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Mercury, May/June 2006 Table of Contents

by Manfred Cuntz and Peter E. Williams

Life on Earth, which to date is the only type of life we know, is exclusively carbon based. Life has taken advantage of the favorable conditions on Earth—such as its abundance of water, its oxygen-rich atmosphere, and climatic stability—to take hold and flourish. But what makes carbon so special? Are there also other types of life possible? What is the definition of life anyhow? Alternative chemistries for life have been the subject of many science fiction novels, but are these credible speculations or simple fantasies? Alternative chemistries also have been the topic of investigation for numerous scientists in recent years who have been considering silicon as the forerunner in alternative biochemistries.

Before proceeding, we should settle on a proper definition of life. This turns out to be much harder than anticipated, considering that even the brightest scientists have not fully agreed on what "really" constitutes life. For our purpose, it is sufficient to adopt either the Property Definition or the Darwinian Definition of Life, as we will focus mostly on the importance of carbon in our discussion. The Darwinian Definition states that life is a system that can reproduce and evolve through natural selection. This definition usually also implies other features such as order and energy utilization, often referred to as "metabolism." It has also been posited that life—or at least "life as we know it"—necessitates the following fundamental requirements: (1) existence of elements to build up life, (2) an energy source to support life, and (3) a liquid medium for transporting life’s constituents. Therefore, the idea that life without carbon may exist is at least in principle possible.

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