A brief history of the classification of living organisms

Large multicellular eukaryotes to which we are accustomed to in daily life, i.e. animals and plants, adopt a variety of forms and behaviors that seem immense to us. This animal and plant biodiversity, which is the most exposed by the media, in fact masks the real extent of the variety of the living world, which is in large covered by that of protists. Indeed, animals and plants represent only a tiny fraction of the total diversity of eukaryotes. Most diversity is in fact concentrated in different groups of protozoa, algae and fungi. In particular, certain so-called “lower” eukaryotes exhibit unique biological characteristics including the functioning of elementary molecular processes (DNA replication, gene expression, metabolism, etc.) that are highly divergent; some have adopted very original and surprising lifestyles (Translator’s note: Terms like “higher organisms” and “lower organisms” should be retired, see here). The study of eukaryotic protists in all their diversity therefore often provides a better understanding of how certain biological processes took place, how they were maintained or how they disappeared during evolution. A prerequisite for understanding this evolution is to have a good knowledge of the eukaryotic biosphere in all its complexity. It is therefore necessary to establish a complete inventory of all eukaryotic biodiversity, and to classify the eukaryotes according to a phylogeny, because it highlights the evolutionary relationships that the organisms have in relation to each other, allowing to better understand the evolution of the living world.

In addition to the fundamental aspect aimed at understanding living things, knowledge of the phylogenetic position of organisms also makes it possible to propose strategies to control them. This is especially true for some parasitic organisms. Their morphology is generally insufficient to classify them and the contribution of molecular data is therefore critical. For example, Blastocystis hominis, a human parasite that inhabits the intestine, has found its phylogenetic position thanks to the sequence of rRNAs. It is part of the Stramenopila, organisms generally possessing two dissimilar flagella, while Blastocystis hominis does not. The same analyzes indicate that it is in fact probably a complex of species. It is the first organism of this phylum described as infecting humans and for which therapeutic strategies therefore remain to be developed based on products effective in other Stramenopila.

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