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Descended from more archaic relatives, the early true mammals were mainly small insect-eating creatures adapted to nighttime activity. They ranged in size from scarcely bigger than a bumblebee to squirrel-sized, keeping out of the way of the predatory dinosaurs. They acquired certain traits that would characterize mammals ever afterward: limbs positioned under the body, an enlarged brain, a more complex physiology, milk-producing glands, and a diverse array of teeth -- incisors, canines, premolars, and molars. Already present were the ancestors of the three major mammalian groups that exist today -- monotremes (platypus and spiny anteater), which lay eggs externally; marsupials (kangaroos, opossums), which carry their young in a pouch; and placental mammals (humans, cows, horses), which retain the fetus internally during long gestation period. In the early Cenozoic era, after the dinosaurs became extinct, the number and diversity of mammals exploded. In just 10 million years -- a brief flash of time by geologic standards -- about 130 genera (groups of related species) had evolved, encompassing some 4,000 species. These included the first fully aquatic mammals (whales) and flying mammals (bats), as well as rodents and primates. This sudden expansion of species diversity into new ways of life is known as adaptive radiation. One way it occurs is in response to events that free up previously occupied environmental zones and roles, making way for many new species that adapt to these vacant living spaces. The extinction of the dinosaurs was one such major event, eliminating a once-dominant group of competitors while some mammals survived. But the mammals did not simply step into ecological roles vacated by the dinosaurs. It took several million years for the mammals to evolve even moderately large body sizes, and the world they inherited was a different place from the one the dinosaurs had dominated. There were new environmental habitats and new food resources to exploit. By the end of the Cretaceous, flowering plants had become dominant, providing food for burgeoning populations of insects, which in turn became another high-quality food source for the mammals, along with fruits and berries. New kinds of forests appeared, offering novel habitats for what would become tree-dwelling mammals -- primates, which first appeared about 50 million years ago, and eventually, some 45 million years later, upright-walking hominids, including us. The astonishing diversity of mammalian species today stems in part from the continuing breakup of the continents that began some 200 million years ago and sent different landmasses moving apart. Australia and South America were isolated from other continents during much of the Tertiary, and marsupial mammals thrived and diversified there, while placental mammals took over similar roles on the other continents. The story of the mammals is only one example of adaptive radiation, a process that has happened again and again, at scales both large and small, in evolutionary history. 
Researchers measuring levels of carbon 13 - a byproduct of photosynthesis - in deep-sea core samples going back 205 million years have identified a sharp rise in oxygen levels about 50 million years ago.
The increase in oxygen levels gave mammals the evolutionary boost they needed to dominate the planet, says researcher Paul Falkowski, from Rutgers University. Because photosynthesis produces oxygen and leaves carbon 13 behind, the presence of carbon 13 in the fossil samples allows precise measurements of how much oxygen was in the atmosphere at any given time. Falkowski said that when dinosaurs flourished the oxygen level was around 10 percent, but increased to 17 percent 50 million years ago and then to 23 percent by 40 million years ago.
While there were placental mammals on Earth at the time of the great extinction of dinosaurs about 65 million years ago, they were small, limited creatures. The dinosaur extinction event itself did little to further the mammalian domination of the planet. It was the subsequent spreading of shallow seas, the increase in plant life - and photosynthesis - in addition to the consequent increase in oxygen content that gave the mammals the boost they needed, according to Falkowski's study in the journal Science. Over the last 10 million years, the oxygen level has decreased to around 21 percent, which many scientists believe was caused by great fires that burned over the earth about 10 million years ago. These fires reduced the number of trees and, therefore, the amount of photosynthesis and oxygen. The geography of the world changed dramatically during the time when animals and plants were evolving. The major continental land masses were initially fused together into one giant continent named Pangaea during the Paleozoic era.  In the Mesozoic, Pangaea gradually broke up into the present-day continents, which have been moving apart from each other, by continental drift, ever since. This idea of continental drift was first based on the remarkably close fit between the coastlines of major continents, most notably the west coast of Africa with the east coast of South America. It is now supported by measurements, which show that the continents on either side of the Atlantic Ocean are still moving apart from one another, at the rate of several centimeters per year. Continental drift was actually a little more complicated, with the North American plate drifting around in the Pacific Ocean for quite a long time. A large chunk of the North American plate was recently found in Argentina, left there after the two continents bumped into each other then moved apart. The separation of the great land mass into different continents allowed biological evolution to take quite different paths in different parts of the world. And the formation of oceanic islands, often by volcanic activity, produced many more isolated areas where evolution could experiment with different forms.
During the Cenozoic era, there was a gradual lowering of temperatures as well as the gradual establishment of different climatic zones of the earth -the tropics, the temperate zones and the cool climates of the higher latitudes. The culmination of the cooling trend was the Pleistocene epoch, or Great Ice Age, of the last 1.8 million years. During this time vast expanses of North America and Eurasia were periodically covered with enormous continental glaciers. These glaciers advanced during the four ice ages (glacial periods) and retreated during the three interglacials. We are now living in the fourth interglacial stage. During the glacial periods the sea level became much lower because so much water was converted to ice. Consequently land bridges, especially the Bering land bridge across the Bering Sea joining Asia with North America, became available for animal migrations. |
