I am working out a future scenario in which humans begin to discover alien life; I want their discoveries to be plausible.
For example, I've frequently heard that alien life will most likely be carbon-based; because carbon can form so many more molecules than any other element.
The question is: What parts of molecular biology are commonly believed to be the most likely to occur if life evolved from scratch elsewhere in the Universe? I put it this way because I am interested in both what we probably have in common, but also what could be different but plausible.
As an aside (and not a separate question) I am interested in any likely ramifications of these molecular-biology similarities and differences on subsequent evolution: Such as if they would lead to oxygen-breathing aliens, or aliens adapted to much hotter or colder conditions, terrestrial vs. aquatic aliens, and so on.