The Contingencies of Molecular Biology

Life on earth uses left handed molecules, but it maybe could use right handed molecules. A biotic processes are known to create all kinds of ameno acids that are not found in life forms. Lab experiments show that you can build proteins out of alternative ameno acids and molecules with right handed carality. But this raises a really interesting question, if these features of molecular biology as we know it are really just contingencies. In other words, if it's just chemically how things happen to shake out when the first life forms were coming together, how do we know that other life forms did not arise?