When it comes to the world of the micro, there’s a little bit of something for everyone.

Do you feel a kinship with chill blobs that contain hidden depths? Well, there’s plenty of amoeba to understand.

Do you love complex choreography? Well, then you might want to catch a performance by bacillaria. And if those don’t draw you in, there are hunters, living jewels, and complicated lifestyles aplenty to explore.

For microbiologist Dr. Lynn Margulis, the draw was symbiosis. The microcosmos is made of many, many organisms who don’t just occupy the same space, they collaborate.

Whether it’s for protection, nutrition, or just some help getting around, these interactions may seem small, but their implications are enormous for all life on earth. Through the second half of the 20th century, Margulis’ dedication to understanding symbiosis transformed our understanding not only of microbes, but of ourselves. Her legacy is a complicated one, rooted in a stubbornness that at times paid off in major scientific understanding, but also made her unwilling to acknowledge the limitations of her work.

At the core of her efforts though is a dedication to microbes and to understanding what they can teach us about the world. Lynn Margulis published many books and articles about microbes for both academic and general audiences. But perhaps her most impactful work was also one of her earliest, an article published in 1967, shortly after she began working at Boston University.

The paper was titled “On the Origin of Mitosing Cells,” and over its roughly 50 pages, Margulis argued that some of the organelles inside eukaryotes were actually once—long, long ago—free-living prokaryotic organisms. What she was describing was the theory of endosymbiosis, the idea that symbiosis between one ancient prokaryote inside another drove the evolution of eukaryotes. We’ve discussed endosymbiosis in its own episode, and you may have noted that it plays a recurring role in the lives of microbes we’ve covered.

This theory is now so well understood, and it has so much explanatory power that, at this point it feels like a thing that we’ve kinda always known. But, indeed, the battle over the acceptance of endosymbiotic theory was still going on during my lifetime. I remember being taught that this was probably a thing, not that it definitely was.

In her paper, Margulis also notes that she was not the first scientist to consider the role of symbiosis in evolution. The theory was first proposed by the Russian botanist Konstantin Sergeevich Mereschkowski in 1905, and later scientists had suggested that the resemblance they observed between certain eukaryotic organelles and certain prokaryotic organisms was more than just coincidence. But while these bits and pieces of endosymbiotic theory had been around before her, Margulis was the one who tied them together. “On the Origins of Mitosing Cells” focuses on three examples of what Margulis thought were possible endosymbiotic events: the evolution of mitochondria, chloroplasts, and flagella from potentially prokaryotic origins.

While the paper didn’t describe any experiments of her own, she drew on the works of her predecessors to provide experimental evidence for her ideas. She also described predictions for future experimental results that would support the theory. The result was a paper that didn’t just present an argument, it laid out a multi-chapter story of evolution that drew on Margulis’ understanding of biology and even atmospheric chemistry to describe how symbiosis may have driven eukaryotic evolution, and how scientists might be able to verify that story.

This paper, “On the Origin of Mitosing Cells” was rejected more than a dozen times until it was finally published in the Journal of Theoretical Biology. And her paper caused quite a stir, highlighted and debated in both popular and academic formats. Some scientists were just as captivated by the idea of endosymbiosis as Margulis was.

Others dismissed her theory outright. In a paper called “The non symbiotic origin of mitochondria,” the authors wrote that “while the symbiotic theory may be aesthetically pleasing, it is not compelling.” But while the conversation may have become contentious, Margulis stuck to her theory. To her, the notion that these organelles could have evolved solely through the accumulation of thousands of mutations was both improbable and unsupported by the evidence.

Slow change was part of the evolutionary story, but it was not all of it. And she thought symbiogenesis would explain these more immense changes that were necessary to the history of eukaryotes. And so Margulis continued to experiment with her microbes and investigate the limits of her theory.

But so did many others. In 1978, Robert M. Schwartz and Margaret O.

Dayhoff published “Origins of Prokaryotes , Eukaryotes, Mitochondria, and Chloroplasts,” which was able to experimentally conclude the prokaryotic origins of chloroplasts and everyone’s favorite powerhouse, the mitochondria. This work was among several that would confirm that Margulis was right about those two organelles in particular. But as important in science as what you get right is what you get wrong.

No experimental evidence has turned up to support Margulis’ theory that the flagella that help many eukaryotic organisms move were originally an organism of their own. She thought they may originally have been spirochetes. But while flagellates and other aspects of her theory have not panned out in experimental testing, Margulis would continue to defend her ideas.

Lynn Margulis eventually moved to the University of Massachusetts Amherst, where she would stay until her death in November, 2011. In an interview she gave to Discover Magazine only a few months before she died, she said, “I don’t consider my ideas controversial. I consider them right.” It’s a statement that may seem admirable when quoted against the history of endosymbiotic theory.

But in that same interview, Margulis insisted that AIDS is not caused by HIV, it’s caused by a syphilis-causing spirochete that has somehow been rendered undetectable to all scientific and medical scrutiny because of...symbiosis? We’re not gonna litigate her statement here because there’s no point in that: at the time she gave the interview, there had been decades of research connecting AIDS to HIV, as well as decades of real-life suffering and death caused by the virus. But this wasn’t the only time Margulis engaged in what some might euphemize as “controversial opinions,” but that at times took her fully into conspiracy theories.

Lynn Margulis’ work was part of a long history of contrarianism that often appeals to us because when they get things right, contrarians push through established beliefs to reveal hidden mechanisms of the world. But when they get things wrong, they do so at the expense of scientific rigor, intellectual honesty, and just plain old decency. And so it can sometimes be unclear how to understand and process their work, unless--perhaps--we take inspiration from Margulis’ fascination with microbes and the communities they form.

Just as the light we shine through our microscope informs the colors and structures that shine on the other side, our understanding of the microcosmos is shaped by the people who study it. Their experience, their personalities, their biases—these all shape the questions they ask, the ways they ask them, and the conclusions they draw. And so, science is at its strongest when, like the symbiotic microcosmos that entranced.

Margulis, it is understood not as the product of a single expert but as a collective expertise—the result of shared experiences that are constantly in conversation with each other. That is what made “On the Origin of Mitosing Cells” so powerful—the way Margulis so effectively synthesized her ideas with other scientists’ work to describe a whole history of evolution that, while not without its flaws, provoked a dialogue that altered our understanding of biology and of the history of life on earth. And this impact makes the frustrations of her legacy all the more acute.

Margulis would continue to draw on what she learned from the world of microbes, co-authoring a number of books with her son Dorion Sagan that brought the wonders of microbial evolution to a wider audience. In one of their books titled Microcosmos, they wrote: “It is tempting, even for scientists, to get carried away by success stories.” It’s a sentence that, against the backdrop of both her scientific and unscientific efforts, reminds us that microbes can teach us a lot about the world, but that there is also a limit to what we learn. And often, that limit is our own selves.

Thank you for coming on this journey with us as we explore the unseen world that surrounds us. Thank you as well to all of the people on the screen right now. Those are our patrons on Patreon.

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