For uncounted millennia, humans have been observing Earth's organisms and trying to make sense of their diversity. Over time, a naming system that lumped all life-forms into "plants" or "animals" gave way to more sophisticated and accurate approaches to classification based on shared traits and ancestry. The modern approach can logically organize millions of species, including those yet to be discovered.

Prehistoric humans survived by hunting animals and gathering plants. Their lives depended on recognizing the edible and the harmful. Indeed, native cultures have passed along a deep store of knowledge about thousands of plant and animal species organized by utility, such as edibility, toxicity, and medicinal value.

The Greek philosopher Aristotle observed and recorded nature in a scholarly way. His student Theophrastus, who lived from 370 bce to 285 bce, recorded 500 plant types, classified into herbs, shrubs, "pre-shrubs," and trees.

Advances in transportation, navigation, and exploration in the Middle Ages greatly expanded people's lists of observed organisms. In 1555, Swiss naturalist Conrad Gessner published Historia Animalium, categorizing thousands of animals into quadrupeds, birds, fish, and snakes. One hundred years later in his Historia Plantarum, English naturalist John Ray organized thousands of plants based on visual similarities and differences. In 1735, Swedish naturalist Carolus Linnaeus produced Systema Naturae, which categorized thousands of organisms into a hierarchy starting with genus and species and building to higher taxa including two kingdoms.

Several refinements have come to these higher taxa. German biologist Ernst Haeckel organized a "Tree of Life" in 1866. In 1894, he redrew its branches into three kingdoms. In 1959, American ecologist R. H. Whittaker established a five-kingdom taxonomy. In 1966, German biologist Willi Hennig invented cladistics, which classifies organisms based on their shared, derived traits in order to reflect their evolutionary history. Molecular data obtained by using ribosomal RNA sequences as an evolutionary measure allowed American molecular biologist Carl Woese to propose in 1977 a six-kingdom system that divided the then-existing kingdom Monera into two new kingdoms: Archaebacteria and Eubacteria. In 1990, Woese introduced the three-domain system that is used today.


1. Describe how prehistoric humans "classified" species.

2. Why has the need for a system of classification grown over the centuries?

3. How do modern classification systems differ from the system of classification used by John Ray?

4. Describe a scientific advancement that enabled Woese to introduce the six-kingdom system.

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