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Unit 1: Many Planets, One Earth // Section 9: The Age of Mammals


The first mammals on Earth were rodent-sized animals that evolved in the shadow of dinosaurs during the Jurassic and Triassic periods. After the K-T boundary extinction eliminated dinosaurs as predators and competitors, mammals radiated widely. Most of the modern mammal orders, from bats to large types like primates and whales, appeared within about 10 million years after dinosaurs died out. Because mammals could maintain a relatively constant internal temperature in hot or cold environments, they were able to adapt to temperature changes more readily than cold-blooded animals like reptiles, amphibians, and fish. This characteristic helped them to populate a wide range of environments.

Another important ecological shift was the spread of angiosperms (flowering plants), which diversified and became the dominant form of land plants. Unlike earlier plants like ferns and conifers, angiosperms' seeds were enclosed within a structure (the flower) that protected developing embryos. Their flower petals and fruits, which grew from plants' fertilized ovaries, attracted animals, birds, and insects that helped plants spread by redistributing pollen and seeds. These advantages enabled angiosperms to spread into more diverse habitats than earlier types of plants.

Earth's climate continued to fluctuate during the Cenozoic, posing challenges for these new life forms. After an abrupt warming about 55 million years ago, the planet entered a pronounced cooling phase that continued up to the modern era. One major cause was the ongoing breakup of Gondwanaland, a supercontinent that contained most of the land masses in today's southern hemisphere, including Africa, South America, Australia, and India (Fig. 20).

Gondwanaland

Figure 20. Gondwanaland
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Source: © United States Geological Survey.

Once these fragments started to separate about 160 million years ago, ocean currents formed around Antarctica. Water trapped in these currents circulated around the pole and became colder and colder. As a result, Antarctica cooled and developed a permanent ice cover, which in turn cooled global atmospheric and ocean temperatures. Climates became dryer, with grasslands and arid habitat spreading into many regions that previously had been forested.

Continued cooling through the Oligocene and Miocene eras, from about 35 million to 5 million years ago, culminated in our planet's most recent ice age: a series of glacial advances and retreats during the Pleistocene era, starting about 3.2 million years ago (Fig. 21). During the last glacial maximum, about 20,000 years ago, ice sheets covered most of Canada and extended into what is now New England and the upper Midwestern states.

Cenozoic cooling

Figure 21. Cenozoic cooling
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Source: © Global Warming Art. GNU License/www.globalwarmingart.com.

Human evolution occurred roughly in parallel with the modern ice age and was markedly influenced by geologic and climate factors. Early hominids (members of the biological family of the great apes) radiated from earlier apes in Africa between 5 and 8 million years ago. Humans' closest ancestor, Australopithecus, was shorter than modern man and is thought to have spent much of its time living in trees. The human genus, Homo, which evolved about 2.5 million years ago, had a larger brain, used hand tools, and ate a diet heavier in meat than Australopithecus. In sum, Homo was better adapted for life on the ground in a cooler, drier climate where forests were contracting and grasslands were expanding.

By 1.9 million years ago, Homo erectus had migrated from Africa to China and Eurasia, perhaps driven partly by climate shifts and resulting changes to local environments. Homo sapiens, the modern human species, is believed to have evolved in Africa about 200,000 years ago. Homo sapiens gradually migrated outward from Africa, following dry land migration routes that were exposed as sea levels fell during glacial expansions. By about 40,000 years ago Homo sapiens had settled Europe, and around 10,000 years ago man reached North America. Today, archaeologists, anthropologists, and geneticists are working to develop more precise maps and histories of the human migration out of Africa, using mitochondrial DNA (maternally inherited genetic material) to assess when various areas were settled.

Early in their history, humans found ways to manipulate and affect their environment. Mass extinctions of large mammals, such as mammoths and saber-toothed cats, occurred in North and South America, Europe, and Australia roughly when humans arrived in these areas. Some researchers believe that over-hunting, alone or in combination with climate change, may have been the cause. After humans depleted wildlife, they went on to domesticate animals, clear forests, and develop agriculture, with steadily expanding impacts on their surroundings that are addressed in units 5 through 12 of this text.

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