Three domains of life

It is believed that life on Earth began in a very simple form, and the constant struggle of early living beings to survive and adapt gave rise to progressively more complex forms — this ongoing process is evolution, and it is responsible for the enormous diversity of life forms we see today.

In 1990, the microbiologist Carl Woese proposed that all life be grouped into three domains: Archaea, Bacteria and Eukarya. A domain is a taxonomic unit broader even than a kingdom. Bacteria and Archaea both have prokaryotic cells (no true nucleus), while Eukarya have eukaryotic cells (a membrane-bound nucleus). All three domains nevertheless share a distinctive kind of ribosomal RNA (rRNA), which is part of the molecular evidence used to define them. Archaea are especially known for surviving extreme conditions — very high temperature, high salinity, high acidity and so on — and although Bacteria are also prokaryotic, they differ from Archaea in the s …