All living things fit naturally into five kingdoms: animals, plants, fungi, protists, and monerans.

Robert Whittaker’s five-kingdom system was a major improvement over older classifications. Animals, plants, and fungi were separated from one another, while most organisms without a cell nucleus were placed in Monera and many other mostly microscopic organisms in Protista.

By the late twentieth century, this became a standard textbook map of life.

The problem was that looking similar does not always mean sharing a close evolutionary history. Researchers increasingly compared DNA and RNA sequences instead. Because related organisms inherit those sequences from common ancestors, the similarities and differences can be used to reconstruct a family tree of life. This approach is called molecular phylogeny.

The old kingdom Monera split dramatically. Organisms without nuclei that had all been treated as bacteria turned out to contain two deeply different lineages: Bacteria and Archaea.

Protista also came apart. It had served as a convenient home for many organisms that were neither animals, plants, nor fungi, but molecular family trees showed that these organisms belonged to many different branches rather than one natural group.

Modern classification therefore uses three domains—Bacteria, Archaea, and Eukarya—along with multiple major groups within the eukaryotes. There is no single universally required number of kingdoms today. The important change is that the five old kingdoms do not map cleanly onto the evolutionary tree.