Evolutionary biologist Lynn Margulis died last week (November 22) at the age of 73. She was best known for proposing the theory of endosymbiosis, which states that rather than evolving via genetic mutation, new species were more likely to have come about via parasitic or symbiotic relationships that became permanently inter-dependent over time.

“She was always stimulating; she always had a new idea, some new connection she had seen and she couldn’t wait to tell you about,” Steve Goodwin, Dean of the College of Natural Resources
 and the Environment told MassLive.com.

Margulis showed early aptitude in science, enrolling at the University of Chicago and earning her bachelor’s degree in zoology by the age of 18. Shortly thereafter she married her first husband, the astronomer Carl Sagan. The marriage ended by the time she got her doctorate in genetics from the University of California, Berkeley, in 1965.

She developed her ideas on symbiosis in the late 1960s, and tried to publish her ideas in 15 journals before finally being accepted by the Journal of Theoretical Biology, according to The New York Times. Though it was highly controversial at the time, serial symbiosis is widely accepted among evolutionary scientists today.

In the 1970s, she became a supporter of James Lovelock’s Gaia hypothesis, which proposed that the earth could be thought of as a complex system whose atmospheric and mineral components existed in symbiosis with living organisms, allowing biota as a whole to self-perpetuate.

She taught evolutionary biology for nearly 40 years, first at the Boston University and then at the University of Massachusetts, where I had the opportunity to experience her carefully crafted course. I came to the class expecting Margulis to expound on the theories that she had championed. Instead, she exposed our small seminar class to the experiments of many researchers whose work provided evidence for her ideas, and invited us to make own conclusions.

“If science doesn’t fit in with the cultural milieu, people dismiss science—they never reject their cultural milieu!” said Margulis in the book The Third Culture: Beyond the Scientific Revolution. In the same chapter, Richard Dawkins wrote: “I greatly admire Lynn Margulis’s sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy.”

According to The New York Times, Margulis died from a stroke. She is survived by a daughter Jennifer Margulis and three sons Dorion Sagan, Jeremy Sagan, Zachary Margulis-Ohnuma.