It's all just chemistry, and the chemistry is extremely conservative all the way back to the first eukaryotes. Once our kind of life figures out a way to do something it's very uncommon that the fundamental pathways change - not by any means impossible.
Cells that have novel things happen to them nearly always die. Others become cancer, which is a career-limiting move for multicellular life at least. So the process of changes to basic chemistry of life is very slow. That's why omnivores can eat almost anything and be fine with it - imagine if different branches of plants and animals had really novel things in them, like a critter with nerve gas for a circulatory fluid.
There's been some discussion of mirror bacteria lately, which have opposite chirality in all their molecules, which would be substantially problematic since we really don't have defenses against that kind of thing. That just shows the degree to which you'd have to rejigger biochemistry to get a truly whole new thing going on.
Cells that have novel things happen to them nearly always die. Others become cancer, which is a career-limiting move for multicellular life at least. So the process of changes to basic chemistry of life is very slow. That's why omnivores can eat almost anything and be fine with it - imagine if different branches of plants and animals had really novel things in them, like a critter with nerve gas for a circulatory fluid.
There's been some discussion of mirror bacteria lately, which have opposite chirality in all their molecules, which would be substantially problematic since we really don't have defenses against that kind of thing. That just shows the degree to which you'd have to rejigger biochemistry to get a truly whole new thing going on.