might be using the black-body radiation from hot sulfides for photosynthesis.4 Newly isolated microorganisms have extended the upper temperature for growth to 121°C and extended the pH limit to below 0. One hyperthermophilic microorganism lacks consensus sequences (nucleotide base sequences in the 16S rRNA gene that are universal in known organisms) in its 16S rRNA and, incidentally, is parasitic on another archaean species.5
To address the second issue, researchers begin by assuming carbon-based life. The key arguments for carbon-based life are the ubiquity of organic (carbon-containing) compounds in the universe and the ability of carbon to form stable compounds with many elements, thus creating the wide variety of structural, catalytic, and informational macromolecules (very large molecules, such as DNA and proteins) that make up Earth life. But how versatile and adequate is the carbon-based life model for environmental conditions that either have not been adequately explored on Earth or extend beyond the bounds found on Earth? Are there alternative carbon-based biochemistries that would allow organisms to exist under more extreme conditions than Earth life can? Embedded in that question are two others: What are the limits of evolutionary innovations in carbon-based life? How do environmental characteristics or extrinsic factors, such as hydrostatic pressure and solute concentrations, affect the limits?
Powerful new molecular methods allow the construction of novel enzymes and, eventually, novel organisms. A viral genome has already been constructed from synthetic oligonucleotides.6 Those “directed evolution” methods have the potential to explore whether it is possible to construct life with novel physiologies, including new metabolic pathways that exploit energy sources not used by extant Earth organisms—such energy sources as ultraviolet radiation, gravity, and thermal gradients. Furthermore, exploration of deep subsurface environments has uncovered microbial communities potentially decoupled from photosynthetic reactions that occur at Earth’s surface. Those communities use magmatic degassing, radiolysis of water, or serpentinization reactions (involving the aqueous alteration of mantle material) to drive their metabolism. The ecosystems persist in environments that have remained essentially unchanged for millions or billions of years and that overlap with conditions favorable for abiotic organic synthesis. Do organisms in those ecosystems contain relict biochemistries that have eluded evolutionary pressures?
Thus, two questions are associated with understanding the limits of carbon-based life: What are the limits of extant Earth life? What are the limits of carbon-based life? With respect to the second question, one of the implications of using Earth-based life as a point of comparison is the need to understand the full range of habitat conditions that can support carbon-based life, including conditions not found on Earth. The tendency is to look for Earth organisms that might be best suited to live under the extreme conditions found on other planets, but that assumes that an extraterrestrial organism would have the same physiological characteristics of an Earth organism if the environmental conditions were the same in the two places. In thinking that way, we assume that life originated in the other place and then evolved physiologies to take advantage of the available habitat conditions. But any planet or moon that has or has had environmental conditions that could support an Earth-like organism might never have had the conditions necessary for a separate origin of life and could be sterile. That may be particularly true for icy planets with liquid water even if they have the cache of chemicals necessary to support life. Could life originate de novo, or would it have to be seeded from a neighboring planet or moon that during its early history had more suitable conditions for spawning life? How many types of environments can lead to the origin of life?
In the case of Earth, various models provide clues to geophysical conditions that may have favored early organic and biochemical stages leading to life. Some models rely on subsurface hydrothermal settings because they can provide all the chemical precursors and catalysts essential to generating complex carbon chemistry. Other models exploit alternative settings, such as meteoritic input, reduced atmospheres, or freshwater ponds. The origin of life is likely to involve multiple environmental conditions that span spatial and temporal dimensions. However, we cannot answer the general question of whether there could be multiple settings for creating different carbon-based life forms. Would a separate origin of life under conditions different from the ones that produced life on Earth create a carbon-based life form capable of different evolutionary innovations, or do rules of organic chemistry limit carbon-based life to the physiological diversity represented by extant Earth life?
Earth may be just one of many models of planets that can evolve complex life. We do not know whether it is even practical or logical to assume that planets that exist outside our perception of a habitable zone could harbor life, particularly life that we know nothing about. Our practical search for extraterrestrial life is focused on water-rich planets and moons because of the possibility that they can support Earth-like life. That does not preclude