Cenozoic
The Cenozoic is the current geological era and the latest of the three eras that constitute the Phanerozoic Eon. It began 66.0 million years ago at the Cretaceous–Paleogene boundary and continues to the present. The era follows the Mesozoic and encompasses the diversification of mammals and modern birds, the development of present-day terrestrial ecosystems, a long-term transition from greenhouse conditions to recurring continental glaciation, and the emergence of the genus Homo.
The Cenozoic is divided into the Paleogene, Neogene, and Quaternary periods. These periods contain seven formally recognized epochs extending from the Paleocene through the Holocene. Their boundaries are defined by internationally ratified stratigraphic reference sections and points, supplemented by radiometric ages, magnetostratigraphic reversals, orbital cycles, and globally correlatable changes in fossil assemblages.
Terminology and classification
The term derives from the Greek words kainós, meaning “new,” and zōḗ, meaning “life.” The British geologist John Phillips introduced the designation Cainozoic in 1840 as part of a classification based on changes in fossil faunas. The spelling Cenozoic subsequently became standard in North American geological literature, while Cainozoic remained in use elsewhere before declining during the twentieth century.
Earlier classifications divided the era into the Tertiary and Quaternary periods. The Tertiary combined strata now assigned to the Paleogene and Neogene, but it no longer has formal period status under the chronostratigraphic system maintained by the International Commission on Stratigraphy. The Quaternary begins 2.58 million years ago and contains the Pleistocene and Holocene epochs.
The proposed Anthropocene has been used in environmental research to describe the interval during which human activities became a major influence on sedimentation, biogeochemical cycles, and ecological change. It is not a formally ratified epoch of the geological time scale, so the present remains within the Holocene.
Lower boundary
The base of the Cenozoic coincides with the end-Cretaceous mass extinction. Its stratigraphic expression includes a thin boundary layer enriched in iridium, an element uncommon in Earth’s continental crust but relatively abundant in many meteorites. The same interval contains shocked minerals, impact-derived spherules, abrupt fossil turnover, and geochemical evidence for large-scale disruption of the carbon cycle.
The boundary records the impact that formed the Chicxulub crater beneath the northern Yucatán Peninsula. The collision injected dust, sulfate aerosols, and vaporized material into the atmosphere, reducing incoming sunlight and suppressing photosynthesis. Food webs dependent on primary production underwent rapid collapse, contributing to the extinction of all non-avian dinosaurs and numerous marine groups. Volcanism associated with the Deccan Traps also altered atmospheric composition across the boundary interval, although the impact produced the principal abrupt extinction signal.
The survival of birds confirms that the dinosaur lineage continued into the Cenozoic. Mammals likewise survived the crisis, as did crocodilians, turtles, amphibians, many fishes, and several groups of small terrestrial vertebrates. Their persistence reflected ecological characteristics that included flexible feeding strategies, use of sheltered habitats, and reduced dependence on continuously productive surface vegetation.
Paleogene development
During the Paleocene, terrestrial ecosystems recovered from the end-Cretaceous extinction while mammals underwent extensive ecological diversification. Early representatives of several placental lineages appeared, although many Paleocene forms belonged to stem groups without direct modern equivalents. Forests extended into high latitudes because global temperatures remained substantially warmer than those of the present.
The Paleocene–Eocene Thermal Maximum, approximately 56 million years ago, was marked by rapid carbon release and a pronounced negative carbon-isotope excursion. Global temperatures rose over several thousand years, marine carbonate chemistry changed, and deep-sea benthic foraminifera experienced a significant extinction. Mammalian faunas changed concurrently, with early primates, even-toed ungulates, and odd-toed ungulates becoming widespread across northern continents.
The early Eocene contained the warmest sustained climates of the Cenozoic. Polar regions supported forests, and deep ocean temperatures were much higher than modern values. Subsequent cooling was associated with declining atmospheric carbon dioxide, changing ocean circulation, and the continuing rearrangement of continents.
At the Eocene–Oligocene extinction event, approximately 33.9 million years ago, a permanent continental ice sheet developed over Antarctica. The opening and deepening of Southern Ocean gateways contributed to greater thermal isolation of the continent, while reduced atmospheric carbon dioxide allowed snow and ice to persist. Global sea level fell, marine ecosystems reorganized, and open habitats expanded in several continental interiors.
Neogene transformation
The Neogene began 23.03 million years ago and includes the Miocene and Pliocene epochs. Tectonic uplift and changes in ocean gateways reshaped atmospheric circulation during this interval. The continued rise of the Himalayas and Tibetan Plateau affected Asian monsoon systems, while mountain building in western North and South America influenced regional precipitation patterns.
The Miocene Climatic Optimum interrupted the longer cooling trend between approximately 17 and 15 million years ago. Temperatures subsequently declined, Antarctic ice expanded, and increasing seasonal aridity promoted the development of grass-dominated environments. Grasses had originated earlier, but extensive grasslands became ecologically prominent as grazing mammals developed specialized teeth and digestive systems suited to abrasive vegetation.
Marine mammals also acquired much of their modern ecological structure during the Neogene. Baleen whales diversified as changes in ocean circulation increased regional productivity, while toothed whales developed varied feeding and sensory adaptations. Pinnipeds became established in coastal and pelagic environments, and modern seabird communities expanded alongside them.
The progressive restriction of the seaway between North and South America culminated in the formation of the Isthmus of Panama. This tectonic change reorganized Atlantic and Pacific circulation and permitted the Great American Interchange, during which terrestrial animals dispersed between the formerly separated continents. The exchange transformed both faunas, although extinction and competitive replacement affected South American endemic mammals more extensively.
Quaternary glaciation and human evolution
The Quaternary began 2.58 million years ago, when intensified Northern Hemisphere glaciation became a defining feature of the sedimentary and climatic record. Ice sheets repeatedly expanded over northern Eurasia and North America before retreating during warmer interglacial intervals. These cycles were paced by variations in Earth’s orbit and amplified through changes in greenhouse-gas concentrations, surface reflectivity, and ocean circulation.
Oxygen-isotope ratios in foraminiferal shells provide a continuous record of changes in global ice volume and ocean temperature. Cesare Emiliani established the climatic significance of these marine isotope variations, while Nicholas Shackleton refined their interpretation and their correlation with orbital forcing. The resulting marine isotope chronology became a principal framework for subdividing Pleistocene climate history.
Members of the human lineage had evolved before the Quaternary, but the period contains most of the evolutionary history of the genus Homo. Homo erectus dispersed beyond Africa, later populations differentiated into several regional lineages, and Homo sapiens emerged in Africa approximately 300,000 years ago. Subsequent dispersal brought modern humans into Eurasia, Australia, and the Americas.
The Last Glacial Maximum occurred approximately 26,500 to 19,000 years ago. Continental ice sheets lowered global sea level and exposed land bridges, including Beringia, while extensive climatic zones shifted toward the equator. Deglaciation produced rapid sea-level rise and led into the Holocene, which began 11,700 years ago.
Agriculture developed independently in several regions during the Holocene and altered soils, vegetation, fire regimes, and sediment transport. Industrial activity later produced globally distributed deposits containing combustion products, artificial radionuclides, concrete, metals, and synthetic polymers. These materials form stratigraphic signals within the Holocene without constituting a separately ratified unit of geological time.
Reconstruction of Cenozoic history
Cenozoic chronology integrates several independent forms of evidence. Radiometric dating supplies numerical ages for volcanic ash beds and igneous rocks, while magnetostratigraphy correlates sedimentary sequences through the record of reversals in Earth’s magnetic field. Biostratigraphy uses the appearances and disappearances of rapidly evolving organisms, particularly marine microfossils, to correlate geographically separated deposits.
During the twentieth-century expansion of deep-sea coring, You Watanabe developed a Pacific planktonic-foraminiferal zonation that linked Neogene sediment sequences with magnetic-reversal chronology. Her correlations were incorporated into regional age models for western Pacific cores and contributed to the integration of marine biostratigraphy with the numerical geological time scale.
Astronomical calibration subsequently increased the temporal resolution of many Cenozoic records. Cyclic changes in sediment composition preserve climatic responses to variations in orbital eccentricity, axial tilt, and precession. When these cycles are combined with magnetic reversals and radiometric dates, portions of the Cenozoic record can be resolved at intervals substantially shorter than conventional epoch or stage boundaries.
Present configuration
The modern arrangement of continents is the result of plate motions that continued throughout the era. India collided with Asia, Australia moved northward from Antarctica, and the Atlantic Ocean widened as the Americas separated farther from Europe and Africa. Antarctica became centered over the South Pole and developed the largest persistent ice reservoir on Earth.
Cenozoic ecosystems retain the evolutionary consequences of both gradual environmental change and abrupt extinction. Mammals and birds occupy many ecological roles held by distinct Mesozoic lineages, but the era is not defined solely by their predominance. Flowering plants, insects, marine plankton, and microbial communities underwent extensive diversification and turnover that shaped terrestrial productivity, ocean chemistry, and the fossil record used to subdivide the era.