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Carbon forms the key component for almost all known naturally occurring life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen, and carbon is able to bond with all of these because of its four valence electrons. Carbon is abundant on earth. It is also lightweight and relatively small in size, making it easier for enzymes to manipulate carbon molecules. It is often assumed in astrobiology that if life exists somewhere else in the universe, it will also be carbon based. This assumption is referred to by critics as carbon chauvinism.
The two most important characteristics of carbon as a basis for the chemistry of life, are that it has four valence bonds and that the energy required to make or break a bond is just at an appropriate level for building molecules which are not only stable, but also reactive. The fact that carbon atoms bond readily to other carbon atoms allows for the building of arbitrarily long complex molecules and polymers.
There are not many other elements which even appear to be promising candidates for supporting life - for example, processes such as metabolism - but the most frequently suggested alternative is silicon. This is in the same group in the Periodic Table of elements and therefore also has four valence bonds. It also bonds to itself, but generally in the form of crystal lattices rather than long chains. Silicon compounds are generally stable but do not support the ability to readily re-combine in different permutations in a manner that would plausibly support lifelike processes.
The most notable groups of chemicals used in the processes of living organisms include:
Silicon has been a theme of non-carbon-based-life since it also has 4 bonding sites and is just below carbon on the periodic table of the elements. This means silicon is very similar to carbon in its chemical characteristics. In cinematic and literary science fiction, when man-made machines cross from nonliving to living, this new form would be an example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as "silicon-based life". Another example of "silicon-based life" is the episode "The Devil in the Dark" from Star Trek: The Original Series, where a living rock creature's biochemistry is based on silicon.