Cenozoic Era

The Cenozoic Era began 66 million years ago and continues today. It is the youngest era of the Phanerozoic Eon and contains the Palaeogene, Neogene and Quaternary periods. Its beginning is fixed at the Cretaceous–Palaeogene boundary, where an impact-driven crisis ended the Mesozoic Era and removed all non-avian dinosaurs.

Mammals expanded dramatically after the extinction, but they did not originate at the boundary. Their ancestors and early members had lived alongside dinosaurs for more than 150 million years. Birds are surviving dinosaurs and also diversified across Cenozoic ecosystems. “Age of mammals” is therefore a useful shorthand only if it does not exclude the rest of life.

The first years after the K–Pg boundary

The Chicxulub impact ejected dust, sulphate aerosols and soot into the atmosphere. Reduced sunlight disrupted photosynthesis and food webs. The losses were severe but selective: some birds, mammals, crocodilians, turtles, amphibians and fishes survived, while many close relatives did not.

Ecological recovery proceeded in stages. Fern-rich vegetation appears in some disturbed landscapes, plankton communities changed and forests rebuilt. Diversity counts could rebound before food webs regained their former structural complexity. The evidence behind the boundary is detailed in why dinosaurs became extinct.

Palaeogene radiations

During the Palaeocene, mammals diversified in body size and diet within recovering ecosystems. Many belonged to extinct branches rather than modern orders. Identifying a fossil as an early relative of living hoofed mammals, carnivorans or primates does not mean it looked or behaved exactly like a modern member.

The early Eocene was one of the warmest intervals of the Cenozoic. Forests reached high latitudes, and warm-adapted groups occupied regions that are now temperate or polar. Whales evolved from terrestrial artiodactyl ancestors through a well-sampled series of amphibious and marine forms. Bats, primates and several other mammal groups also have informative Eocene records.

At about 34 Ma, the Eocene–Oligocene transition coincided with major cooling and the expansion of Antarctic ice. Changing atmospheric carbon dioxide, ocean gateways and circulation all form part of the explanation. The transition affected sea level and habitats but was not identical in every region.

Neogene grasslands and changing continents

Open habitats expanded through parts of the Miocene and Pliocene, although grass had originated much earlier and forests remained extensive. Grazing mammals evolved specialised high-crowned teeth repeatedly, while predators and fast-running herbivores changed alongside more seasonal environments. Tooth form alone, however, does not identify every food item; microscopic wear and chemical evidence provide additional tests.

Continental movement altered ocean circulation. The collision of India and Asia, uplift of major mountain systems and changing seaways affected regional monsoons and global heat transport. South America’s later connection with North America allowed animals to disperse in both directions, but this interchange occurred through multiple waves rather than one simultaneous migration.

Hominins emerged within African ape evolution during the Neogene. Bipedal anatomy developed before the large brain and technology characteristic of later Homo. Human evolution is a branching history containing coexistence and extinction, not a ladder leading inevitably to one species.

Quaternary glacial cycles

The Quaternary began 2.58 Ma. Ice sheets expanded and retreated repeatedly as changes in Earth’s orbit altered the geographical and seasonal distribution of sunlight. Greenhouse gases, ice reflectivity and ocean circulation amplified these orbital signals. “Ice Age” therefore describes a climatic condition and its cycles, not an official synonym for every part of the period.

Cold, dry steppe environments supported mammoths, woolly rhinoceroses, horses, bison and many smaller animals across large parts of northern Eurasia and North America. Other regions retained forests, deserts, tropical ecosystems or distinctive island faunas. Even at a glacial maximum, the world was not one continuous frozen plain.

Megafaunal losses were not one global event

Many large-bodied animals disappeared during the Late Pleistocene and early Holocene. The timing varied by continent and island. Human hunting, habitat change and rapid climate shifts have different evidential weight in different regions; a single universal cause does not fit every extinction.

Large animals often reproduce slowly and need extensive habitat, which can make populations vulnerable to combined pressure. Archaeological associations, cut marks, population histories and dated last occurrences must be evaluated regionally. The youngest fossil yet found is not automatically the exact moment the final individual died.

Humans occupy a very short interval

Homo sapiens originated roughly 300,000 years ago, only about 0.45% of the Cenozoic. Agriculture began far later, and industrialisation occupies an almost invisible sliver on a geological chart. This small duration does not imply a small effect: human activity now alters climate, erosion, biogeochemical cycles and species distributions at globally measurable rates.

The Holocene Epoch began 11,700 years ago. We currently live in its Meghalayan Age. Anthropocene is widely used for human-driven planetary change, but the proposal to make it a formal epoch was not ratified. Environmental evidence and formal chronostratigraphic rank answer related but distinct questions.

How Cenozoic climates are reconstructed

Marine microfossils, oxygen isotopes, organic molecules, fossil leaves, soils, ice cores and models sample different parts of the climate system. Ice cores provide exceptional resolution but cover only the latest fraction. Ocean sediments reach farther back but require assumptions about seawater and later alteration.

A global curve cannot describe every region. Elevation, currents, seasonality and continental position matter. Reliable summaries therefore separate a long-term tendency towards cooling from shorter warm events, local exceptions and the uncertainty of each proxy.

Tertiary, Quaternary and the current scale

Tertiary once covered most of the Cenozoic before the Quaternary. It remains common in older books and informal names but is not a formal period on the current international chart. Its former interval is now divided mainly between the Palaeogene and Neogene.

The Cenozoic continues. Enter a numerical age in the geological time calculator to distinguish its periods and epochs, especially near the 23.04 and 2.58 Ma boundaries.