Cenozoic

• 1. Introduction

  ›

  The Cenozoic Era began with an asteroid strike. Sixty-six million years ago, a large celestial body known as the Chicxulub impactor slammed into Earth, ending the reign of the non-avian dinosaurs and triggering one of the most consequential extinction events in planetary history. That catastrophe marks the opening chapter of the era we are living in right now.

  The Cenozoic is Earth's current geological era. It encompasses the last 66 million years of the planet's history, and it belongs to the Phanerozoic Eon, the most recent and best-documented stretch of geologic time. The name itself comes from Ancient Greek: kainós, meaning 'new', and zoe, meaning 'life'. British geologist John Phillips proposed it in 1840, originally spelling it Kainozoic.

  What follows the extinction is not a story of loss. It is a story of radical reinvention. Mammals, birds, insects, and flowering plants moved into the spaces left behind by the dinosaurs. Continents drifted into the shapes we recognize on maps today. Ice ages came and went. And one primate lineage, beginning in Africa, eventually gave rise to a species capable of altering the planet itself.

  How did mammals come to dominate two hemispheres? What drove the climate from tropical warmth to repeated glaciations? And what does it mean that we are still inside this era, living within an epoch that began only 11,700 years ago?

• 2. After the Impact

  ›

  The Paleocene Epoch began immediately after the K-Pg extinction event, stretching from 66 million to 56 million years ago. The first thing that epoch inherited was devastation. Large herbivores had been wiped out, and dense but species-poor forests spread into the gaps they left behind.

  The oceans told a similar story of transition. Sharks became dominant there, filling the predator role that large reptiles had previously held. On land, archaic mammals called creodonts, carnivores unrelated to any living Carnivora, spread across the recovering world.

  One striking geographic fact defined this epoch: India was still an island. Afro-Eurasia was separated from Asia by the Tethys Sea. The Americas were split by open water where the Isthmus of Panama had not yet formed. Modern placental mammals originated during this time, though the continents that would carry them were still far apart.

  The Paleocene ran warm. Jungles spread toward the poles as temperatures climbed. Then, between 56 and 33.9 million years ago, the Eocene Epoch carried that warming to an extreme. Carbon dioxide levels reached approximately 1,400 ppm. The average temperature hit 30 degrees Celsius, with almost no gradient between the equator and the poles.

  Dense Eocene forests kept animals small, but the fauna was already diversifying. Early primates, early whales, and early horses all lived during this time. At the top of the food chain sat giant birds such as Paracrax. Then a geological shift changed everything: the Antarctic Circumpolar Current formed between Australia and Antarctica, disrupting ocean circulation worldwide and triggering global cooling. Mammals responded by growing larger. Whales became nearly fully aquatic. The mammal Andrewsarchus sat atop the food chain. By the Late Eocene, seasons returned, savannas expanded, and grasses evolved for the first time.

• 3. Grasslands and New Orders

  ›

  The Oligocene, running from 33.9 to 23.03 million years ago, was defined by grass. As grasslands expanded across the cooling planet, entirely new lineages found their footing. The first elephants appeared. The first cats and dogs emerged. Marsupials diversified into new forms.

  The Neogene picked up where the Oligocene left off. Spanning from 23.03 to 2.58 million years ago, it contains two epochs: the Miocene and the Pliocene. During the Miocene, grasses spread further and claimed large portions of the world at the expense of forests. Kelp forests evolved in the oceans, and with them came new species including sea otters. Apes diversified into 30 species. Perissodactyla, the group that includes horses and rhinos, thrived and branched into many varieties.

  By the end of the Miocene, 95% of modern seed plant families were already present. The Tethys Sea finally closed entirely with the formation of the Arabian Peninsula, leaving behind only remnants: the Black, Red, Mediterranean, and Caspian Seas. That closure increased aridity across vast regions.

  The Pliocene, from 5.333 to 2.58 million years ago, brought some of the most dramatic geographic shifts of the era. The Mediterranean Sea dried up for several million years after sea level drops disconnected it from the Atlantic and evaporation outpaced river inflow. In Africa, Australopithecus evolved, beginning the human branch of the primate family. The Isthmus of Panama formed, linking North and South America. Animals crossed in both directions during what is called the great American interchange, with severe consequences for local ecologies. Savannas spread. Indian monsoons began. The Sahara started forming. The world's geography had essentially reached its modern configuration.

• 4. Ice, Humans, and Extinction

  ›

  The Quaternary Period, officially recognized by the International Commission on Stratigraphy in June 2009, is the shortest geological period in the Phanerozoic Eon. It spans from 2.58 million years ago to the present.

  The Pleistocene, which lasted from 2.58 million to 11,700 years ago, was an age of ice. At least four separate glaciation periods sent ice caps advancing as far south as 40 degrees North in mountainous areas. Africa responded differently, undergoing a desiccation trend that produced the Sahara, Namib, and Kalahari deserts. A hundred thousand years ago, one of the worst droughts in Africa's history ended, and primitive humans began expanding.

  The fauna of the Pleistocene reads like a catalog of the spectacular: mammoths, giant ground sloths, dire wolves, and sabre-toothed cats all lived alongside Homo sapiens. Neanderthals were also present. As the epoch drew to a close, a mass extinction swept through the world's megafauna. Every continent felt the loss, though Africa was affected less severely, which is why it still holds large animals like hippos.

  The Holocene began 11,700 years ago and continues today. All of recorded human history falls within it. Human activity is linked to a mass extinction that began roughly 10,000 years ago, though extinctions only began being formally recorded after the Industrial Revolution. Over 322 species have been documented as extinct due to human activity since that time, though the true rate may encompass as many as 500 vertebrate species alone, with the majority of losses occurring after 1900. This ongoing event is sometimes called the Sixth Extinction.

• 5. Drifting Continents

  ›

  Australia-New Guinea split from the supercontinent Pangea during the early Cretaceous and spent the Cenozoic drifting northward until it eventually collided with Southeast Asia. Antarctica moved into position over the South Pole. The Atlantic Ocean widened steadily. Around 2.8 million years ago, South America finally joined North America through the Isthmus of Panama.

  India's journey had even larger consequences. Its collision with Asia raised the Himalayas, and those mountains did more than reshape the landscape. As the upraised rocks eroded, they reacted with gases in the atmosphere, drawing down carbon dioxide over millions of years and contributing directly to the long-term cooling trend of the Cenozoic.

  Arabia's collision with Eurasia closed the Tethys Ocean and built the Zagros Mountains. That same closure reshaped drainage patterns across Africa. The break-up of Gondwana in the Late Cretaceous and into the Cenozoic redirected the courses of major rivers, including the Congo, Niger, Nile, Orange, Limpopo, and Zambezi.

  The Quaternary glaciations added their own geographic signatures. The Great Lakes, Hudson Bay, and the Baltic Sea are all products of ice-age activity during this period, forming long after the Pliocene configurations had settled into place.

• 6. The Age of Mammals

  ›

  Early in the Cenozoic, following the extinction of the non-avian dinosaurs, the dominant land animals in the Northern Hemisphere were eutherians, the placental mammals. In the Southern Hemisphere, metatherians, the marsupials, held sway. Marsupials are now mainly restricted to Australia and, to a lesser extent, South America.

  Some of the most striking early predators were not mammals at all. Gastornithid birds dominated parts of the early Cenozoic landscape. Terrestrial crocodylians like Pristichampsus were formidable. In the oceans, large sharks such as Otodus were apex predators. Primitive large mammal groups including uintatheres, mesonychians, and pantodonts rounded out the fauna.

  As forests gave way to open grasslands and the climate cooled, mammals diversified into an extraordinary range of forms. The Cenozoic roster includes chalicotheres, creodonts, entelodonts, mastodons, mammoths, three-toed horses, and the giant rhinoceros Paraceratherium. South America produced strange groups entirely its own: pyrotheres, which were vaguely elephant-like, and borhyaenids, dog-like marsupial relatives. Australia harbored both monotremes and marsupials in isolation.

  Snakes also diversified enormously during this era. Their expansion tracked the rise of rodents, which became their primary prey. Some flightless birds grew larger than humans; these are sometimes called terror birds, and they were active predators. The Cenozoic is as much an age of savannas, co-dependent flowering plants and insects, and birds as it is an age of mammals. Grasses shaped the evolutionary paths of every creature that fed on them.

  Calcareous nannoplankton in the oceans experienced rapid speciation but declining diversity during the Eocene and Neogene. Diatoms took a different path, diversifying sharply through the Eocene, especially at high latitudes, with a concentrated burst at the Eocene-Oligocene boundary. A second major pulse of diatom diversification followed during the Middle and Late Miocene.