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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". 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.
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