Norman R. Pace
Department of Molecular, Cellular, and Developmental Biology
University of Colorado
The search for life beyond Earth requires understanding the basic chemical requirements for life and the fundamental molecular structures upon which life is likely to be based. Life on Earth is a self-replicating, evolving system based on the element carbon, and life elsewhere is highly likely to be based on carbon as well. Life anywhere will be based on macromolecules (polymers) because of the high degree of specificity required in carrying out reactions in self-sustaining living organisms and the demand for storage of large amounts of information required for living organisms. Because of their ubiquity and simplicity, amino acids, purines, and pyrimidines are likely to be the universal monomeric foundation of polymers upon which life is based. Differences in evolutionary systems will lie at the higher-order levels, the structures of the large molecules assembled from the simple units, and the mechanisms through which they are assembled and in which they participate. Techniques for detecting life beyond Earth could take advantage of the expected universality of the foundations of biology, but must account for likely variations associated with diverse evolutionary pathways.
Humans 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 like our own elsewhere in the physical universe. Speculation on the nature of life elsewhere generally has paid little heed to constraints imposed by the nature of biochemistry, however. A century of fanciful science fiction writings has resulted not only in social enthusiasm for the quest for extraterrestrial life, but also in fanciful notions of the chemical and physical forms that life can take, what the nature of life can be.
At the current stage of the exploration of life in the solar system we are, for the first time, confronting realistically the simple question: How to detect life regardless of its nature and origin? As we undertake detection of extraterrestrial life on nearby bodies in the solar system such as Mars and Europa, or terrestrial life on outbound or returning missions, it is instructive to try to put constraints on what the nature of life can be. The search for life elsewhere than Earth will be conducted at the chemical level, so we need to try to understand what are the basic chemical requirements for life and what are the forms that life can take.
An early question that we must pose, indeed one which in the last analysis requires definition, is: What is life? Most would agree that self-replication is the fundamental goal of the life process. Most would also agree that the definition of life should include the capacity for evolution. Indeed the mechanism of evolution, natural selection, is a consequence of the competing drives for self-replication that are manifest in all organisms. The definition based on those processes, then, would be that life is any self-replicating, evolving system.
The processes of self-replication and evolution are not reliably detectable, however, so we need to incorporate into the definition of life information on the nature of the chemical reactions that provide the basis for self-replication and, consequently, evolution. Based on the expected properties of molecules likely to be needed to replicate an evolving entity, life that we encounter anywhere in the universe, and can recognize, is likely to be composed of organic chemicals that follow the same general principles as our own terrestrial kind of organic-based life. The operational definition of life then becomes the following: Life is a self-replicating, evolving system based on organic chemistry. This is what we need to search for.