Lynn Margulis died on Tuesday, the 22nd of November.

She is most famous for her theory of endosymbiosis, which says that eukaryotic cells—the complicated cells of protozoa, animals, plants, and fungi—were formed in a series of stages where one organism swallowed another but, instead of digesting it, took it on as a symbiotic partner!

As a young faculty member at Boston University, she wrote a paper on this theory. It was rejected by about fifteen scientific journals, but eventually accepted:

• Lynn Sagan, On the origin of mitosing cells, J. Theor. Bio. 14 (1967), 255–274.

(At the time she was married to the well-known astronomer Carl Sagan; she later wrote a number of books with their son Dorion.)

While Lynn Margulis was not the first to suggest that endosymbiosis played a major role in evolution, her brilliant and endlessly energetic advocacy of this idea helped trigger a scientific revolution. By now, almost all biologists agree that chloroplasts, the little green packages where photosynthesis happens in plant cells, were originally free-living bacteria of their own. Here’s a model of a chloroplast:

As you can see, it’s quite a complex world of its own. Biologists also believe that mitochondria, the ‘powerhouses’ of eukaryotic cells:

were originally bacteria.

Indeed, both chloroplasts and mitochondria still have their own DNA, and they reproduce like bacteria, through fission. This is very useful in research on human populations, since mitochondrial DNA is passed down, not through the sperm cells, but only through the egg cells—which means it’s passed down from mothers to daughters, with sons as a mere sideshow. On the other hand, Y chromosomes go down from father to son.

There’s a lot more evidence for endosymbiosis as the origin of mitochondria and chloroplasts… and in fact, we can see this process going on today in various organisms. There are also many other ways in which organisms pass genes to each other.

This led Margulis to advocate a worldview in which the ‘tree’ of life is replaced by something more like a ‘thicket’:

It also suggests that competition between organisms is part of a bigger story where the formation of partnerships plays a major role. She has written many popular books on this, and they make great reading, even though some of her ideas are still controversial.

In 1995, the famous evolutionary biologist 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. I’m referring to the theory that the eukaryotic cell is a symbiotic union of primitive prokaryotic cells. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it.

I’ll end by quoting an excerpt from this essay, which you can read in its entirety online:

• Lynn Margulis, Gaia is a tough bitch, in The Third Culture: Beyond the Scientific Revolution, ed. John Brockman, Simon & Schuster, 1995.

I think it’s a good sample of how she wasn’t afraid to stir up controversy. Indeed, you’ll see her poking Dawkins with a sharp stick at the same time he was complimenting her!

More importantly, you’ll get a sense of how her work helped push evolutionary biology away from neo-Darwinism, also known as the Modern Synthesis. This approach tried to explain all evolution as a process of populations slowly changing through random mutation and natural selection. Now biologists are struggling to formulate an Extended Synthesis which takes many more processes into account.

Her remarks also some contain good warnings for mathematicians and physicists, such as myself, who want to dabble in biology. Physics may be defined as ‘the study of natural systems that can be accurately modeled by beautiful mathematics’. That’s why we can learn a surprising amount about physics just by sitting around scribbling. But living systems always surprise us, since they’re always more rich and complex than our models. So, biology requires more emphasis on experiment, observation, and perhaps a feeling for the organism:

I work in evolutionary biology, but with cells and microorganisms. Richard Dawkins, John Maynard Smith, George Williams, Richard Lewontin, Niles Eldredge, and Stephen Jay Gould all come out of the zoological tradition, which suggests to me that, in the words of our colleague Simon Robson, they deal with a data set some three billion years out of date. Eldredge and Gould and their many colleagues tend to codify an incredible ignorance of where the real action is in evolution, as they limit the domain of interest to animals—including, of course, people. All very interesting, but animals are very tardy on the evolutionary scene, and they give us little real insight into the major sources of evolution’s creativity. It’s as if you wrote a four-volume tome supposedly on world history but beginning in the year 1800 at Fort Dearborn and the founding of Chicago. You might be entirely correct about the nineteenth-century transformation of Fort Dearborn into a thriving lakeside metropolis, but it would hardly be world history. By “codifying ignorance” I refer in part to the fact that they miss four out of the five kingdoms of life. Animals are only one of these kingdoms. They miss bacteria, protoctista, fungi, and plants. They take a small and interesting chapter in the book of evolution and extrapolate it into the entire encyclopedia of life. Skewed and limited in their perspective, they are not wrong so much as grossly uninformed. Of what are they ignorant? Chemistry, primarily, because the language of evolutionary biology is the language of chemistry, and most of them ignore chemistry. I don’t want to lump them all together, because, first of all, Gould and Eldredge have found out very clearly that gradual evolutionary changes through time, expected by Darwin to be documented in the fossil record, are not the way it happened. Fossil morphologies persist for long periods of time, and after stasis, discontinuities are observed. I don’t think these observations are even debatable. John Maynard Smith, an engineer by training, knows much of his biology secondhand. He seldom deals with live organisms. He computes and he reads. I suspect that it’s very hard for him to have insight into any group of organisms when he does not deal with them directly. Biologists, especially, need direct sensory communication with the live beings they study and about which they write. Reconstructing evolutionary history through fossils—paleontology—is a valid approach, in my opinion, but paleontologists must work simultaneously with modern-counterpart organisms and with “neontologists”—that is, biologists. Gould, Eldredge, and Lewontin have made very valuable contributions. But the Dawkins–Williams–Maynard Smith tradition emerges from a history that I doubt they see in its Anglophone social context. Darwin claimed that populations of organisms change gradually through time as their members are weeded out, which is his basic idea of evolution through natural selection. Mendel, who developed the rules for genetic traits passing from one generation to another, made it very clear that while those traits reassort, they don’t change over time. A white flower mated to a red flower has pink offspring, and if that pink flower is crossed with another pink flower the offspring that result are just as red or just as white or just as pink as the original parent or grandparent. Species of organisms, Mendel insisted, don’t change through time. The mixture or blending that produced the pink is superficial. The genes are simply shuffled around to come out in different combinations, but those same combinations generate exactly the same types. Mendel’s observations are incontrovertible. So J. B. S. Haldane, without a doubt a brilliant person, and R. A. Fisher, a mathematician, generated an entire school of English-speaking evolutionists, as they developed the neo-Darwinist population-genetic analysis to reconcile two unreconcilable views: Darwin’s evolutionary view with Mendel’s pragmatic, anti-evolutionary concept. They invented a language of population genetics in the 1920s to 1950s called neo-Darwinism, to rationalize these two fields. They mathematized their work and began to believe in it, spreading the word widely in Great Britain, the United States, and beyond. France and other countries resisted neo-Darwinism, but some Japanese and other investigators joined in the “explanation” activity. Both Dawkins and Lewontin, who consider themselves far apart from each other in many respects, belong to this tradition. Lewontin visited an economics class at the University of Massachusetts a few years ago to talk to the students. In a kind of neo-Darwinian jockeying, he said that evolutionary changes are due to the Fisher–Haldane mechanisms: mutation, emigration, immigration, and the like. At the end of the hour, he said that none of the consequences of the details of his analysis had been shown empirically. His elaborate cost-benefit mathematical treatment was devoid of chemistry and biology. I asked him why, if none of it could be shown experimentally or in the field, he was so wedded to presenting a cost-benefit explanation derived from phony human social-economic “theory.” Why, when he himself was pointing to serious flaws related to the fundamental assumptions, did he want to teach this nonsense? His response was that there were two reasons: the first was “P.E.” “P.E.?,” I asked. “What is P.E.? Population explosion? Punctuated equilibrium? Physical education?” “No,” he replied, “P.E. is `physics envy,'” which is a syndrome in which scientists in other disciplines yearn for the mathematically explicit models of physics. His second reason was even more insidious: if he didn’t couch his studies in the neo-Darwinist thought style (archaic and totally inappropriate language, in my opinion), he wouldn’t be able to obtain grant money that was set up to support this kind of work. The neo-Darwinist population-genetics tradition is reminiscent of phrenology, I think, and is a kind of science that can expect exactly the same fate. It will look ridiculous in retrospect, because it is ridiculous. I’ve always felt that way, even as a more-than-adequate student of population genetics with a superb teacher—James F. Crow, at the University of Wisconsin, Madison. At the very end of the semester, the last week was spent on discussing the actual observational and experimental studies related to the models, but none of the outcomes of the experiments matched the theory.

This passage shows her tough side—these are the top names in evolutionary biology that she’s criticizing here, after all. But when I saw her speak, she was engaging and fun! You can see that yourself in these interviews. Hear how she started as a bad student in 4th grade, why her laboratory budget got cut to $0 in 2004… and get a sense of her career, personality, and ideas.

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