The universal nature of biochemistry
Norman R. Pace
University of Colorado, Boulder, CO 80309-0347
PNAS, January 30, 2001, vol. 98, no. 3, 805-808
People have long speculated about the possibility of life in settings other than Earth. Only in the past few centuries, however,have we been able to conceive of the specific nature of such settings:other planets around our own sun and solar systems similar toour own elsewhere in the physical universe. Speculation on thenature of life elsewhere often has paid little heed to constraintsimposed by the nature of biochemistry, however. A century of fancifulscience fiction has resulted not only in social enthusiasm forthe quest for extraterrestrial life, but also in fanciful notionsof the chemical and physical forms that life can take, what thenature of life can be. Since the time of the Viking missions toMars, in the mid-1970s, our view of life's diversity on Earthhas expanded significantly, and we have a better understandingof the extreme conditions that limit life. Consequently, our searchfor extant life elsewhere in the solar system can now be conductedwith broader perspective thanbefore.
How can life be detected regardless of its nature and origin? Considering the recent spectacular advances in observationalastronomy, it seems likely that the first sign of life elsewherewill be the spectroscopic detection of co-occurring nonequilibriumgases, for instance oxygen and methane, in the atmosphere of aplanet around some distant star. Co-occurrence of such gases wouldindicate that they are replenished, perhaps most readily explainedby the influence of life (1). By observation of oxygen andmethane, Earth could possibly be seen as a home for life evenfrom distant galaxies. Other potential habitats for life in thissolar system, such as Mars and Europa, however, are not so obvious.The search for life on those bodies will be conducted at the levelof analytical chemistry. As we undertake the detection of extraterrestriallife, it is instructive to try to put constraints on what thenature of life can be. These constraints, the requirements forlife, tell us where and how to look for life, and the forms thatit cantake.