Arsenic is a poison because it is chemically similar to phosphorus which is essential to all life forms. Researchers have discovered (in arsenic-rich Mono Lake) a bacterium that can use arsenic and phosphorus interchangeably, so that when it is given a diet of arsenic it can do without phosphorus entirely — even its DNA and ATP, which are as fundamental as life gets, use arsenic instead (so technically it is ATA instead of ATP).
This is said to have implications for alien life on other planets, but it suggests to me that there may be microscopic “alien” life forms already here on earth which we have never recognized because they have different chemical signatures.
UPDATE: Here is the full research paper.
Next question: the other essential elements for life are hydrogen, carbon, nitrogen, oxygen, and sulfur. The first 4 are obviously unique, but sulfur’s role in proteins (it allows amino acids to cross-link thus creating tertiary structure) can be replaced by selenium in some cases. Could there be life forms that can substitute selenium for all their sulfur?