Cenozoic Era (65 mya-present)
"Age of the Mammals"
Cenozoic Era in a Nutshell
The Cenozoic Era, which began about 65 million years ago and continues into the present, is the third documented era in the history of Earth. The current locations of the continents and their modern-day inhabitants, including humans, can be traced to this period.
The era began on a big down note, catching the tail end of the Cretaceous-Paleogene extinction event at the close of the Cretaceous Period that wiped out the remaining non-avian dinosaurs.
1. Paleogene Period (65-23 million years ago)
During the Paleogene the continents drifted farther apart, heading toward their modern positions. Oceans widened the gaps, Europe severed its last ties with North America, and Australia and Antarctica finally parted ways.
The cooling and drying trend began in earnest following a sudden temperature spike about 55 million years ago. Sea surface temperatures rose between 9 and 14 degrees Fahrenheit (5 and 8 degrees Celsius) over a period of a few thousand years, killing off numerous single-celled marine organisms called foraminifera, along with some other invertebrates. This event also profoundly affected northern forests, previously full of deciduous hardwoods with sequoias and pines. The new, more humid subtropical conditions nurtured abundant palms and guavas. Land mammals responded in kind, radiating and diversifying into many new forms.
The climate significantly cooled and dried, sea levels continued to drop from late Cretaceous levels, draining most interior seaways.
At the dawn of the Paleogene—the beginning of the Cenozoic era—dinosaurs, pterosaurs, and giant marine reptiles were conspicuously absent from the face of the Earth. Rodent-size (and perhaps larger) mammals emerged from the shadow of the night, suddenly free to fill the void. Over the next 42 million years, they grew in size, number, and diversity. As the period came to a close, life-forms still common today filled the seas, dominated the land, and had taken to the air.
As the climate cooled and dried following the warming, forests gave way to open woodlands and grasslands in the northern hemisphere and started to support thundering herds of grazing mammals.
Fish filled in the oceans, food to fuel sharks, which were fast ruling the waters in the absence of the giant mosasaurs and plesiosaurs of the Cretaceous. Squid and other soft-bodied cephalopods replaced their shelled relatives, which once filled the middle rung on the food chain. Sea snails and bivalves that were similar to modern forms lurked on the ocean bottom. New types of foraminifera and sea urchins replaced those that had died off in earlier mass extinctions.
But the biggest development in the seas was the appearance of whales in the mid- to late Paleogene. The huge animals evolved from land mammals that took to the seas.
Meanwhile, smaller reptiles that survived the Cretaceous, such as turtles, snakes, crocodiles, and lizards, basked in the tropical warmth along the coasts. Birds, the holdouts of the dinosaur age, diversified and flourished in the skies. But the rapidly evolving mammals stole the show. Starting from a fairly humble position 65 million years ago, primates, horses, bats, pigs, cats, and dogs had all evolved by the close of the period, 23 million years ago.
2. Neogene Period (23-2.6 million years ago)
From afar, Earth looked much as it does today when the Neogene period began. But looks are deceiving. Mountains rose, and sea levels fell. The climate cooled and dried. Species were forced to adapt or die.
Though close to where they are today, the continents began the Neogene by crashing into each other. India continued its slow-moving collision with Asia, which had already started the giant push-up of the Himalaya that continues today. Italy pushed into Europe, giving rise to the Alps. Spain butted France, and the Pyrenees rose. Faulting, stretching, thinning, and lifting created parts of the Rocky, Sierra Nevada, and Cascade Mountains in North America.
The high mountains altered air circulation and weather patterns, contributing to the drier and cooler climate.
The Arctic ice cap grew and thickened. Snow and ice fell on the high mountains, locking up water far from the oceans. Sea levels plummeted, exposing land bridges between Africa and Eurasia and between Eurasia and North America. Eventually, South America moved north and merged with North America, forming the Isthmus of Panama.
The continental connections gave animals that had evolved in isolation access to new lands. Elephants and apes wandered from Africa to Eurasia. Rabbits, pigs, saber-toothed cats, and rhinos went to Africa. Elephants and rhinos continued across the Bering Strait to North America. Horses went the other way. Ground sloths migrated from South America to North America; raccoons scurried south. Even rodents may have hopped Pacific islands en route to Australia from Southeast Asia.
As the climate changed, many of the great forests that carpeted the continents from shore to shore and from Pole to Pole slowly gave way to grasslands, a habitat more suited to the cooler and drier weather. But that hardiness came with less nutrition. Plant-eating animals had to adapt in order to survive. Horses evolved stronger, enamel-protected teeth and flourished. So too did ruminants such as bison, camels, sheep, and giraffes, whose compartmentalized stomachs are well adapted to digesting grass. Many of the grazers were quick and roamed in herds—new tricks for survival out in the open. Their predators were also forced to adapt.
In the oceans, a new type of large brown algae, called kelp, latched onto rocks and corals in cool shallow waters, establishing a new habitat favored by sea otters and dugongs, a marine mammal related to the elephant. Sharks grew and dominated the seas once again. Megalodon, the biggest shark of all, was nearly 50 feet (15 meters) long.
Meanwhile on land, Asian and African apes diverged and then, several million years later, hominins split from their closest African ape ancestors, the chimpanzees. Adapted to two-footed walking, early hominins dropped out of the trees and started to carry food and tools in their hands. These new species were poised to alter the planet unlike any other in the centuries to come.
3. Quaternary Period (2.6 million years ago to the present)
At the start of the Quaternary, the continents were just about where they are today, slowing inching here and there as the forces of plate tectonics push and tug them about. Sea levels rose rapidly about 10,000 years ago, and the continents achieved their present-day outline.
Glaciers advance from the Poles and then retreat, carving and molding the land with each pulse. Sea levels fall and rise with each period of freezing and thawing. Some mammals get massive, grow furry coats, and then disappear. Humans evolve to their modern form, traipse around the globe, and make a mark on just about every Earth system, including the climate. But throughout the period, the planet has wobbled on its path around the sun. The slight shifts cause ice ages to come and go. By 800,000 years ago, a cyclical pattern had emerged: Ice ages last about 100,000 years followed by warmer interglacials of 10,000 to 15,000 years each. The last ice age ended about 10,000 years ago.
When the temperatures drop, ice sheets spread from the Poles and cover much of North America and Europe, parts of Asia and South America, and all of Antarctica. With so much water locked up as ice, sea levels fall. Land bridges form between the continents like the currently submerged connector across the Bering Strait between Asia and North America. The land bridges allow animals and humans to migrate from one landmass to another.
During warm spells, the ice retreats and exposes reshaped mountains striped with new rivers draining to giant basins like today's Great Lakes. Plants and animals that sought warmth and comfort toward the Equator return to the higher latitudes. In fact, each shift alters global winds and ocean currents that in turn alter patterns of precipitation and aridity around the world.
Since the outset of the Quaternary, whales and sharks have ruled the seas, topping a food chain with otters, seals, dugongs, fish, squid, crustaceans, urchins, and microscopic plankton filling in the descending rungs.
On land, the chilliest stretches of the Quaternary saw mammals like mammoths, rhinos, bison, and oxen grow massive and don shaggy coats of hair. They fed on small shrubs and grasses that grew at the ever moving edges of the ice sheets. About 10,000 years ago, the climate began to warm, and most of these so-called megafauna went extinct. Only a handful of smaller, though still impressively large, representatives remain, such as Africa's elephants, rhinoceroses, and hippopotamuses. Scientists are uncertain whether the warming climate is to blame for the extinction at the end of the last ice age. At the time, modern humans were rapidly spreading around the globe and some studies link the disappearance of the big mammals with the arrival of humans and their hunting ways.
In fact, the Quaternary is often considered the "Age of Humans." Homo erectusappeared in Africa at the start of the period, and as time marched on the hominid line evolved bigger brains and higher intelligence. The first modern humans evolved in Africa about 190,000 years ago and dispersed to Europe and Asia and then on to Australia and the Americas. Along the way the species has altered the composition of life in the seas, on land, and in the air—and now, scientists believe, we're causing the planet to warm.