Students of molecular biology are taught early on that proteins are built using an "alphabet" of just 20 amino acids. Many excellent web-sites already exist to describe this standard alphabet (example).
However, by itself, this information is misleading! While it is true that most life uses a standard alphabet of just 20 amino acids for the purposes of genetic (de)coding, life uses hundreds, probably thousands of amino acids in metabolism (source)... . Understanding the difference between these two statements took an entire book chapter to explain (see here), but the short version is this:
Around 3 billion years ago, the life-form(s) from which today's biosphere evolved were using this set of 20 amino acids as encoded "meanings" for the genetic code-words (codons) of their genetic material.
Abundant evidence from multiple sources shows us that many other amino acids were possible, both at the start of life and during life's early evolution. Recent research has uncovered a 21st and 22nd amino acid used in the genetic coding of some lineages (source) - these have been added since the time of the standard genetic code. Furthermore, the emerging frontier of "synthetic biology" has nowplaced more than 70 "unatural" (human engineered) amino acids into the genetic codes of organisms as diverse as bacteria, fungi and mammals (source).
Relatively little research has asked the question "why did life select one particular alphabet for use in standard genetic coding?" That is one of the key areas of research for our group at present.
For a recent video presentation of our research into the evolution of the standard amino acid alphabet, see here. Associated Publications: