The most common other proposed basis is silicon, since silicon has many similar chemical properties to carbon. Silicon has a number of handicaps as a carbon analogue, however. Silanes (hydrogen-silicon compounds analogous to the alkane hydrocarbons) are highly reactive with water, and long-chain silanes spontaneously decompose. Molecules incorporating Si-O-Si bonds (known collectively as silicones) instead of Si-Si bonds are much more stable; ordinary sand is one such example. However, silicon dioxide (the analogue of carbon dioxide) is a non-soluble solid at the temperature range where liquid water is possible making it difficult for silicon to be introduced into water-based biochemical systems even if the necessary range of biochemical molecules could be constructed out of it.
In general, complex long-chain silicone-based molecules are still more unstable than their carbon counterparts.
Finally, of the molecules identified in interstellar space as of 1998, 84 are based on carbon and 8 are based on silicon. Moreover, of the eight Si-based compounds, four also include carbon. This suggests a greater abundance of complex carbon compounds throuhout the cosmos, providing less of a foundation upon which to build silicon-based biologies. It is possible that silicon compounds may be biologically useful under certain exotic environmental conditions, however, either in conjunction with carbon or in a role less directly analogous to carbon. A simple real-world example is the silicate skeletal structure of diatoms.
Nitrogen and phosphorus also offer possibilities as the basis for biochemical molecules. Phosphorus can form long chain molecules on its own like carbon, and so potentially could be built up into complex macromolecules, but phosphorus is fairly reactive. In combination with nitrogen, however, it can form much more stable phosphorus-nitrogen (P-N) bonds; compounds containing these can form a wide range of molecules, including rings.
Earth's atmosphere is approximately 80% nitrogen, but this would probably not be much use to a P-N lifeform since molecular nitrogen (N2) is very inert and energetically expensive to "fix" (certain Earth plants such as legumes can fix nitrogen using symbiotic anaerobic bacteria contained within their root nodules). A nitrogen dioxide (NO2) or ammonia (NH3) atmosphere would be more useful (Nitrogen actually forms a number of oxides with oxygen (N2O, N2O4), and all would be present in a nitrogen dioxide-rich atmosphere).
In a nitrogen dioxide atmosphere, phosphorus-nitrogen-based plant analogues could absorb nitrogen dioxide from the atmosphere and phosphorus from the ground. The nitrogen dioxide would be reduced, P-N sugar analogues being produced in the process, and waste oxygen would be released into the atmosphere. P-N animal analogues would consume the plants, use atmospheric oxygen to metabolize the P-N sugar analogues, exhaling nitrogen dioxide and depositing phosphorus (or phosphorus rich material) as solid waste.
In an ammonia atmosphere, P-N plants would absorb ammonia from the atmosphere and phosphorus from the ground, then oxidize the ammonia to produce P-N sugars and release hydrogen waste. P-N animals are now the reducers, breathing in hydrogen and converting the P-N sugars to ammonia and phosphorus. This is the opposite pattern of oxidation and reduction from a nitrogen dioxide world, and indeed from the known biochemistry of Earth; it would be analogous to Earth's atmospheric carbon supply being in the form of methane instead of carbon dioxide.
Debate continues as several aspects of a P-N cycle biology would be energy deficient.
Unfortunately, nitrogen and phosphorus are not likely to be found in the ratios and quantity required in the real universe. Carbon, being preferentially formed during nuclear fusion, is more abundant and is more likely to end up in a preferred location.
Chlorine is sometimes proposed as a biological alternative to oxygen, either in carbon-based biologies or hypothetical non-carbon-based ones. Chlorine is much less abundant than oxygen in the universe, however, and so it is unlikely that a planet will be able to form which has a large enough concentration of chlorine available on its surface to form the basis of a biochemistry. Chlorine will instead likely be bound up in the form of salts and other inert compounds.
Sulfur is also able to form long-chain molecules, but suffers from the same high reactivity problems that phosphorus and silanes do.
Another approach to alternative biochemistry is the idea that the basis of evolution need not be natural but could instead be deliberately created by an intelligent designer, opening up a wider range of possibilities.
For example, it is possible in principle to construct a robot or a system of robots that is capable of replicating itself from raw ores and natural energy sources without any external direction or assistance (a "clanking replicator"). Such a machine system could be considered alive, in that it is capable of evolution through mutational errors in its inherited design patterns, but is in no way required to be composed of carbon-based compounds. The most detailed proposition for machine life made so far considered self-replicating lunar factories, which were composed primarily of refined metal and cast basalt since the Earth's moon is extremely carbon-poor.
Related to macroscopic machine life is the concept of self-replicating nanotechnology, sometimes referred to as "grey goo" when it is operating without programmed limitations. Nanotechnology, like larger scale machines, could potentially be made of non-carbon-containing materials (including any of the other elements already mentioned earlier). Both diamondoid and carbon nanotubes are also commonly proposed materials for use in nanomachines, both of which are forms of carbon and so may not ideally serve as a counterexample to carbon chauvinism. But neither of these forms of carbon is used by life as it is currently known, and furthermore it is often proposed that carbon-based nanotechnological devices will operate without the water environment that life as it is currently known requires, so they might likewise be considered an "alternative basis" for life. Any of the other life-bases mentioned previously could also serve as the basis for an artificial life form.
These beings, although they could not have evolved without the help of carbon-based (or other) beings, being irreducibly complex, could potentially out-compete them or destroy their creators if robustly enough designed. They would then become the inheritors of the world their creators evolved on, and be indistinguishable to most outsiders from native beings. This is the Intelligent Design form of creationism - the proposal that an intelligent creator designed human beings and deliberately placed us here.
Scientifically, the relevance of this possibility is that high intelligence in a transition species may be the means by which an otherwise-impractical basis for life gets started, after which it can continue to evolve by more natural means. Ethically, this could be considered as an argument for carbon chauvinism, or at least for teaching it to any artificial life forms that human beings may create.
Silicon biochemisty
Nitrogen/Phosphorus biochemistry
Other exotic biochemical elements
Artificial life
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