The Cenozoic Era: An Age of Mammals and Modern Life

Leave a comment / / Updated on: 4th January 2024

cenozoic era

The current era of geologic time is called the Cenozoic Era.

It began 66 million years ago after the end of the Mesozoic Era.

The Cenozoic Era is the last of the three eras of the Phanerozoic Eon and is the shortest so far.

The term “Cenozoic” means “new life.”

The name was proposed by British Geologist John Philips in 1840.

The era is further divided into sub-eras or periods: Paleogene, Neogene, and Quaternary. 

Also known as the “Age of Mammals,” the Cenozoic Era saw the rise of the eutherian and metatherian mammals in the Northern and Southern hemispheres of the Earth.

The mammals came to prominence after the disappearance of the dinosaurs and other apex reptiles of the Mesozoic era.

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Birds and flowering parts also became more abundant during this era of Earth’s history. 

Earth’s continents moved into their current configuration during the Cenozoic Era.

The movement of the continents to their current positions was characterized by major volcanic and Earth-building activities over the course of the era.

Earth’s climate has also changed significantly, moving from the warm and humid climate of the early Cenozoic to the cool and dry climate that we currently have.

In this article, we’ll discuss the timeline of the Cenozoic Era and provide a comprehensive overview of how the climate, geography, and biological life of the planet has changed within the past 66 million years. 

Timeline of the Cenozoic Era

Rock deposits from the Cenozoic Era (Torre Sant’Andrea, Salento, Italy) | Petrescu via Wikipedia

The Cenozoic Era is an interval of 66 million years starting from the end of the Cretaceous Period to the present time.

Cenozoic means “new life,”  and this contrasts with the previous eras, whose names translate as middle life (Mesozoic) and old life (Paleozoic), respectively. 

The Cenozoic Era is divided into three periods, which are further split into seven epochs.

Subdividing the Cenozoic periods into epochs has made it easier for scientists to organize and better understand the events that have occurred over the comparatively short timeline of the Cenozoic Era. 

Sub-eras of the Cenozoic Era 

Coryphodons during the Cenozoic Era | Photo via CK-12 College Human Biology

The Paleogene Period is the first and longest of Cenozoic’s sub-eras.

It began 65 million years ago and ended 23 million years ago.

The Paleogene is further divided into Paleocene, Eocene, and Oligocene epochs. 

The Neogene Period, which is the second sub-era of the Cenozoic Era, is further divided into the Miocene and Pliocene epochs.

The period began 23 million years ago and lasted till about 2.6 million years ago. 

The last sub-era of the Cenozoic Era, which we’re currently in, is known as the Quaternary Period.

It began 2.6 million years ago till the present time.

The period is further divided into Pleistocene and Holocene epochs. 

Although Quaternary is the last recognized period of the Cenozoic Era, some scientists have argued that we are now in a new phase of the Earth’s history known as the Anthropocene Epoch.

However, this has not been officially designated. 

Paleogene Period

Two Uintatherium walk the Eocene of North America. | Artwork by Mauricio Antón via Darwin’s Door

The Paleogene Period is a 43-year period starting from the end of the Mesozoic about 66 million years ago till the Neogene started 23 million years ago.

The period kicked off with a major extinction known as the Cretaceous-Paleogene extinction event.

This event which took place at the close of the Mesozoic Era wiped out all the non-avian dinosaurs and other large reptiles of that period. 

The disappearance of the apex predators of the Mesozoic paved the way for smaller animals (especially mammals) to evolve significantly during the Paleogene.

New mammals evolved, and existing groups became more dominant.

The Paleogene climate was generally warmer than today, with tropical and subtropical conditions all over the world.

The Earth’s continents continued to shift throughout the Paleogene Period, with the separation of South America and Antarctica and the formation of the Himalayas.

The period is further divided into Paleocene, Eocene, and Oligocene epochs.

Paleocene Epoch (66–56 Million Years Ago)

Badlands near Drumheller, Alberta, where erosion has exposed the K–Pg boundary; | Glenlarson via Wikipedia

The first 10 million years of the Paleogene Period (known as the Paleocene Epoch) was a time of recovery for the planet after the devastation caused by the K–Pg extinction event.

The sea level began to fall, and dry lands started appearing in regions previously covered by a large inland sea. 

Sharks were the dominant species in Earth’s receding oceans, replacing the large reptiles as the apex predators.

A similar replacement was taking place on land as well. With the dinosaurs gone, many archaic mammalian groups emerged to fill the available niches.

These included large rodents, armadillos, and primitive primates.

But the Paleocene animals were nowhere close to the Mesozoic beasts that roamed the Earth in terms of their size.

The biggest of them was about the size of a small bear. 

Plant life evolved rapidly during the Paleocene as dry land appeared on the continents.

Pines, cacti, and palm trees appeared, and flowering plants that were already in existence diversified even further. 

The continents began to take shape similar to their current forms, but each one existed as a separate landmass.

The Tethys Sea separated the Afro-Eurasia continents, while North and South America were separated by the strait of Panama.

Earth’s climate was generally warm and tropical. 

Eocene Epoch (56-34 million years ago) 

An extract from Rudolph F. Zallinger’s mural “The Age of Mammals,” showing reconstructions of Eocene mammals. | Yale University via American Museum of National History

The Eocene Epoch started 56 million years ago and lasted until 32 million years ago.

During this epoch, there were significant changes in the continents’ configuration, which had a resultant effect on the Eocene climate. 

The Indian continent, which was one of the major fragments of the old Gondwana supercontinent, collided with the Asian continent creating the Himalayan Mountains.

Australia and Antarctica also started to drift apart.

The gap created between them created a shift in oceanic currents that would later cool down the planet towards the end of the Eocene Epoch. 

This cooling event caused the Earth’s tropical jungles to shrink.

Mammals that once lived in Earth’s dense tropical forest were able to evolve into larger sizes as the jungles shrunk.

Large carnivores like the Andrewsarchus replaced big birds like the Paracrax as the dominant predators on land.

Some large mammals returned to the oceans during the Eocene, adopting a fully aquatic lifestyle.

These were the ancestors of modern whales. 

Oligocene Epoch (34-23 Million Years Ago)

Restoration of Nimravus (far left) and other animals from the Turtle Cove Formation | Turtle Cove mural – Roger Witter via Wikipedia

The Oligocene Epoch, which began 34 million years ago, was ushered in by an extinction event caused by changing climatic conditions.

The changing climate caused seasonal variations from one part of the planet to the other, introducing savanna-like conditions in many places.

This paved the way for the evolution of grasslands and their associated habitats. 

New animal species also emerged to fill the new continental niches. Dogs, cats, marsupials, and other mammalian species emerged during this time.

Bigger animals like elephants and mammoths emerged as well. 

Seasonal variations persisted throughout the Oligocene Epoch, and shifting continents soon triggered another worldwide ice age.

The entire continent of Antarctica was covered in huge sheets of ice glaciers.

This caused a drop in sea level across the planet. More dry land emerged, and there was a significant rise in volcanic activity across Europe and North America.

Neogene Period (23–2.6 Million Years Ago)

Neogene Period | Jay Matternes via Earthly Universe

This is the second period of the Cenozoic Era, and it began 23 million years ago.

During the Neogene Period, mammalian diversification, which began during the Paleogene, progressed further.

The cooling effect, which began during the Oligocene Epoch, continued as well, leading to the development of more diverse ecosystems. 

The Neogene Period is divided into two epochs: the Miocene and the Pliocene.

The Miocene Epoch saw the evolution of apes and the emergence of early human ancestors, while the Pliocene Epoch witnessed the appearance of more advanced hominin species.

Marine life also flourished during this time, with the evolution of modern forms of whales and dolphins.

The Neogene Period also witnessed significant geological events, such as the uplift of the Andes Mountains and the formation of the Great Rift Valley in Africa.

Miocene Epoch (23.03–5.33 Million Years Ago)

Miocene Epoch | Heinrich Harder via Earthly Universe

The volcanic activities that began during the previous epoch across the continents of North America, Europe, South America, and Africa continued throughout the Miocene Epoch.

These mountain-building activities and a combination of other factors, such as shifting oceanic currents and the presence of polar ice in Antarctica, caused a drop in temperature and increased seasonality on the continents. 

The tropical tree forests continued to shrink, and large grasslands evolved in their place, triggering the emergence of new species, such as the hoofed mammals with multiple stomachs adapted for digesting tough grasses.

In the seas, Kelp forests emerged during the Miocene, allowing the evolution of new marine species like sea otters.

The Miocene Epoch had the highest abundance and diversity of mammalian species in all of Earth’s history. 

Hoofed mammals were quite abundant; up to 30 ape species have been identified from this epoch.

The most significant evolution of the Miocene Epoch was the emergence of human-like apes.

The continents of Asia and Africa, as well as North America and Siberia, were linked by land bridges.

As a result, many mammals were able to migrate into new territories via land bridges.

For instance, elephants migrated to North America for the first time during the Miocene Epoch. 

Pliocene Epoch (5.33 to 2.58 Million Years Ago)

During the early Pliocene Epoch, australopithecus evolved | Matheusvieeira via Earthly Universe

The later part of the Neogene Period is known as the Pliocene Epoch, and it began 5.3 million years ago.

This relatively short period was characterized by major changes in climatic conditions and the emergence of many of the modern plant and animal species that are still living. 

During the first half of the Pliocene Epoch, the Earth was warmer, and the sea levels were higher.

But as the epoch progressed, the climatic conditions shifted drastically. The icecaps of the south pole grew larger as temperatures plummeted across the planet.

An ice cap also appeared at the north pole. 

Expanding glaciers on the continents caused the sea level to drop.

The Mediterranean Sea was completely dried out for several million years during the Pliocene Epoch.

There was significant fauna exchange across the continents thanks to the emergence of a land bridge connecting them.

The latest of these land bridges was the Isthmus of Panama which connected North and South America.

This and the other existing land bridges allowed animal species to migrate into new regions, leading to competition and the extinction of some species. 

Most of the plant and animal groups of the Pliocene are similar to present-day varieties, even though individual species were different.

The ancestors of modern humans, Australopithecus, evolved in Africa during the Pliocene Epoch.

These early hominids migrated to other parts of the world and evolved into several distinct species.

However, only one of them has survived into the present day as modern humans.

Quaternary Period (2.58 Million Years – Present)

Artist’s impression of Earth during the Last Glacial Maximum | Ittiz via Wikipedia (CC BY-SA 3.0)

The Quaternary is the current period of the Cenozoic Era, which started about 2.58 million years ago.

It is the shortest geological period in Earth’s history known for repeated glaciations, which led to the formation of ice sheets.

The ice age had a profound impact on global climate and ecosystems. 

The period saw the presence of large mammals like mammoths, saber-toothed cats, and giant sloths.

It was also marked by the presence of early humans and the development of sophisticated stone tools. 

The Pleistocene Epoch (2.58 Million to 11,700 Years Ago)

Pleistocene of Northern Spain, including woolly mammoth, cave lions eating a reindeer, horses, and woolly rhinoceros | Art by Mauricio Antón (CC BY 2.5)

The Pleistocene Epoch lasted until about 11,700 years ago.

The Earth’s continents were now in their current position, but their outline changed over the years as global sea levels rose and dropped.

The Pleistocene climate was characterized by alternating periods of global cooling and warming.

During periods of extensive glaciation, up to 30% of the Earth’s entire land area and a significant portion of the northern oceans were covered by ice sheets. 

As a result of these glaciation events, the planet generally grew more arid. Africa was particularly affected by the emergence of desert lands such as the Sahara, Kalahari, and Namib on the continent.

While many animals survived the ice age and the period of desiccation that followed, several animal species have gone extinct. 

Some of the animal species that evolved and went extinct during the Pleistocene include mammoths, dire wolves, saber-toothed cats, giant ground sloths, and various hominid species.

The period also saw the expansion of primitive humans out of Africa about 100,000 years ago.

A global extinction at the end of the Pleistocene Epoch wiped out most of the planet’s megafauna, including some of the ancestors of modern humans like the Neanderthals. 

Holocene Epoch (11,700 Years Ago Till Present)

Holocene cinder cone volcano on Utah State Route 18 near Veyo | Wilson44691 via Wikipedia

The Holocene Epoch is the last epoch of the Quaternary Period. Most of human history lies within this epoch which began about 11,700 years ago.

The epoch has been marked by a relatively stable and warm climate compared to the preceding glacial periods of the Pleistocene Epoch.

However, there have been major fluctuations in the Holocene climate, a lot of which have been blamed on human activities. 

Ongoing anthropogenic climate change and the extinction of several plant and animal species have been blamed on human activities such as deforestation, urbanization, industrialization, and the burning of fossil fuel. 

At least 322 record species have gone extinct within the past few thousand years. 

Climate and Geography of the Cenozoic Era

The breakup of Pangaea over time | This Dynamic Earth: The Story of Plate Tectonics via Wikipedia

Over the course of the Cenozoic Era, the climate and geography of the Earth have undergone significant changes influenced by changes in the position of the continents, volcanic activities, greenhouse effects, and other related factors. 

At the start of the Cenozoic Era, the Earth was warm and humid. Climatic conditions were also consistent across various locations with limited seasonal or geographical variation.

However, as the period progressed, the climate gradually cooled. 

The continents continued to drift and separate away from each other throughout the Paleogene Period.

South America separated from Antarctica. The Indian and Eurasian tectonic plates also collided to form the Himalayan mountain range. 

During the Neogene, the planets experienced a gradual cooling trend that led to the development of a more seasonal climate.

This cooling trend eventually led to the establishment of ice sheets in the polar regions and the occurrence of intermittent glaciations as the period progressed.

The movement of tectonic plates shaped the geography during the Neogene Period.

The closure of the Central American Seaway between North and South America created a connection between the Atlantic and Pacific Oceans, leading to changes in oceanic circulation and climate of the Neogene.

Pleistocene of South America, including Megatherium and two Glyptodon | DiBgd via Wikipedia (CC BY-SA 3.0)

The ongoing Quaternary Period has been characterized by several fluctuations in climate patterns.

During the Early Quaternary, there were glaciations (Ice Ages) which resulted in the development of ice sheets at the poles and lower global sea levels.

Large areas of land were covered in ice; as these ice sheets expanded, global sea levels dropped.

The drop in sea level exposed land bridges that connected different continents and allowed the migration of species between them. 

Overall, the Cenozoic Era witnessed a transition from a unified warm and tropical climate to a more diverse range of climates that now varies across different locations.

The movement of tectonic plates was the major factor that influenced the geography of the Earth, shaping mountain ranges and altering oceanic connections in the early part of the Era.

In the later years of the Cenozoic Era, cooling trends characterized by intermittent glaciations have been responsible for shaping the geography and climate of the planet. 

Key Events and Developments of the Cenozoic Era

The Cenozoic Era is a relatively short span of time compared to the rest of geologic time.

Yet, some major events and developments have been recorded within the past 65 million years that are quite noteworthy.

Some of these evolutionary and geological events have shaped the Earth’s biology and altered the trajectory of life on Earth significantly. 

Extinction of the Dinosaurs (66 Million Years Ago)

Artist’s rendering of an asteroid a few kilometers across colliding with the Earth. | Fredrik via Wikipedia

Although this technically happened during the Mesozoic Era, the extinction of the dinosaur is the most significant event that determined the direction of life on Earth during the Cenozoic. 

The mass extinction event, which resulted in the demise of non-avian dinosaurs and various other species, paved the way for the rise of mammals and their subsequent dominance throughout the Cenozoic Era.

Radiation of Mammals

The Whales of Today | Gabriel Dizzi via Unsplash

Although mammals first evolved about 178 million years ago, the earliest mammals remained at the bottom of the food chain for several million years.

Many mammalian groups were also wiped out in the end-Cretaceous extinction event, but some of them persisted into the Cenozoic.

With the apex predators gone, the remaining mammals diversified rapidly and adapted to the changing Cenozoic environment.

They soon rose to fill various ecological niches.

Some of them, like the whales, moved to the seas while groups like the primates, rodents, and carnivores became the dominant species on land. 

Paleocene-Eocene Thermal Maximum (55.5 Million Years Ago)

Azolla floating ferns, fossils of this genus indicate subtropical weather at the North Pole | Kurt Stüber via Wikipedia (CC BY-SA 3.0)

During the Paleocene-Eocene transition, a significant global warming event known as the Paleocene-Eocene Thermal Maximum (PETM) occurred.

During this period, large amounts of greenhouse gasses were released into the atmosphere, causing a spike in temperatures and ocean acidification.

This event had a profound impact on marine and terrestrial ecosystems, leading to changes in species distribution.

Glacial-Interglacial Cycles

Icebergs | Mark Olsen via Unsplash

Throughout the Cenozoic Era, there have been periods of glaciation followed by warmer interglacial periods.

The Quaternary Period, which constitutes the latter part of the Cenozoic Era, is particularly known for its ice ages.

These glacial-interglacial cycles were influenced by changes in orbital parameters and greenhouse gas concentrations, leading to fluctuations in global climate and sea levels.

Major Groups of Organisms in the Cenozoic Era

Although the Cenozoic Era is often referred to as the “Age of Mammals,” the period was also characterized by the diversification and emergence of several other major groups of organisms.

Here are a few animal groups that emerged during the Cenozoic Era:


Comparison of a woolly mammoth (left) and an American mastodon (right). | Dantheman9758 via Wikipedia (CC BY-SA 3.0)

Mammals, in general, were the big winners of the Cenozoic Era.

They evolved rapidly after the extinction of the dinosaurs and became the dominant group within the first few million years of the Cenozoic.

Some of the major mammalian groups of the Cenozoic include primates, rodents, carnivores, ungulates (hooved mammals), cetaceans (whales and dolphins), and many others.


Waterfowl | wang binghua via Unsplash

Birds are a diverse group of vertebrates that evolved from the same ancestors as theropod dinosaurs.

The evolution of the birds began during the Mesozoic Era. However, the Cenozoic Era was when the group really diversified into various ecological niches.

With the non-avian dinosaurs and flying reptiles of the Mesozoic gone, many modern bird groups, such as songbirds, raptors, and waterfowl, emerged and diversified.

Various bed species have become adapted to various habitats, including forests, grasslands, and the aquatic environments of the Cenozoic.

Flowering Plants

Dryas octopetala, the mountain avens, lives in cold arctic and montane habitats in the far north of America and Eurasia. | Ethan Rose via Wikipedia (CC BY 4.0)

Angiosperms, or flowering plants, first evolved during the Paleozoic.

But they did not become a dominant group until the Cenozoic Era.

Various flowering plant groups, including grasses, legumes, and fruit-bearing trees, formed during this period.

Angiosperms played a crucial role in shaping terrestrial ecosystems, providing habitats and food sources for various animal groups throughout the Cenozoic Era.


Butterflies | Shiebi AL via Unsplash

Insects, the most diverse group of animals on Earth, continued to diversify during the Cenozoic Era.

The evolution and radiation of flowering plants provided new ecological opportunities for insects.

Pollinating insects, such as bees, butterflies, and beetles, played a vital role in the reproduction and spread of flowering plants during the Cenozoic Era.


Reconstruction of Patriofelis ferox (creodont) | Dmitry Bogdanov via Wikipedia (CC BY 3.0)

The Cenozoic Era saw the rise of various carnivorous lineages that became the dominant predator species after the dinosaurs and other giant reptiles disappeared.

The early carnivores known as “creodonts” were widespread during the Paleogene Period.

However, as the Cenozoic progressed, creodonts declined in diversity and were eventually replaced by other carnivorous lineages.

The Oligocene and Miocene epochs saw the rise of modern carnivore lineages (order Carnivora) that are still present today. 

The various carnivore lineages of the Cenozoic also evolved many specialized adaptations that helped them adapt to specific habitats and lifestyles.

For instance, marine carnivores, such as seals, sea lions, and walruses, developed streamlined bodies, flippers, and other adaptations for swimming and hunting in marine environments.

Similarly, Saber-toothed carnivores that lived alongside large megafauna species like the mammoths during the Late Eocene to the

Pleistocene developed elongated, saber-like canine teeth adapted for stabbing and killing large prey.


Plesiadapis life restoration at MUSE – Science Museum in Trento | Matteo De Stefano via Wikipedia (CC BY-SA 3.0)

The Cenozoic Era saw the rise and evolution of primates.

This includes early primate forms like the Plesiadapiforms and later groups like adapids and omomyids.

These early primates played a crucial role in the subsequent diversification and development of primates, eventually leading to the emergence of monkeys, the big apes, and humans.


Original skull of Mrs. Ples, a female A. africanus | José Braga;Didier Descouens via Wikipedia (CC BY-SA 4.0)

The evolution of hominins, the lineage that includes humans and our closest relatives, took place during the Cenozoic Era.

Fossil evidence shows the emergence of early hominin species, such as Ardipithecus, Australopithecus, and eventually Homo, within the last  2 million years of the Cenozoic Era.

The Cenozoic Era witnessed a gradual increase in brain size, bipedal locomotion, and the development of stone tools among hominin species.

Extinction Events of the Cenozoic Era

There were at least three significant extinction events that shaped the course of life on Earth during the Cenozoic.

Although these events were not as catastrophic as the one that occurred at the end of the preceding Mesozoic Era, they still had significant impacts on various groups of organisms that were alive during the Cenozoic. 

The Eocene-Oligocene Extinction Event (34–33.5 Million Years Ago)

Global cooling | Gabriel Garcia Marengo via Unsplash

Changes in climatic conditions during the Eocene and Oligocene led to the decline and extinction of various marine and terrestrial species.

The causes of this extinction event are not yet fully understood, but factors such as global cooling, changes in ocean circulation patterns, and tectonic events probably played a role.

This event impacted many marine organisms, including coral reefs and foraminifera, as well as land-dwelling mammals.

The Middle Miocene Extinction Event (14.8 Million Years Ago)

Miocene Epoch | John Sibbick from Natural History Museum via Dinopedia

This extinction took place about 15 million years ago during the Miocene Epoch in Eurasia and North America.

It resulted in the extinction of several groups on these continents, such as the primitive rhinoceroses and large carnivores like bear dogs.

Climate change, shifting vegetation patterns, and competition between migrating mammalian groups may have contributed to this extinction event.

The Late Pleistocene Extinction Event (Approximately 50,000 to 12,000 Years Ago)

Illustration of two woolly mammoths fighting during an ice age. | Dottedhippo via Getty Images

Also known as the Quaternary extinction event, this event is the most recent major extinction event.

It resulted in the loss of most of Earth’s megafauna across several continents, especially North America, South America, Australia, and Eurasia.

Species such as mammoths, giant sloths, saber-toothed cats, and wooly rhinoceroses went extinct during this period.

This event has been attributed to climate change and the spread of human populations. 


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