because both water and carbon have very specific, near-unique properties at both the molecular and macro level that make them unique candidates for forming the

because both water and carbon have very specific, near-unique properties at both the molecular and macro level that make them unique candidates for forming the basis of life. Silicon and ammonia share some of these properties, so they potentially are competitors, but what we observe not just on earth, but as gas in interstellar space, is that complex molecules are all carbon-based.