Possibilities for life based on other chain-molecules are surveyed on this wikipedia page: - http://en.wikipedia.org/wiki/Alternative_biochemistry - The science section is followed by a comprehensive list of examples from science fiction. - It seems that carbon and oxygen based life is the most likely, simply because these elements are the most common and most stable throughout the universe. - There are also speculations about non-molecular life forms, such as based on electromagnetic connections within plasma. - The possibilities for imaginative fiction are endless.