. To the philosophical the  earth is eternal, while the human race — presumptive  keeper of the world's history — is a mere speck  in the rich stream of life. It is known that  nothing upon Earth is forever; geography, climate, and  plant and animal life are all subject to radical  change. On five occasions in the past, catastrophic  natural events have caused mass extinctions on  Earth. But today humans stand alone, in dubious  distinction, among Earth's species: possesses the ability to destroy  entire species at will, to trigger the sixth  extinction in the history of life. In , Richard Leakey and Roger Lewin  consider how the grand sprawl of human life is  inexorably wreaking havoc around the world. The  authors of and , unimpeachable  authorities on the human fossil record, turn their  attention to the most uncharted anthropological territory  of all: the future, and man's role in defining it.  According to Leakey and Lewin, man and his  surrounding species are end products of history and  chance. Now, however, humans have the unique  opportunity to recognize their influence on the global  ecosystem, and consciously steer the outcome in order  to avoid triggering an unimaginable upheaval.