Margulis, Lynn  ( born March 5, 1938 , Chicago, Illinois, U.S.—died November 22, 2011 , Amherst, Massachusetts )  American biologist whose serial endosymbiotic theory of eukaryotic cell development revolutionized the modern concept of how life arose on Earth.

Margulis was raised in Chicago. Intellectually precocious, she graduated with a bachelor’s degree from the University of Chicago in 1957. Soon after, she married American astronomer Carl Sagan, with whom she had two children; one, Dorion, would become her frequent collaborator. The couple divorced in 1964. Margulis earned a master’s degree in zoology and genetics from the University of Wisconsin at Madison in 1960 and a Ph.D. in genetics from the University of California, Berkeley, in 1965. She joined the biology department of Boston University in 1966 and taught there until 1988, when she was named distinguished university professor in the department of botany at the University of Massachusetts at Amherst. She retained that title when her affiliation at the university changed to the department of biology in 1993 and then to the department of geosciences in 1997.

Throughout most of her career, Margulis was considered a radical by peers who pursued traditional Darwinian “survival of the fittest” approaches to biology. Her ideas, which focused on symbiosis—a living arrangement of two different organisms in an association that can be either beneficial or unfavourable—were frequently greeted with skepticism and even hostility. Among her most important work was the development of the serial endosymbiotic theory (SET) of the origin of cells, which posits that eukaryotic cells (cells with nuclei) evolved from the symbiotic merger of nonnucleated bacteria that had previously existed independently. In this theory, mitochondria and chloroplasts, two major organelles of eukaryotic cells, are descendants of once free-living bacterial species. She explained the concept in her first book, Origin of Eukaryotic Cells (1970). At the time, her theory was regarded as far-fetched, but it has since been widely accepted. She elaborated in her 1981 classic, Symbiosis in Cell Evolution, proposing that another symbiotic merger of cells with bacteria—this time spirochetes, a type of bacterium that undulates rapidly—developed into the internal transportation system of the nucleated cell. Margulis further postulated that eukaryotic cilia were also originally spirochetes and that cytoplasm evolved from a symbiotic relationship between eubacteria and archaebacteria (see archaea).

Her 1982 book Five Kingdoms, written with American biologist Karlene V. Schwartz, articulates a five-kingdom system of classifying life on Earth—animals, plants, bacteria (prokaryotes), fungi, and protoctists. The protist kingdom, which comprises most unicellular organisms (and multicellular algae) in other systems, is rejected as too general. Many of the organisms usually categorized as protists are placed in one of the other four kingdoms; protoctists make up the remaining organisms, which are all aquatic, and include algae and slime molds. Margulis edited portions of the compendium Handbook of Protoctista (1990).

Another area of interest for Margulis was her long collaboration with British scientist James Lovelock on the controversial Gaia hypothesis. This proposes that the Earth can be viewed as a single self-regulating organism—that is, a complex entity whose living and inorganic elements are interdependent and whose life-forms actively modify the environment to maintain hospitable conditions.

In addition to Margulis’s scholarly publications, she wrote numerous books interpreting scientific concepts and quandaries for a popular audience. Among them were Mystery Dance: On the Evolution of Human Sexuality (1991), What Is Life? (1995), What Is Sex? (1997), and Dazzle Gradually: Reflections on Nature in Nature (2007), all cowritten with her son. She also wrote a book of stories, Luminous Fish (2007). Her later efforts were published under the Sciencewriters Books imprint of Chelsea Green Publishing, which she cofounded with Dorion in 2006.

Margulis was elected to the National Academy of Sciences in 1983 and was one of three American members of the Russian Academy of Natural Sciences. She was awarded the William Procter Prize of Sigma Xi, an international research society, and the U.S. National Medal of Science in 1999. In 2008 she received the Darwin-Wallace Medal of the Linnean Society of London. She was a coauthor, with Dorion, of Encyclopædia Britannica’s article on life.