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( Alternative biochemistry)
Alternative biochemistry is the speculative biochemistry of alien life forms that differ radically from those on Earth. It includes biochemistries that use elements other than carbon to construct primary cellular structures and/or use solvents besides water. Theories about extraterrestrial life based on alternative biochemistries are common in science fiction. Perhaps the least unusual "alternative" biochemistry would be one with differing chirality of its biomolecules. In known Earth-based life, amino acids are almost universally of the L form and sugars are of the D form. Molecules of opposite chirality have identical chemical properties to their mirrored forms, so life that used D amino acids and/or L sugars may be possible. Scientists have speculated about the pros and cons of using atoms other than carbon to form the molecular structures necessary for life, but no one has proposed a theory employing such atoms to form all the molecular machinery necessary for life. Since humans are carbon-based beings and have never encountered any life that has evolved outside the earth’s environment, excluding the possibility of all other elements may be considered carbon chauvinism. The most commonly proposed basis for an alternative biochemical system is the silicon atom, since silicon has many chemical properties similar to carbon and is in the same periodic table group, the carbon group.
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