FIGURE 3.12 Evolution of Caribbean land bridge through plate tectonic processes over the past 30-million-years. This changed ocean circulation and may have led to the accumulation of continental ice sheets.

changes operate. These questions not only bear on past rates and patterns of evolution and extinction, as revealed in the fossil record, but also illuminate impending extinctions that humans may be able to prevent. Some predictions suggest that half of all living species may disappear within the next 50 to 100 years. This rapid disappearance is supposed to be biased toward loss of terrestrial species, especially those of the tropics. However, there has been little work on present rain forest diversity or on natural rates of extinction in rain forest areas during the past 50,000 years. The little research that has been done produced conflicting results. Conservation programs can only be improved by learning more about the environments and biology of these areas during the recent past.

Specifically, we need to find out the climatic and vegetational histories of the Amazon Basin and of similar rain forests in western Africa and Asia. The Amazon rain forest shrank and broke up into pockets of verdant growth separated by stretches of savannah as the building northern glaciers repeatedly removed much of the oceanic and atmospheric water from free circulation. Some researchers claim that there was a 90 percent reduction in rain forest area during this period of Northern Hemisphere glaciation, with the inevitable accompanying extinctions.

The modern world is impoverished in terrestrial species of mammals, the class to which humans belong. Only 12,000 years ago the American landmasses were populated by beavers the size of black

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