Life on Earth is driven by energy. Autotrophs take it from solar radiation and heterotrophs take it from autotrophs. Energy captured slowly by photosynthesis is stored up, and as denser reservoirs of energy have come into being over the course of Earth's history, heterotrophs that could use more energy evolved to exploit them.Homo sapiens is such a heterotroph; indeed, the ability to use energy extra-somatically (outside the body) enables human beings to use far more energy than any other heterotroph that has ever evolved. The control of fire and the exploitation of fossil fuels have made it possible forHomo sapiens to release, in a short time, vast amounts of energy that accumulated long before the species appeared. By using extrasomatic energy to modify more and more of its environment to suit human needs, the human population effectively expanded its resource base so that for long periods it has exceeded contemporary requirements. This allowed an expansion of population similar to that of species introduced into extremely propitious new habitats, such as rabbits in Australia or Japanese beetles in the United States. The world's present population of over 5.5 billion is sustained and continues to grow through the use of extrasomatic energy. But the exhaustion of fossil fuels, which supply three quarters of this energy, is not far off, and no other energy source is abundant and cheap enough to take their place. A collapse of the earth's human population cannot be more than a few years away. If there are survivors, they will not be able to carry on the cultural traditions of civilization, which require abundant, cheap energy. It is unlikely, however, that the species itself can long persist without the energy whose exploitation is so much a part of itsmodus vivendi. The human species may be seen as having evolved in the service of entropy, and it cannot be expected to outlast the dense accumulations of energy that have helped define its niche. Human beings like to believe they are in control of their destiny, but when the history of life on Earth is seen in perspective, the evolution ofHomo sapiens is merely a transient episode that acts to redress the planet's energy balance.