are we in right now?

We are in the Cenozoic Era, in the Quaternary Period, and in the Holocene Epoch, which began about 11,700 years ago. A proposal to name a separate Anthropocene epoch was rejected as a formal unit in 2024, though the word is still used informally.

Why is the Precambrian not divided into periods like the Phanerozoic?

Precambrian rocks contain few easily identified fossils, so the fine subdivisions that rely on fossil sequences are not possible. They are defined mostly by broad geologic and chemical events and by radiometric ages, which is why the units are larger.

What is the difference between an era and a period?

An era is a larger unit that contains several periods. For example, the Mesozoic Era includes the Triassic, Jurassic, and Cretaceous periods. Periods in turn are divided into epochs.

Were humans alive at the same time as dinosaurs?

No. Non-avian dinosaurs died out about 66 million years ago, while our species appeared only a few hundred thousand years ago, so the two are separated by tens of millions of years.