Concept preview

What you will be able to do

1. 01Name the three domains of life and the four kingdoms of Eukarya.
2. 02Explain why Archaea were separated from Bacteria.
3. 03Describe how molecular data (Woese and rRNA) revised the tree of life.
4. 04Trace the shift from two kingdoms to five kingdoms to three domains.

At the very top of the classification hierarchy sit the domains, the broadest and most fundamental divisions of all life. But biology took a long time to reach them. For most of history, living things were sorted into just two kingdoms: plants and animals. Anything green and rooted was a plant; anything that moved about and ate was an animal. This simple scheme worked well enough for the large, familiar organisms Linnaeus knew, but it began to crack as microscopes revealed a hidden world of bacteria, protists and fungi that fit neither box comfortably. A mushroom does not photosynthesise like a plant, yet it does not move and hunt like an animal; a bacterium is clearly neither. The two-kingdom system, built almost entirely on appearances, simply could not cope with the true diversity of life once the microscopic world came into view.