A Brief Yet Discerning History of Scientific Classification

The great Greek philosopher Plato believed the world as perceived by our senses was but a shadowy and incomplete version of the truth. His brilliant student, Aristotle, had other ideas. He said that nature could only be understood through observation, analysis, and classification. 

Aristotle’s De historia animalium divided animals into those with and without blood—roughly corresponding to modern categories of vertebrates and invertebrates. He classified animals using hierarchies and relationships, forging the way for future biologists to try to organize nature as they saw it. 

Swiss naturalist Conrad Gessner’s Historia animalium—an immense 16th-century work that attempted to catalog all known animals—organized species not by any systematic scientific classification, but in alphabetical order. That might not seem like particularly rigorous biology, but, to his credit, Gessner also included details on nomenclature, distribution, and behavior—and tried to group animals based on broad characteristics, such as number of limbs or if they were land- or water-dwellers.

Gessner’s contemporary, the Renaissance naturalist and philosophy professor Ulisse Aldrovandi, is known for assembling a spectacular teatro della natura, one of history’s most famous “cabinets of curiosity.”

But Aldrovandi didn’t just collect; he categorized. He wrote influential volumes on animal classification, borrowing heavily from Gessner, but with a more scientific approach to organization. His Ornithologiae classified birds based largely on habitat and behavior, rather than anatomical features. Groupings included birds with powerful beaks, birds that bathe in dust, and even a section on bats.

When English naturalist Robert Hooke used an early microscope to produce detailed illustrations for his 1665 work Micrographia, he opened a portal into a previously invisible world. 

It is the minuscule writ large: the whiskery legs of a flea, dainty stalks of bread mold, and a seemingly enormous louse clinging to a single strand of human hair. Scientists suddenly had an abundance of new organisms to classify.  They also had a new word to deploy: Micrographia includes the first recorded use in biology of the word “cell,” which Hooke used to describe the pattern he observed on a thin slice of cork.

Little Carl Linnaeus might have been great at fantasy football (or, being Swedish, fantasy fotboll). But instead of mastering stats, he memorized plant names, a particularly difficult task at the time given that plants had elaborate Latin names. The tomato, for example, was called Solanum caule inermi herbaceo, foliis pinnatis incises (“solanum with the smooth stem which is herbaceous and has incised pinnate leaves.”) 

Despite this verbosity—or perhaps because of it—classification fascinated Linnaeus throughout his life. He developed the system of binomial nomenclature—formally assigning two names to each species—that is still in use today, and he popularized rank-based classification (the kingdom, phylum, class, order structure that you may have learned in grade school). The tenth edition of his Systema naturae, published in 1758, contains descriptions and the first scientific names of some 4,400 animals ranging from the narwhal to the dodo.

A popularizer of the then-recent theory of evolution, German biologist Ernst Haeckel helped put Charles Darwin on the map. His 1866 illustration in Generelle Morphologie der Organismen is often cited as the first published depiction of a phylogenetic tree of all life—a map of the evolutionary development of species. 

Haeckel is also known for his visually stunning Kunstformen der Natur and for his formulation  “ontogeny recapitulates phylogeny” (the idea, since rejected in biology, that as embryos develop, they pass through stages of evolution.

Willi Hennig, who was trained as an entomologist, outlined the biological classification method called cladistics in Theorie der phylogenetzschen Systematik (1950).  

This approach uses shared derived characteristics—that is, characteristics found in members of a particular group (say, hominids) but not in the larger group of related animals (great apes)—to group organisms and gives insights into the distribution and diversity of life on Earth. Unfortunately, the first edition in German was largely ignored, and few scientists understood the implications for taxonomy until the work appeared in English in the mid 1960s.

Just as microscopes opened new worlds for Hooke, technology is unlocking new lines of research for today’s taxonomists. Web-based technology and advances in DNA analysis now support online databases for classification and systematics. The web application MorphoBank recently allowed Museum researchers and their colleagues to build a new tree of life for mammals and in the process, reconstruct the common ancestor of all placental mammals.

This new technology comes at a turning point, as there is a renewed urgency for taxonomic work. For all the centuries we’ve spent classifying life, we’re now presiding over a great extinction event. There are potentially 8 million species on Earth, and researchers have only described about 1.5 million. Many will go extinct before we can even give them fancy Latin names. In the face of this biodiversity crisis, taxonomists are trying to record life as it disappears—and perhaps help save some species along the way.

Curator Darrel Frost created and manages Amphibian Species of the World, an online database listing all scientific and English names for more than 7,000 amphibian species. While the database was designed for professional systematists—lots of text, no pretty pictures—it gets more than a million visits a year from scientists, conservationists, and policymakers. Frost says it’s become, “a unifying influence for conservationists to talk to one another.” Organizations such as CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora) and the IUCN (International Union for the Conservation of Nature) use the database in trade regulation of amphibian species and to inform habitat conservation efforts around the world.