On Earth, all known living things have a carbon-based structure and system. 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 necessary structures. However, as Carl Sagan argued, it is very difficult to be certain whether a statement that applies to all life on Earth will turn out to apply to all life throughout the universe.[7] Sagan used the term "carbon chauvinism" for such an assumption.[8] Carl Sagan regarded silicon and germanium as conceivable alternatives to carbon;[8] but, on the other hand, he noted that carbon does seem more chemically versatile and is more abundant in the cosmos.[9]