The Evolution of Biological Classification: From Appearance to Genes and Domains

During the 17th century, the arrival of new plant and animal species in Europe sparked the need for classification systems. In 1735, Carl Linnaeus, an influential figure in the field of taxonomy, created a hierarchical system of biological nomenclature that included two kingdoms: animal and vegetable (plant), where the highest rank encompassed all the lower levels. As the understanding of unicellular organisms grew, Ernst Haeckel recognized the need for a third kingdom, Protista, which he proposed in 1866.

The colors of Grand Prismatic Spring in Yellowstone National Park are produced by thermophilic microbes residing within it. These extremophiles, belonging to the Kingdom Archaea, thrive in temperatures ranging from 1,880°F (870°C) at the center to 1,470°F (640°C) at the rim, resulting in a stunning display of rainbow hues.

During the 1960s, Roger Y. Stanier and C.B. van Niel created a classification system consisting of four kingdoms, which was based on the differentiation between prokaryotic and eukaryotic cells. Eukaryotic cells have a cell membrane surrounding their nucleus, while prokaryotic cells do not. They also proposed a higher rank known as superdomain or empire, which consisted of two empires - Empire Prokarya and Empire Eukarya. The Kingdom Monera (bacteria) was included in Empire Prokarya, while Kingdoms Plantae, Animalia, and Protista were grouped together in Empire Eukarya.

Prior to the mid-1970s, taxonomic classifications were primarily based on observable traits such as morphology, anatomy, embryology, and cell structure. However, in 1977, Carl Woese and George E. Fox at the University of Illinois at Urbana-Champaign revolutionized the field by classifying organisms based on a molecular level comparison of their genes. They focused on the nucleotide sequences in a subunit of ribosomal rRNA, which undergoes evolutionary changes. The outcome was the proposal of three domains of cellular life: Domain Archaea, a diverse group of prokaryotic organisms among the oldest on Earth capable of adapting to extreme environments (extremophiles); Domain Bacteria; and Domain Eukarya, which was further divided into the Kingdoms Fungi (yeasts, molds), Plantae (flowering plants, ferns), and Animalia (vertebrates, invertebrates). Woese and Fox’s Protista Kingdom has also undergone further subdivisions. Taxonomic systems containing between two to eight kingdoms have been proposed, highlighting the fact that the final chapter on classification has yet to be written.