Carbon forms the backbone of biology for all life on Earth. Complex molecules are made up of carbon bonded with other elements, especially oxygen, hydrogen and nitrogen. It is these elements that living organisms need, among others, and carbon is able to bond with all of these because of its four valence electrons. Since no life has been observed that is not carbon-based, it is sometimes assumed in astrobiology that life elsewhere in the universe will also be carbon-based. This assumption is referred to by critics as carbon chauvinism, as it may be possible for life to form that is not based on carbon, even though it has never been observed.
In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living is often posited, this new form being the first example of non-carbon-based life. Since the advent of the microprocessor in the late 1960s, these machines are often classed as computers (or computer-guided robots) and filed under "silicon-based life", even though the silicon backing matrix of these processors is not nearly as fundamental to their operation as carbon is for "wet life".
Characteristics of carbon as a basis for life
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 and complex molecules.
There are not many other elements which appear to be even promising candidates for supporting life-like behavior, but the most frequent alternative suggested 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. However, its compounds are generally highly stable and do not support the ability to readily re-combine in different permutations in a manner that would plausibly support life-like processes.
This speculation of a life based on the chemistry of silicon is clearly distinct from "silicon-based life" in the above sense of artificial intelligence based on electronic processes utilizing silicon integrated circuits.
Key carbon-based molecules in the life processes
The most notable groups of chemicals used in the processes of living organisms include:
- Proteins, which are the building blocks from which the structures of living organisms are constructed
- Nucleic acids, which carry genetic information
- Enzymes, which catalyse organic chemical reactions
- Carbohydrates, which store energy in a form that can be used by living cells
- Fats, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.
- "Encyclopedia of Astrobiology, Astronomy & Spaceflight". Retrieved 2006-03-14.