The committee found no compelling reason to limit the environment for life to water as a solvent, even if life is constrained to use carbon as the scaffolding element for most of its biomolecules. In water, a wide array of molecular structures are conceivable that could (in principle) support life but be so different from those for life on Earth that they would be overlooked by unsophisticated life-detection tools. Evidence suggests that Darwinian processes require water, or a solvent like water, if they are supported by organic biopolymers (such as DNA). Although macromolecules that use silicon are known, few thoughts suggest how they might have emerged spontaneously to support a biosphere.
Many of the definitions of life include the phrase undergoes Darwinian evolution. The implication is that phenotypic changes and adaptation are necessary to exploit unstable environmental conditions, to function optimally in the environment, and to provide a mechanism to increase biological complexity. The canonical characteristics of life are inherent capacities to adapt to changing environmental conditions and to increase in complexity by multiple mechanisms, particularly by interactions with other living organisms.
One of the apparent generalizations that can be drawn from knowledge of Earth life is that lateral gene transfer is an ancient and efficient mechanism for rapidly creating diversity and complexity. The unity of biochemistry among all Earth’s organisms emphasizes the ability of organisms to interact with other organisms to form coevolving communities, to acquire and transmit new genes, to use old genes in new ways, to exploit new habitats, and, most important, to evolve mechanisms to help to control their own evolution. Those characteristics are likely to be present in extraterrestrial life even if it has had a separate origin and a very different unified biochemistry from that of Earth life.
Because we have only one example of biomolecular structures that solve problems posed by life and because the human mind finds it difficult to create ideas truly different from what it already knows, it is difficult for us to imagine how life might look in environments very different from what we find on Earth. Recognizing the challenges in mitigating that difficulty, the committee chose instead to embrace it. In constructing its outlook, it exploited a strategy that began by characterizing the terran life that humankind has known well, first because of its macroscopic visibility and then through microscopic observation that began in earnest 4 centuries ago.
As the next step in its strategic process, the committee assembled a set of observations about life that is considered exotic when compared with human-like life. The committee asked, Can we identify environments on Earth where Darwinian processes exploiting human-like biochemistry cannot exploit available thermodynamic disequilibria? The answer to that question is an only slightly qualified no. It appears that wherever the thermodynamic minimum for life is met on Earth and water is present, life is found. Furthermore, the life that is found appears to be descendent from an ancestral life form that also served as the ancestor of humankind (we might not have recognized it if its ancestry were otherwise), and it exploits fundamentally human-like biochemistry. The committee reviewed evidence about abiotic processes that manipulate organic material in a planetary environment. It asked whether the molecules that we see in contemporary terran life might be understood as the inevitable consequences of abiotic reactivity.
The committee then surveyed the inventory of environments in the solar system and asked which ones might be suitable for life of the terran type. The survey made clear that most locales in the solar system are at thermodynamic disequilibrium, an absolute requirement for chemical life, and that many locales at thermodynamic disequilibrium also have solvents in liquid form and environments where the covalent bonds between carbon and other lighter elements are stable. Those are weaker requirements for life, but the three together appear, perhaps simplistically, to be sufficient for life. The committee asked whether it could conceive of a biochemistry adapted to those exotic environments, much as human-like biochemistry is adapted to terran environments. Because few detailed hypotheses are available, the committee reviewed what is known, or might be speculated, and considered research directions that might expand or constrain understanding about the possibility of life in such exotic environments. Finally, the committee considered more exotic solutions to problems that must be solved to create the emergent properties that we agree characterize life.
The committee found that using thermal and chemical energy to maintain thermodynamic disequilibria, covalent bonds between carbon atoms, water as the liquid, and DNA as a molecular system to support Darwinian evolution is not the only way to create phenomena that would be recognized as life. Indeed, the emerging field of synthetic biology has already provided laboratory examples of alternative chemical structures that support genetics, catalysis, and