Margulis, Lynn Alexander (1938-), an American biologist, helped advance the study of the origins of cells. She developed the symbiotic theory, which states that bacteria played a major role in the development of living cells. This theory has become known as the serial endosymbiosis theory, or SET.

Margulis was born on March 5, 1938, in Chicago. She was the oldest of four daughters of Morris Alexander, a lawyer and businessman, and Leone Wise Alexander, a travel agent. At age 15, Lynn completed her sophomore year at Hyde Park High School, and enrolled in a special early admission program at the nearby University of Chicago (UC). There Lynn enjoyed excellent science classes that included reading original works of famous scientists.

At the University of Chicago, Lynn met Carl Edward Sagan, then a graduate student in physics, who would later become a famous astronomer and author. Margulis graduated with a liberal arts degree in 1957, and she and Sagan married that June. They moved to Madison, Wisconsin, where she earned her master's degree in science in zoology and genetics from the University of Wisconsin in 1960.

She and Sagan next moved to California, where Margulis received a doctoral degree from the University of California at Berkeley in 1965. By that time, her marriage to Sagan had ended, and she moved to Massachusetts with her two sons.

Margftlis taught at Boston University for 22 years, from 1966 until 1988. Her second marriage, to Thomas N. Margulis, a crystallographer, lasted from 1967 to 1980. The couple had one son and a daughter. From 1988 on, Margulis taught with the department of botany at the University of Massachusetts at Amherst.

In graduate school, Margulis became intrigued by reproduction that involved genetic material not found in a cell's nucleus, but in its cytoplasm, which is all the material within a cell's membrane other than the nucleus. In the early 1960's, Margulis and other researchers presented evidence of genes in chloroplasts, specialized structures within the cells of plants.

Margulis focused on symbiosis, which is the scientific term for two different species of organisms living closely together in a way that benefits at least one of the species. Examples of symbiosis are a species of small fish that cleans parasites from much larger fish, or a fungus that grows on a person's skin. Margulis theorized that eukaryotic cells (cells with nuclei) evolved from a symbiosis of bacteria without nuclei that had previously lived independently. In this theory, both chloroplasts and other structures found in cells, called mitochondria, evolved from once free-living bacterial species. This serial endosymbiotic theory, or SET, gave a new view of evolution and helped explain the origin of cells with nuclei. Margulis's idea was revolutionary: That cells with nuclei, including all the cells in the human body, descended from bacteria that formed symbiotic relationships more than 2 billion years ago.

In the 1960's and 1970's, Margulis endured doubt and even ridicule from other scientists as she pursued her theory. She went on to compile her work on the symbiotic theory in the book Origin of Eukaryotic Cells (1970). She elaborated on this work in a revised version, Symbiosis in Cell Evolution (1981). Margulis's theory has since been widely accepted.

Her holistic view of biology led Margulis to embrace the Gaia hypothesis, which holds that the earth is a living organism that functions as a unified whole. British biologist James Lovelock presented his hypothesis in 1968. According to his theory, all living things interact to create the conditions that are needed for life to continue. Lovelock's theory aroused controversy among scientists, and when Margulis helped Lovelock develop the Gaia hypothesis, she earned further disapproval. However, some scientists believe that researching the Gaia concept could lead to a greater understanding of complex environmental problems.

Margulis has given talks all over the world and served on numerous associations and committees. She worked with NASA and wrote or cowrote many books, film scripts, and articles. Her son, Dorion Sagan, collaborated with her on Origins of Sex: Three Billion Years of Genetic Recombination (1986), Microcosmos Coloring Book (1988), Mystery Dance: On the Evolution of Human Sexuality (1991), and What Is Life? (1995).

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 also received honorary doctorates from several universities. In March 2000, President Bill Clinton presented the U.S. National Medal of Science to Margulis and 11 other recipients.