The Geologic Time Scale (GTS) is the framework scientists use to organize and understand Earth’s 4.54-billion-year history. Because this immense expanse of time cannot be described using human calendars, geologists developed a comprehensive system of divisions. The GTS uses a hierarchical structure to categorize this deep history based on the physical and biological changes preserved in the planet’s rocks. The largest and most encompassing of these divisions is the eon, which serves as the fundamental building block for all other, shorter time units.
What Defines an Eon in Geologic Time
An eon represents the largest unit in the geochronologic hierarchy. While a human lifetime spans decades, an eon lasts for hundreds of millions to billions of years. The boundaries between these time blocks are not arbitrary dates but are determined by profound and irreversible global events that fundamentally changed Earth’s geology, atmosphere, or biology.
The formal standardization of the Geologic Time Scale, including the definition and naming of eons, is managed by the International Commission on Stratigraphy (ICS). This global body divides the planet’s history using two complementary concepts: chronostratigraphy and geochronology. Chronostratigraphy refers to the physical rock layers themselves, known as eonothems, while geochronology refers to the abstract time unit, the eon, to which those rock layers correspond.
Eons sit at the top of the time-scale structure, encompassing the smaller, more familiar divisions like eras, periods, and epochs. For example, the current eon contains three eras, each of which is further broken down into multiple periods. This system ensures that every rock layer and every event in Earth’s past can be precisely located within a standardized chronological context.
The Four Recognized Eons
There are four formally recognized eons that span the entire history of the planet. These four divisions are named sequentially from the oldest to the current one. The immense time preceding the current eon is often collectively referred to as the Precambrian, which incorporates the first three eons and accounts for nearly 88% of all geologic time.
The oldest is the Hadean Eon, which began with the formation of Earth approximately 4.54 billion years ago and lasted until about 4.0 billion years ago. The Hadean, named for the Greek god of the underworld, was a chaotic time when the planet was largely molten, experiencing intense volcanism and frequent impacts from space debris. Few rock records survive from this eon, but it was the time when Earth differentiated into its core, mantle, and crust, and when the Moon likely formed.
The Archean Eon followed, beginning 4.0 billion years ago and concluding 2.5 billion years ago. This era saw the cooling of the planet’s surface, allowing for the formation of stable continental crust and the condensation of water to form the first oceans. The Archean is marked by the appearance of the first single-celled, prokaryotic life forms, such as bacteria and archaea, which began to colonize the primordial seas.
Next came the Proterozoic Eon, which extended from 2.5 billion years ago to 538.8 million years ago. This eon is characterized by the expansion of microbial life, leading to the first significant modification of the global environment. The Proterozoic also saw the evolution of eukaryotic cells, which possess a nucleus and other internal structures, setting the stage for the emergence of multicellular organisms.
The fourth and current eon is the Phanerozoic Eon, which started 538.8 million years ago and continues to the present day. Its name translates to “visible life,” which is fitting because this eon encompasses the time when complex, multicellular life forms diversified rapidly and became widespread. All familiar forms of life, including fishes, dinosaurs, mammals, and flowering plants, evolved and flourished during the Phanerozoic.
Key Planetary Shifts Marking Eon Boundaries
The transitions between the four eons are anchored by global shifts that irreversibly altered planetary development. The first major transition, from the Hadean to the Archean, was defined by the planet’s cooling enough to support a stable, solid crust and liquid water. This geological stabilization allowed the first continental fragments to form and provided the necessary environment for the earliest life to take hold.
The boundary between the Archean and the Proterozoic is marked by the Great Oxidation Event (GOE), a profound atmospheric transformation. Early photosynthetic organisms, primarily cyanobacteria, began producing free oxygen as a waste product, which gradually accumulated in the atmosphere starting around 2.4 billion years ago. This rise in oxygen levels was toxic to much of the existing anaerobic life but created the conditions required for the eventual evolution of organisms that use aerobic respiration.
The final major shift occurred at the transition from the Proterozoic to the Phanerozoic, defined by the Cambrian Explosion. Starting around 538.8 million years ago, this event was a relatively short span of time in which most major animal phyla appeared in the fossil record. The sudden emergence of complex, hard-shelled, and motile organisms created a stark difference in the fossil record compared to the preceding eon. These three shifts—geological stabilization, atmospheric oxygenation, and the diversification of complex life—represent global, irreversible changes that justify the separation of Earth’s history into four distinct eons.