Biological classification organizes living organisms into hierarchical groups to understand their relationships and evolutionary history. This system helps make sense of the millions of species on Earth, from the smallest microorganisms to the largest animals. By categorizing life forms, researchers identify commonalities and differences, aiding comprehension of the intricate web of life.
The Three Domains of Life
The highest rank in biological classification is the domain. Life on Earth is categorized into three fundamental domains: Bacteria, Archaea, and Eukarya. Carl Woese and colleagues proposed this three-domain system in 1990, based on significant differences in ribosomal RNA (rRNA) sequences. Previously, life was often divided into five kingdoms or simply into prokaryotes and eukaryotes. Distinct rRNA sequences revealed that prokaryotic organisms, which lack a membrane-bound nucleus, were not a single group. Instead, they comprised two separate evolutionary lineages: Bacteria and Archaea. Eubacteria, also known as “true bacteria,” belong to the Domain Bacteria. This reclassification highlighted that Archaea are genetically and biochemically more closely related to Eukarya than to Bacteria, despite their similar appearance.
Defining Eubacteria (Bacteria)
Organisms within the Domain Bacteria are single-celled prokaryotes. Their cells lack a membrane-bound nucleus and other membrane-enclosed organelles. Genetic material, a single circular chromosome, resides in the cytoplasm. A distinguishing feature of bacteria is their cell wall, which contains peptidoglycan. This polymer provides structural strength and protects the cell from osmotic lysis. Bacteria are small, ranging from 0.2 to 50 micrometers. They exhibit metabolic diversity, with species capable of photosynthesis, chemosynthesis, or heterotrophy, allowing them to thrive in nearly every environment. Bacteria are ubiquitous, playing roles in nutrient cycling, decomposition, and influencing human health as both beneficial gut bacteria and pathogens.
Distinguishing Bacteria from Archaea and Eukarya
While both Bacteria and Archaea are prokaryotic, their cell wall compositions differ. Bacteria possess peptidoglycan in their cell walls, whereas archaea do not, instead having cell walls made of pseudopeptidoglycan, proteins, or other complex carbohydrates. Eukarya, including plants, animals, fungi, and protists, are characterized by a membrane-bound nucleus and organelles like mitochondria and chloroplasts. Differences also exist in their membrane lipids. Bacterial membranes consist of unbranched fatty acid chains linked to glycerol by ester bonds. In contrast, archaeal membranes are composed of branched isoprenoid chains attached to glycerol by ether linkages, contributing to their ability to survive in extreme environments. RNA polymerase enzymes also show variations; bacteria have a single type, while archaea and eukaryotes have more complex, multi-subunit RNA polymerases that share structural similarities. Introns, non-coding sequences within genes, are common in eukaryotes and some archaea, but rare in bacteria.