The rise and fall of Monera, the kingdom that once held all bacteria

Nature · Published 3 October 2026 Swipe more like this →

In short: For decades, biology textbooks taught that life was divided into five kingdoms, with Monera serving as the catch-all home for every single-celled organism lacking a nucleus. Today, that kingdom has been dismantled, replaced by a more complex understanding of life that separates bacteria from the ancient, distinct domain of archaea.

The concept of Monera began in 1866 when Ernst Haeckel proposed a three-kingdom system. He envisioned Monera as a collection of structureless, homogeneous organisms, though his initial classification was messy, grouping together true bacteria with various eukaryotic organisms like amoebas. For a long time, microscopic life was difficult to categorize; Antonie van Leeuwenhoek, who discovered bacteria in 1675, simply labeled them animalcules and tucked them into the animal kingdom.

By the 1920s, the importance of distinguishing between organisms with and without a nucleus became clear. Édouard Chatton formalized the divide between prokaryotes and eukaryotes in 1925, and the kingdom Monera eventually became the standard home for all prokaryotes in Robert Whittaker’s widely adopted five-kingdom system of 1969. This classification grouped bacteria and blue-green algae—later renamed cyanobacteria—together based on their shared lack of a nucleus and their method of reproduction.

The kingdom’s collapse began in 1977, when Carl Woese and George Fox published findings showing that archaea were not closely related to bacteria. This discovery, which initially sparked significant controversy, revealed that Monera was paraphyletic, meaning it failed to include all descendants of a common ancestor. Modern taxonomy now recognizes three domains: Bacteria, Archaea, and Eukarya. While some researchers, such as Thomas Cavalier-Smith, have argued against this division, the scientific consensus has shifted away from the Monera model, proving that even the most fundamental biological categories are subject to revision as our tools for tracing evolutionary history improve.

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