The Retirement of the Monera Kingdom: Advances in Prokaryotic Classification

Introduction

The term Monera has long been a staple in biological classification, but recent advancements in our understanding of the diversity and evolutionary relationships among prokaryotic organisms have prompted a reevaluation of this designation.

Why the Monera Kingdom Was Removed

The Monera kingdom, which included all prokaryotes such as bacteria and archaea, was a simple yet inadequate classification. Here are the key reasons for its removal:

Genetic Differences

Molecular studies, particularly those involving ribosomal RNA sequences, revealed substantial genetic differences between bacteria and archaea. These differences indicate that they represent distinct lineages in the tree of life, leading to the acknowledgment that they are not closely related.

Evolutionary Relationships

The introduction of the three-domain system by Carl Woese in the late 20th century provided a much more accurate representation of evolutionary relationships. This system recognizes that archaea are more closely related to eukaryotes than to bacteria, overturning the previous belief that they shared a common lineage with bacteria.

Complexity and Diversity

The prokaryotic world is incredibly diverse, with numerous metabolic and structural variations. Classifying them into a single kingdom oversimplifies this complexity, reducing the richness and diversity of the prokaryotic domain.

The Three-Domain System

As a result of these insights, the scientific community has largely adopted the three-domain system: Bacteria, Archaea, and Eukarya. This system better reflects the evolutionary history and diversity of life on Earth.

Advantages of the Three-Domain System

Modern biological classifications are most useful when they have predictive value, allowing taxonomists to make distinct statements about shared biology. The three-domain system meets this criterion more effectively than the Monera kingdom did.

The Transition from Monera to the Three-Domain System

Taxonomists used to work entirely with morphology and anatomy, which works well for macroscopic organisms and also has the useful property of working with fossils. However, microscopic creatures posed a challenge. Different classification schemes were proposed, one of which was to place all organisms lacking a nucleus together. This led to the proposed kingdom Monera which included both bacteria and archaea.

Then molecular sequence methods revolutionized classification. Ribosomal RNA sequences of different prokaryotes revealed that what Carl Woese named Archaea were as different from bacteria as bacteria are from eukaryotes. Further analysis has underscored this point, showing that archaea are distinct from bacteria in numerous biochemical and molecular ways.

These findings make a kingdom Monera nonsensical at multiple levels. It lumps together two dissimilar groups based on a shared non-feature—the lack of membrane-bounded organelles. Furthermore, the evolutionary split is as old or even older than the split from eukaryotes, so there is no justification for lumping them together.

The Impact on Textbooks and Publications

Decades have passed since biologists abandoned the concept of Monera, but textbooks are painfully slow to update. Consequently, the zombie concept of Monera persists in places like Quora. However, look on PubMed, the premiere biomedical database, and you'll find only a handful of publications mentioning Monera, with many authors named Monera appearing in the results.