There's a thoughtful argument on the possible structures and mechanisms on life, a subject on which you might recall I have some interest. Kelly suggests that the form of life may be inevitable.

[Besides DNA] there may indeed be other molecules in the universe capable of such autonomous self-replicating feats as life demands, but they may not be able to spontaneously self-organize the first time... The alternative minority view is that unrepeatability of variety is true in the short term, but that at the large metascale, at the scale of billions of years, the constraints of physics dictate certain possibilities. Chemistry may decide how many ways molecules can spontaneously self-organize into life.