Classification Rush

Darwin led lots of bright people to classify life based on their observable similarities and dissimilarities. Thanks to these attempts, numerous trees of life (ToLs) with different goals, ie. understanding the emergence of the human species or exploring major transitions in evolutionary history, were generated. 1879 was a significant year for microbial taxonomy, not just because Ernst Haeckal published another ToL putting humans to the very pinnacle of evolution, but also because he positioned microscopic amoeba to the root of ToL illustrating human evolution. Interestingly, Haeckal’s suggestion still appears true today since phylogenetic trees based on molecular techniques illustrate amoeba and animals diverging from a common ancestor. Haeckal’s ToL illustrates 2 kingdoms of the living: protists and animals. Following Haeckal’s ToL exploring microscopic organisms (Protists), Mereschkowski published another ToL suggesting that complex life forms such as animal and plants arose as a result of endosymbiosis of microscopic bacterial species, which in fact is partially correct. Today we know that eukaryotes arose as a result of an endosymbiosis between a bacterium and an archeon, and plants arose owing to an endosymbiosis between a eukaryote and a cholorophyllic bacterium thanks to developments in DNA-based molecular techniques. Hence, the lack of molecular techniques in 1905 may redeem Mereschkowski.

The ToL of Haeckal showing the evolution of human, The Evolution of Man, Haeckal, 1879

The ToL of Mereschkowski showing serial endosymbiosis that gave rise to eukaryotes and plants, Über Natur und Ursprung der Chromatophoren im Pflanzenreiche, Mereschkowski, 1905, Courtesy of Bill Martin

Nonetheless, Mereschowski neither knew what an archeon is nor that Archeae have stark similarities and dissimilarities to Bacteria. In 1925, Chatton realized that microscopic species had vast variation in their morphology and physiology, which suggested that classifying all microbes in the same domain is incorrect. Consequently, Chatton became the first to categorize all life into 2 domains, Eukaryota and Prokaryota, based on their nuclear morphology. After this division into two domains, inconsistencies due to morphology and physiology-based taxonomic attempts fueled later debates on how to branch these domains into kingdoms.