In the tenth edition of his book Systema Naturae (1758), Swedish botanist Carolus Linnaeus developed a biological classification system (now known as the Linnaean System) that placed all organisms into seven hierarchical groupings. He suggested that all organisms could be classified as belonging to two kingdoms, the Plantae (plants) and the Animalia (animals), and that members of these kingdoms could be distinguished by whether they have the ability to "sense" (both plants and animals grow and live, but only animals sense). This two-kingdom classification system remained virtually unchallenged for over a century.

Today, most biologists group living things into five kingdoms: the Monera, the Protista, Fungi, Plants, and Animals. The Monera includes all prokaryotes. The monerans include the most ancient forms of life and were the only organisms on Earth from around 3.5 to 1.5 billion years ago. The Protista is a diverse group of single-celled eukaryotes that originally derived from...