It has been proposed that the early Earth hosted multiple origins of life, some of which produced chemical variations on life as we know it. Some argue that these alternative life forms could have become extinct, either by being out-competed by other forms of life, or they might have become one with the present day life via mechanisms like
lateral gene transfer. Other proposed candidates for a shadow biosphere include organisms using different suites of amino acids in their proteins or different molecular units (e.g., bases or sugars) in their nucleic acids, having a
chirality opposite of ours, using some of the
nonstandard amino acids, or using
arsenic instead of
phosphorus, having a different
genetic code, or even another kind of chemical for its genetic material that are not nucleic acids (
DNA nor
RNA) chains or
biopolymers. It is suggested that if an alternate form of microbial life on Earth is discovered, the odds are good that life is also common elsewhere in the universe. ==Criticism==