Mammals Take the Field

When the skies cleared after the impact, the Cretaceous period was over. The giants were gone. The long‑necked sauropods that converted whole forests into muscle and bone were extinct, along with Tyrannosaurus rex and the other big meat‑eating dinosaurs. Giant marine reptiles and spiral‑shelled ammonites disappeared from the seas. After a few thousand years the plankton in the oceans rebounded, as did the plants on land. But the ecosystems of the early Tertiary period were bottom‑heavy and top‑light.

Once again, a mass extinction had cleared the way for a new burst of evolution: the age of dinosaurs was followed by the age of mammals. “The death of the dinosaurs allowed mammals to evolve into many ecological niches that were not available for them,” says Ward. “It was the removal of the dinosaurs through mass extinction that allowed so many lineages of mammals to come about through the evolutionary process. In that sense, it’s really a good thing. There would not be humans here but for that mass extinction.”

Mammals suffered along with the rest of life during the K‑T extinction, as an estimated two‑thirds of all mammal species disappeared. But the ones that survived inherited the earth. Within 15 million years after the K‑T extinction, they had evolved into all 20 orders of the living placental mammals, along with many other orders that are now extinct. At first these new mammals remained small. Hoofed mammals as big as raccoons browsed on low leaves and were hunted by predators the size of weasels. But within a few million years they evolved out of their old niches into the ones that dinosaurs had dominated. Giant relatives of rhinoceroses and elephants browsed the leaves of shrubs and trees. Ancestors of today’s cats and dogs stalked the herbivores; some mammals became scavengers, stripping corpses and crushing bones. Primates raced through the trees, using their color vision to choose the ripest fruits. Bats evolved from shrewlike tree dwellers into hundreds of flying species, some searching for fruit, others using echolocation to catch insects and frogs. Whales and the ancestors of today’s dugongs and manatees colonized the oceans.

While mammals have remained the dominant land vertebrates over the past 65 million years, they’ve also had to weather their own evolutionary shocks. Today’s climate bears little resemblance to the one in which the age began. Between 65 and 55 million years ago, volcanoes spewed carbon dioxide into the atmosphere, gradually warming the planet until palm trees could grow north of the Arctic Circle and Canada looked more like Costa Rica does today. Wyoming was home to lemur‑like primates, jumping through jungle canopies.

Earth would never be so warm again. For the past 50 million years the world’s average temperature has been dropping, with occasional hiccups of warmth along the way. We may have the Himalayas partly to blame. When India collided with Asia, the crash created the craggy mountain range. The rains that fell on these fresh new slopes carried dissolved carbon dioxide; the gas reacted with the rock and formed compounds that were carried away by streams and rivers to be buried in the sea. The Himalayas may have withdrawn so much carbon dioxide from the atmosphere that the climate gradually cooled. At the same time, the collision also pushed up the Tibetan plateau, just north of the Himalayas. This enormous dome began rerouting weather patterns throughout southern Asia. Air that passed over the plateau warmed and rose from the ground, pulling in moist air from the oceans to take its place. This pattern created the monsoons of India and Bangladesh and brought more rain to the Himalayas. The removal of carbon dioxide sped up even more, making the planet’s greenhouse effect even weaker.

Changes were taking place in the oceans as well. During the Cretaceous, Antarctica was much farther north than it is today, and it was so warm that dinosaurs and trees thrived on its coasts. Eventually, though, the continent pulled away from Australia and moved south, becoming more and more isolated, until it reached the South Pole. Permanent ice began building up on Antarctica, reflecting sunlight back into space and cooling the atmosphere.

As winters grew colder, the tropical forests of North America disintegrated. Mammals that could not survive without them, such as primates, disappeared as well. The jungle gave way to broad‑leaved trees much like those alive today, interspersed with scrublands. As carbon dioxide levels continued to fall, new kinds of plants evolved that could absorb the gas more efficiently. Among these new plants were grasses, which formed the first major grasslands about 8 million years ago.

Grass is loaded with tough cellulose and sprinkled with bits of glasslike silica, making it far harder to eat than the soft fruits and leaves that were abundant when the planet was warmer. Some mammals, such as horses, managed to survive on this diet thanks to their high‑crowned teeth, which could grind down the plants. The ancestors of cows and camels were also prepared for the tough grasses because they had altered their digestive system to let bacteria help them break down the tough plants. But many lineages couldn’t adapt to the cooling climate and the shifting vegetation and went extinct.

Geography also drove some mammals into oblivion. Before 7 million years ago, North and South America were separated by ocean, but continental drift gradually drew them together. At first islands formed between them, and then, 3 million years ago, the Isthmus of Panama emerged and joined the continents. The mammals from each landmass could spread into new territory to compete with species they had never encountered before.

During the 60 million years that South America had been isolated from other continents, it had developed an ecosystem unlike any other on Earth. The top predators were coyote‑sized opossums and giant, fast‑running flightless birds. When the continents were linked, a few species of South American mammals such as opossums, sloths, monkeys, and armadillos moved north. But the mammals that moved from north to south were much more successful. The opossum coyotes of South America were driven extinct along with all the other marsupial carnivores, their place taken by cats and dogs. The hoofed mammals of South America were replaced by horses and deer.

“The 65‑million‑year history of mammals, after the K‑T extinction event, is marked by invasions,” says Novacek. “There are lots of mammals moving from one continent to another. You can almost visualize these as armies marching across the continents. Some of these mammals that enter into new continents show a high degree of success in dominance soon after the invasion. But it’s very difficult to really understand why invasions work this way. We really don’t know. It may be that many of the animals that tend to invade are more mobile, or more flexible with environmental change, and that confers some kind of competitive advantage.”

Around the same time as the Great American Interchange, the global climate began to swing into a new pattern. The glaciers at the poles spread toward the equator and then pulled back in a cycle of ice ages. This cycle probably is controlled by the changing orbit of Earth around the sun. Earth moves closer and farther away from the sun in a 100,000‑year cycle. At the same time, its tilted axis draws out a circle like a spinning top every 26,000 years. And every 41,000 years the angle of the tilt changes too, between 21 and 25 degrees (today we are at 23 degrees). These cycles combine to change the amount of sunlight that the earth receives over the course of the year.

Nicholas Shackleton, a paleooceanographer at the University of Cambridge, has studied the effects of these wobblings in ancient Antarctic ice and in seafloor mud. He has found that when the wobbles bring less sunlight to Earth, carbon dioxide levels in the atmosphere fall. Researchers don’t know for sure how the former can drive the latter; it’s possible that a drop in sunlight alters the way plants and plankton grow. In any case, with less carbon dioxide in the atmosphere, the planet cools. Each summer less polar ice melts, so the glaciers get larger. Eventually they march thousands of miles toward the tropics, until something triggers them to retreat–perhaps an increase in carbon dioxide in the atmosphere. As the glaciers retreat, forests can expand again. We live in one of these pleasant interglacials, which has lasted 11,000 years and may last for a few thousand more.

A snapshot of North America, taken just on the verge of the arrival of humans at the end of the last Ice Age, would have shown a landscape crowded with giant mammals. Saber‑toothed cats, jaguars, cheetahs, short‑faced bears, dire wolves, and other carnivores now hunted for the grazing and browsing mammals. These included mammoths grazing on the grasslands, and mastodons wandering the forests and swamps, as well as camels, horses, rhinos, and ground sloths. The North American mammals were part of a global boom of biodiversity. Studies of the fossil record suggest that the number of species has been climbing over the past 100 million years. One reason for this rise, researchers suspect, is that Pangaea has been disintegrating into smaller and smaller pieces. On a united supercontinent, there were fewer barriers to animals and plants, so versatile species could invade the territories of more specialized ones and outcompete them. As Pangaea split apart, it created more isolated habitats where more species could thrive, and more coastlines where marine life could evolve. By the time modern humans evolved around 100,000 years ago, the world may have harbored the most diversity of life in its entire history. It is a rich legacy to inherit, and a terrible one to squander.