Final answer:
The 21st amino acid referred to is selenocysteine, known for its distinctive incorporation into proteins and believed to have evolved in Earth's early reducing atmosphere before oxygenic photosynthesis, allowing for a wider array of chemical reactions.
Step-by-step explanation:
The 21st amino acid, often referred to in scientific and educational literature, is selenocysteine. Selenocysteine was discovered in 1986 and is sometimes referred to as the 21st amino acid because it is incorporated into proteins via a unique mechanism that involves the recoding of a stop codon. While all proteins are made from a standard set of 20 amino acids, selenocysteine is an exception as it is encoded in the genetic code in a distinctive manner and is considered non-standard.
The evolution of selenocysteine is presumed to have its origins when the Earth's atmosphere was still a reducing atmosphere, before the advent of oxygenic photosynthesis. During this time, the absence of oxygen allowed for complex chemical reactions that were conducive to the formation of more diverse chemical building blocks of life. This is supported by evidence that the earliest proteins likely contained fewer amino acids, expanding the genetic code as biological complexity increased over time.