Friday, April 2, 2010

Carbon chauvinism

Carbon chauvinism is a relatively new term meant to disparage the assumption that extraterrestrial life will resemble life on Earth.

Carbon chauvinism is applied to those who assume that the molecules responsible for the chemical processes of life must be constructed primarily from carbon. It suggests that human beings, as carbon-based life forms who have never encountered any life that has evolved outside the earth’s environment, may find it difficult to envision radically different biochemistries. The term was used as early as 1973, when Carl Sagan described it and other human chauvinisms that limit imagination of possible extraterrestrial life in his Cosmic Connection.
In a 1999 Reason magazine article discussing the theory of a fine-tuned universe, Kenneth Silber quotes astrophysicist Victor J. Stenger using the term:

"There is no good reason, says Stenger, to "assume that there's only one kind of life possible" - we know far too little about life in our own universe, let alone "other" universes, to reach such a conclusion. Stenger denounces as "carbon chauvinism" the assumption that life requires carbon; other chemical elements, such as silicon, can also form molecules of considerable complexity. Indeed, Stenger ventures, it is "molecular chauvinism" to assume that molecules are required at all; in a universe with different properties, atomic nuclei or other structures might assemble in totally unfamiliar ways".

Carbon has unique features that make it suitable for life possessed by no other element. Only two elements, carbon and silicon, can create molecules that are sufficiently large enough to carry biological information. However, carbon, unlike silicon has the important property that it can form chemical bonds with diverse types of other atoms and so create the chemical versatility needed to enable the chemical reactions needed for biology such as metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and diverse metals, such as iron, magnesium, and zinc. The only alternative, silicon, interacts with very few other types of atoms. Moreover, where it does it creates molecules that "are monotonous compared with the combinatorial universe of organic macromolecules."

Carbon-based life
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, and carbon is able to bond with all of these because of its four valence electrons. 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.
In cinematic and literary science fiction, a moment when man-made machines cross from nonliving to living, is often posited, this new form would be 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, less amenable to a complete source of life, 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:
a) Proteins, which are the building blocks from which the structures of living organisms are constructed (this includes almost all enzymes, which catalyse organic chemical reactions
b) Nucleic acids, which carry genetic information
c) Carbohydrates, which store energy in a form that can be used by living cells
d) Fats, which also store energy, but in a more concentrated form, and which may be stored for extended periods in the bodies of animals.
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