Scientists who study life desire an unambiguous definition, and they adopt two complementary approaches in their efforts to distinguish that which is alive from that which is not. Many apply the "top-down" approach. They scrutinise all manner of living and fossil organisms to identify characteristics of the most primitive entities that are, or were, alive. This strategy is limited, however, because all known life forms, whether living or fossil, are based on sophisticated cells containing DNA and proteins. Any definition of life based on top-down research is correspondingly myopic.

By contrast, a small army of investigators pursues the so-called "bottom-up" approach. They devise laboratory experiments to mimic the chemistry of Earth's ancient environments. Eventually, the bottom-up goal is to create a living chemical system in the laboratory from scratch - an effort that might clarify the transition from non-life to life. Such research leads to an amusing range of opinions regarding what is alive, because each scientist tends to define life in terms of his or her own chosen speciality - cell membranes, metabolic cycles, RNA, viruses and even silicon-based artificial intelligence all have their passionate proponents.
“Eventually, the bottom-up goal is to create a living chemical system in the laboratory from scratch”

Into this mix, philosophers and theologians inject a more abstract view and speculate on the full range of phenomena that might be said to be alive - robotic life, computer life, even a self-aware internet. Such debates can at times sound like a science fiction convention, but defining life is no idle exercise. After all, if NASA is to look for life on other worlds, a clear definition is essential for planning future missions.

Scientists excel at many things, but compromise is not always one of them. Nevertheless, Gerald Joyce of The Scripps Research Institute, serving on a NASA exobiology panel, has tried to achieve this. He proposed one of the descriptions in Lahav's list as a "working definition" for life in the context of space exploration: "Life is a self-sustained chemical system capable of undergoing Darwinian evolution."

This succinct and widely cited metric combines three distinct characteristics. First, any form of life must be a chemical system. Accordingly, computer programs, robots or other electronic entities are not alive. Life also grows and sustains itself by gathering energy and atoms from its surroundings - the essence of metabolism. Finally, living entities must display variation. Natural selection of more fit individuals inevitably leads to evolution and the emergence of more complex entities. This NASA-inspired definition is probably as general, useful and concise as any we are likely to come up with - at least until we discover more about what is actually out there.