The Three Domains of Life

For much of the twentieth century, the broadest classification of life divided organisms into two great categories based on cell structure: prokaryotes (cells without a true, membrane-bound nucleus, such as bacteria) and eukaryotes (cells with a true nucleus and membrane-bound organelles, such as plants, animals, fungi and protists). This two-way split seemed to capture the most fundamental division in the living world.

In 1977, the American microbiologist Carl Woese, studying differences in ribosomal RNA sequences — a molecule common to all cellular life and therefore ideal for comparing very distantly related organisms — discovered that what had been lumped together as a single group of "prokaryotes" was not one natural group at all. A distinctive set of prokaryote-like organisms, many of them found in extreme environments such as hot springs, salt lakes and the guts of animals, turned out to be as genetically different from ordinary bacteria as bacteria are from eukaryotes. Woese proposed elevating this group to its own domain, which he called the Archaea (from a Greek word meaning "ancient").

This led to the three-domain system of classification, now widely accepted as the broadest, most fundamental level at which all cellular life is organised:

• Domain Bacteria — the "true" bacteria, prokaryotic organisms found in essentially every habitat on Earth, including many that live inside or alongside other organisms.
• Domain Archaea — prokaryotic in basic cell structure but biochemically and genetically distinct from Bacteria, notably including many organisms adapted to extreme conditions (extremophiles), though Archaea are now known to be widespread in ordinary environments too.
• Domain Eukarya — all organisms whose cells possess a true, membrane-bound nucleus, encompassing protists, fungi, plants and animals — the domain that includes every organism most students encounter directly. …