When Lynn Margulis was a young woman, all life was divided into two great kingdoms, known as plants and animals. But Margulis and others saw that this division did not accurately reflect the diversity of life: many organisms are neither. To combat this, Carl Woese in 1990 introduced the three-domain system, meant to expand on the previously narrow categories of life. In 1969, life on earth was classified into five kingdoms (Plants, Animals, Fungi, Protists and Monera, as introduced by Robert Whittaker.
Illustration depicting Whittaker’s 5 kingdom classification
Margulis became the most important supporter and defender of this proposed theory, as she rejected the three-domain system which gained wide acceptance, believing that there were more domains that were neglected by this system.
However, while in support of parts of the new classification as it was superior to the three domain system, Margulis was also a big critic of Whittaker’s newly introduced 5 kingdom classification and she was the first to recognise the limitations of Whittaker’s classification of microbes. She introduced a modified classification by which all life forms, including the newly discovered, could be integrated into the classical five kingdoms. According to her the main problem, archaea, falls under the kingdom Prokaryotae alongside bacteria (in contrast to the three-domain system, which treats archaea as a higher taxon than kingdom, or the six-kingdom system, which holds that it is a separate kingdom).
Her concept is given in detail in her book, “Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth”, written with Karlene V. Schwartz. Stephen Jay Gould, in the Foreword, called the book the “rarest of intellectual treasures” and drew upon its contents in pointing out that the greatest division among living beings was not “between plants and animals, but within the once- ignored microorganisms—the prokaryotic Bacteria and the eukaryotic Protoctista.” The famous hand symbol used to illustrate the 5 classifications, where each part of the hand represents one of the tenets of the proposed classification, is now globally recognised.
Margulis’ book, Five Kingdoms, written with Karlene V. Schwartz
It is believed that it is mainly because of Margulis and her propagation of this theory that the five-kingdom system survived. But later discoveries of new organisms, such as archaea, and emergence of molecular taxonomy challenged the concept. By the mid-2000s, most scientists began to agree that there are more than five kingdoms.