Lynn Margulis, 73, a rebel within the realm of science, whose determined advocacy of her ideas about how new species arise helped change evolutionary biology, died Nov. 22 at her home in Amherst, Mass.

The University of Massachusetts at Amherst, where she had been a professor, announced her death. News accounts reported that she suffered a stroke.

Dr. Margulis was best known for demonstrating a theory of biological change, called the endosymbiotic theory, that appears to suggest a degree of cooperation between organisms.

She advanced the idea that, at a cellular level, more complex forms of life could arise from the merger of simpler forms with one another. Each simple form of life supplied some of the ingredients for creation of something new.

In biological terms, Dr. Margulis showed that eukaryotic cells — the more complex organisms — could develop from a blend of less complex cells — prokaryotes, such as bacteria. The process was known as symbiogenesis.

Symbiogenesis was considered a possible solution to one of the perplexities of evolutionary biology: How lower-order cells that lacked a nucleus made the jump to a higher order of cells that possessed a nucleus.

Dr. Margulis’s ideas have been seen as offering an alternative to the prevailing scientific belief that random mutation was the only basis for species variation and evolutionary change.

Her revolutionary 1967 paper setting forth her ideas was turned down by more than a dozen different scientific publications before running in the Journal of Theoretical Biology. She wrote a more detailed version of her endosymbiotic theory in her first book, “Origin of Eukaryotic Cells” (1970), and expanded on it with other publications through the 1980s.

In time, her theory was hailed as a great achievement of evolutionary biology and put Dr. Margulis in the top rank of scientific thinkers. She was elected in 1983 to the National Academy of Sciences and was presented the National Medal of Science in 1999 by President Bill Clinton.

Nevertheless, Dr. Margulis appeared to remain a lightning rod for controversy, and in some areas, she met vigorous challenge.

Some scientists were put off by her embrace of the Gaia hypothesis, suggesting that both the animate and inanimate worlds can be viewed as a single organism. In one interview, published in Discover magazine, she was asked whether she ever tired of being called controversial.

“I don’t consider my ideas controversial,” she replied. “I consider them right.”

Lynn Petra Alexander was born in Chicago on March 5, 1938, and enrolled as a teenager in an early entrance program at the University of Chicago.

In a campus building one day, she met her future husband, Carl Sagan, while she was on her way upstairs and he was headed down. A student of the cosmos, Sagan became an astronomer and won fame as a popularizer of science. The two were married in 1957, the year she received her bachelor’s degree.

She obtained a master’s degree in zoology and genetics from the University of Wisconsin in 1960 and received a PhD from the University of California at Berkeley, where she did research in genetics. By the time she got her doctorate in 1965, the marriage had ended.

After a postdoctoral period at Brandeis University in Massachusetts, Dr. Margulis began teaching in the biology department at Boston University. After 22 years in Boston, she joined the faculty of the University of Massachusetts at Amherst in 1988.

Her second marriage, to scientist Thomas N. Margulis, ended in divorce in the early 1980s.

Survivors include two children from her first marriage, Dorion Sagan and Jeremy Sagan; two children from her second marriage, Zachary Margulis-Ohnuma and Jennifer Margulis di Properzio; three sisters; and nine grandchildren.

Many of those who knew Dr. Margulis viewed her as outspoken, energetic, hard-working and a constant source of ideas. But she did not consider herself a superwoman.

“I quit my job as a wife twice,” she once said about the difficulties of balancing science and domestic life. “It’s not humanly possible to be a good wife, a good mother and a first-class scientist. No one can do it — something has to go.”