Cenozoic

The Cenozoic Era is the most recent of the three major subdivisions of geological time, spanning from approximately 66 million years ago to the present day. It is characterized by the diversification and dominance of mammals, birds, and flowering plants on land, as well as the emergence of modern ocean ecosystems following the Cretaceous–Paleogene mass extinction. The Cenozoic is often referred to as the "Age of Mammals," though it equally marks the radiation of insects, modern coral reefs, and grasslands.

Etymology and early definitions

The term "Cenozoic" was introduced in 1840 by the British geologist John Phillips, deriving from the Greek words kainós (καινός, "new") and zōḗ (ζωή, "life"), thus meaning "new life." Phillips proposed the term to describe the most recent chapter of Earth history, in which the fossil record shows the rise of life forms resembling those of the modern world. It was one of the three eras he defined for the Phanerozoic Eon, alongside the Paleozoic ("ancient life") and Mesozoic ("middle life"). The spelling "Cainozoic" is sometimes used, especially in older British literature, but "Cenozoic" has become the internationally accepted form.

Divisions

The Cenozoic is divided into three periods, which are further subdivided into epochs. The International Commission on Stratigraphy currently recognizes the following hierarchy:

Paleogene Period (66.0 – 23.03 Ma)

The Paleogene spans the interval from the immediate aftermath of the end-Cretaceous mass extinction to the beginning of the Neogene. It contains three epochs:

• Paleocene (66.0 – 56.0 Ma): A time of recovery and rapid mammalian evolution in the absence of non-avian dinosaurs. Modern mammal orders began to appear, but many forms were archaic.
• Eocene (56.0 – 33.9 Ma): Marked by intensely warm global climates, including the Paleocene–Eocene Thermal Maximum. Mammals reached their peak of archaic diversity, and modern orders such as perissodactyls, artiodactyls, and primates diversified. The first grasses appeared.
• Oligocene (33.9 – 23.03 Ma): A period of climatic cooling and drying, which saw the expansion of grasslands and the evolution of many modern mammal families. Antarctica became permanently glaciated near the epoch's end.

Neogene Period (23.03 – 2.58 Ma)

The Neogene is characterized by further cooling, the spread of open landscapes, and the continued evolution of modern flora and fauna. Its two epochs are:

• Miocene (23.03 – 5.333 Ma): Warmer than the Oligocene at first, then progressively cooler and drier. Grasses expanded dramatically, leading to the evolution of grazing mammals. Apes diversified in Africa and Eurasia. The Mediterranean Sea temporarily dried up during the Messinian salinity crisis near the end.
• Pliocene (5.333 – 2.58 Ma): Cooling intensified, with Northern Hemisphere ice sheets beginning to form. The Isthmus of Panama closed, altering ocean circulation. Early hominins appeared in Africa. The modern marine fauna became established.

Quaternary Period (2.58 Ma – present)

The Quaternary is the shortest and most recent period, defined by the onset of persistent continental glaciation in the Northern Hemisphere and the evolution of the genus Homo. It comprises two epochs:

• Pleistocene (2.58 – 0.0117 Ma): Commonly known as the Ice Age, this epoch was marked by repeated glacial–interglacial cycles driven by Milankovitch orbital variations. Large mammals such as mammoths, woolly rhinoceroses, and saber-toothed cats roamed the Northern Hemisphere. Anatomically modern humans (Homo sapiens) evolved in Africa and spread across the globe near the epoch's end.
• Holocene (0.0117 Ma – present): The current interglacial period, which began with the end of the last glacial phase. All recorded human history falls within the Holocene. Its beginning is now formally defined by a GSSP in the Greenland ice core record. Some scientists propose an Anthropocene epoch to reflect humanity's significant global impact, but this has not yet been ratified by the ICS.

Paleogeography and climate

The geography of the Cenozoic saw the final breakup of the supercontinent Pangaea and the movement of continents to their present positions. Key events include:

• The separation of Australia from Antarctica in the early Eocene, which allowed the Antarctic Circumpolar Current to form and thermally isolate Antarctica, leading to its permanent glaciation.
• The collision of India with Asia, beginning in the Paleogene and still ongoing, which uplifted the Himalayan–Tibetan orogen and profoundly influenced global atmospheric circulation.
• The formation of the Isthmus of Panama during the late Pliocene, closing the marine connection between the Atlantic and Pacific and intensifying the Gulf Stream, which contributed to Northern Hemisphere glaciation.
• The opening of the Greenland–Scotland Ridge and the Fram Strait, allowing deep-water exchange between the Arctic and Atlantic Oceans.

The Cenozoic climate trend was one of long-term cooling from the early Eocene hothouse, with peak warmth around 55–50 Ma, toward the Quaternary icehouse. Superimposed on this trend were abrupt events such as the Paleocene–Eocene Thermal Maximum, the Middle Miocene Climate Optimum, and the multiple glacial–interglacial oscillations of the Pleistocene. The cooling is attributed primarily to declining atmospheric CO₂ due to enhanced silicate weathering linked to mountain uplift, and to changes in ocean gateways.

Life

Flora

The Cenozoic witnessed the rise of angiosperms (flowering plants) to complete dominance across most terrestrial ecosystems. In the Paleogene, tropical and subtropical forests extended to high latitudes. As the climate cooled and dried, forests retreated and grasslands expanded, especially from the late Oligocene onward. The evolution of C4 photosynthetic pathways in grasses during the Miocene allowed them to thrive under low CO₂ and arid conditions, transforming global terrestrial biomes. Modern plant communities such as temperate deciduous forests, taiga, tundra, and savannas became widespread during the Neogene and Quaternary.

Fauna

The defining faunal turnover event was the Cretaceous–Paleogene extinction, which eliminated all non-avian dinosaurs, pterosaurs, mosasaurs, plesiosaurs, and ammonites. In their absence, mammals underwent an explosive adaptive radiation. During the Paleogene, placental mammals evolved into a wide array of forms, including early primates, rodents, bats, cetaceans, proboscideans, and large herbivores such as uintatheres and brontotheres. Marsupials and monotremes diversified on the isolated continents of Australia and South America.

The Neogene saw the modernization of mammalian families. Ungulates diversified into grazing and browsing forms as grasslands spread. Carnivora, including cats, dogs, bears, and seals, evolved into their modern families. The interchange of faunas between continents, particularly the Great American Interchange following the connection of North and South America, reshuffled regional ecosystems. The Quaternary is notable for the evolution of large megafauna and the rise of Homo sapiens, whose global expansion coincided with the late Pleistocene megafaunal extinctions. In the oceans, teleost fishes diversified enormously, cetaceans returned to the water and reached modern forms, and modern coral reefs became established.

Significant events

• K-Pg mass extinction (66 Ma): Marks the base of the Cenozoic; caused by a bolide impact and massive volcanism, it opened ecological niches for mammals.
• Paleocene–Eocene Thermal Maximum (~56 Ma): A rapid global warming event that triggered turnovers in mammal, insect, and plant communities.
• Azolla event (middle Eocene): Massive blooms of the freshwater fern Azolla in the Arctic Ocean sequestered huge amounts of carbon, contributing to cooling.
• Antarctic glaciation (~34 Ma): At the Eocene–Oligocene boundary, ice sheets formed on Antarctica, initiating a major phase of global cooling.
• Messinian salinity crisis (5.96–5.33 Ma): The Mediterranean Sea partly or wholly dried up, leaving thick evaporite deposits.
• Quaternary glaciations: Cyclical Northern Hemisphere ice ages began around 2.58 Ma, profoundly shaping landscapes, sea levels, and species distributions.
• Human dispersal and megafauna extinctions: The spread of Homo sapiens to all continents except Antarctica correlates with the extinction of many large vertebrates in the late Pleistocene and early Holocene.

Ongoing relevance

The Cenozoic is not merely an interval of academic geological interest; its sediments, landforms, and fossils form the immediate foundation for modern soils, coastlines, groundwater systems, and hydrocarbon reserves. Understanding Cenozoic climate dynamics provides essential context for interpreting current global warming, while the evolutionary history of its biota underpins modern ecology and conservation biology. The era encompasses all of human existence and continues to unfold as the present-day geological chapter of Earth history.