To the best of our ability to tell, everything on Earth shares a few common features. It encodes information in DNA using a set of four bases, A, T, C, and G. Sets of three bases are used to code for a single amino acid, and most organisms use a set of 20 amino acids to build proteins. These features appear everywhere, from plants and animals to bacteria and viruses, suggesting that they appeared in the last common ancestor of life on Earth.

This raises a question that comes up a lot in evolutionary studies: are these features used because they’re in some way efficient, or did we end up stuck with them as a result of some historic accident?

A team of California-based researchers has been building an argument that it’s an accident. And it’s doing so by expanding life beyond the limitations inherited from its common ancestor. After having expanded the genetic alphabet to six letters, the team has now engineered a bacterial strain that uses the extra letters to put an unnatural amino acid into proteins.