The Tangled Tree

by David Quammen

  By then, he had moved to Geneva and holed up in a picturesque hotel. He tried to arrange some lecturing on symbiogenesis but was thwarted by a professor at the university, a botanist, who presumably viewed him as a blackguard or a flake, very possibly both. It was a difficult time for many people, just after the Great War, and not least difficult for expatriate Russian biologists with unsavory backstories, unorthodox theories, and delusions of grandeur. As he went broke, Merezhkowsky blamed that condition on the war. The only things he seems to have seen clearly, at this point, were chloroplasts and his own approaching end.

  Sapp’s group found an obituary note, with unusual detail, published in a Geneva newspaper, La Suisse, on January 11, 1921. Two days earlier, police had been called, after a porter at the hotel noticed a letter shoved out under the door of room 58, Merezhkowsky’s. It warned: “Do not enter my room. The air in it is poisoned. It will be dangerous to enter for several hours.” The police duly waited two hours. Then they went in, finding Merezhkowsky elaborately arranged and dead.

  He had mixed up a brew of chloroform and several kinds of acid. He had poured that into a container rigged to the wall above his bed, like an IV drip. Instead of a needle for his arm, there was a mask for his face. But first he had sealed off the room, lain down, and tied himself to the bed, leaving one arm free. How do you tie yourself to a bed? Merezhkowsky was enterprising. How do you concoct such a recipe for death? Merezhkowsky was a scientist. Sapp and colleagues think it was a ritual suicide of some sort, related to his delusional metaphysics. Maybe so. The magistrate on the scene, according to La Suisse, found an inscription in Latin pinned to a cord. It might have been more esoteric ravings, or just a final yelp of despair, pretentiously expressed. But the Latin note disappeared when Merezhkowsky’s Geneva police file was destroyed. Ritual or not, he had his plan. He put the mask on his face and opened a valve.

  This made for a lurid ending to a strange life, but probably the strangest point in the whole story of Constantin Merezhkowsky is that, on the subject of chloroplast origins in plants, the pillar of his symbiogenesis theory, he was right. Fifty-four years later, that idea would be confirmed, with molecular data, using the methodology invented by Carl Woese.

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  The other pioneering theorist in this field who rose to a certain threshold of notice, the American, was Ivan E. Wallin. He was a son of Swedish immigrants but a corn-fed midwesterner, born in Iowa, eventually an anatomist at the University of Colorado School of Medicine. Wallin’s publications in the 1920s, like Merezhkowsky’s before him, became just well enough known to be mentioned, but not discussed, in the early works of Lynn Margulis.

  Wallin offered a version of endosymbiosis that differed from, but complemented, Merezhkowsky’s. He argued that mitochondria in all complex organisms, not just chloroplasts in plants and algae, are descended from captured bacteria. Mitochondria, you’ll recall, are the little particles within cells that burn food and oxygen, packaging that energy into units of ATP (the energy-carrier molecule) for fueling the life of the cell. They perform other services too, but neither Wallin nor anyone else had yet figured out their functions. Wallin cared more about their origin. He wasn’t the first biologist who, viewing mitochondria through a microscope, found their resemblance to bacteria striking. But for him, it became the crux of a research program. He set out to prove, with an exhaustive campaign of experiments, that the resemblance was more than coincidental.

  Beginning around 1920, he became fascinated with the idea, mentioned by at least one earlier researcher but never persuasively developed, that mitochondria are the descendants of internalized bacteria. He launched a course of experimental investigations. His tools were simple, the basics of microscopy and microbial culturing in his day. He had no grant funding, just a bit of money given “from time to time” by a wealthy patron. He had no collaborators and no graduate students, just a pair of technical assistants, one also named Ivan. He was geographically isolated, in Colorado, from the leading centers of cell research on the East Coast, and he evidently didn’t relieve his isolation (as many scientists did, including Darwin) by establishing close relations with colleagues through the mail. Wallin’s day job was anatomy professor. His lab was a shed behind the medical school’s classrooms. He set to work and, between 1922 and 1927, produced nine papers and then a book, making his case not just that mitochondria derive from bacterial symbionts but also that such partnerships had repeatedly changed the course of the history of life.

  He coined a fancy term for this broad phenomenon, of which the bacterial origin of mitochondria was a cardinal instance: symbionticism. He defined symbionticism as an intimate and “absolute” symbiosis, one creature taking residence within the cells of another, in which the inner partner is always a bacterium. It was essentially the same as what Merezhkowsky had called symbiogenesis. Wallin wanted his own label. His penchant for coining new jargon, along with his vast claims about the implications of his idea, combined to earn him dismissal in his own time and footnote status in the longer view. In 1927, just after the series of articles, he published a compendium of his experimental findings and his theorizing as a book, Symbionticism and the Origin of Species. By now, you certainly recognize the echo in the title. He was implying, as Margulis would imply with her 1967 title, I walk in the footsteps of Darwin. He was suggesting, beyond that: My idea explains, even better than Darwin’s, the origin of diversity, complexity, and adaptation on Earth.

  Symbionticism, Wallin declared, was “the fundamental principle controlling the origin of species.” Darwin’s 1859 idea, natural selection, was secondary, determining only the retention or destruction of species once they have arisen. And there was a third force, an “unknown principle,” accounting for evolutionary progress toward better and more complex forms. Symbionticism brought the emergence of new species by creating radical points of divergence. Natural selection eliminated the worst, least-promising of those innovations. And what was that third, unknown principle? Wallin didn’t say.

  He was devoted to empirical evidence, except when he wasn’t. For that reason and others, Symbionticism and the Origin of Species, delivering its grandiose assertions, landed in January 1927 with a dull thud.

  “Dr. Wallin’s writings stirred up much interest, but little enthusiasm,” according to one friendly account. A conspicuous review, in the journal Nature, brought London disdain against Colorado noodling. Ivan E. Wallin “asks us to believe” that mitochondria are bacteria, and he claims that “the origin of species has taken place largely owing to the activity of these bacterial symbiotes.” That process, the reviewer sneered, “is called ‘symbionticism,’ a new and horrid word.” These pounding rejections seem to have flattened Wallin’s zeal for research, and for the next twenty-four years, until retirement, he contented himself with teaching anatomy.

  By the mid-1960s, Wallin’s ideas and those of Merezhkowsky and the other early seers of endosymbiosis had fallen beyond disrepute into forgetting. If you were a young biologist being educated at that time, you might never have heard those names or been exposed to those wild notions unless you happened to be at Wisconsin, taking a course from Hans Ris. One scientist that year, invoking the bygone theories of endosymbiosis for purposes of a broader argument, called them “certainly defunct.” A year later, the paper by Lynn Margulis (under her Sagan name) announced their comeback.

  The idea of mitochondria as captured bacteria traced to Wallin, and she knew it. Chloroplasts as captured bacteria traced to Merezhkowsky, and she knew it. In this 1967 paper, she added something more: another aspect of eukaryotic cells, possibly also originating from endosymbiosis. Her addition comprised three features she saw as related: the flagella of tiny swimming eukaryotes such as Euglena gracilis (on which she had done her dissertation); the cilia, little hairs that project from virtually every eukaryotic cell, including the cells of your body; and the centrioles, tiny structures in a cell, which I mentioned earlier. Flagella are threads that wiggle back and forth, poweri
ng single-celled organisms through liquid like the tail of a fish. Cilia (from the Latin for “eyelashes”) serve various important purposes in larger eukaryotes, including mammals: moving mucus and unwelcome debris along the windpipe, for instance. Centrioles are cylindrical bodies that help organize and distribute the chromosomes during cell division.

  Flagella, cilia, and centrioles share certain similarities, not just with one another but with bacteria of the group called spirochetes, which tend to be long, spiral, or corkscrewy in shape, capable of twisting motions that allow them to move. Can you see where this is going? Many spirochetes are parasitic, invading other creatures and, in humans, causing afflictions such as syphilis, yaws, leptospirosis, and Lyme disease. Margulis’s innovative idea was that these three other crucial mechanisms in eukaryotic cells—the flagella, the cilia, and the centrioles—are also descended from captured bacteria. Maybe something wiggly and mobile, she suggested, like a spirochete.

  She hypothesized that an ancient amoeboid creature, an early eukaryote, had acquired the wiggly thing by eating it. Or perhaps the wiggly thing had attached itself to the outside of the eukaryotic cell. Instead of being digested (if it was inside) or causing harm as an internal parasite, or being sloughed off (if it was externally attached), in at least one fateful case, it had become domesticated. It stuck, it stayed, it assimilated. Some of its genes, including those that coded for a particular structural feature Margulis noticed, were incorporated somehow into the coding genome of the host. Those genes were put to three purposes: building flagella, cilia, and centrioles, all of which helped lead the way to glorious new possibilities for the eukaryotic lineage.

  Spirochetes, with their bad reputation, commonly viewed as nasty pathogens, were a counterintuitive pick for partners in the rise of complex life. That wouldn’t have discouraged Lynn Margulis. Apart from the structural evidence, which she found so compelling, this idea had the merit of being outrageous.

  If it was true, it was vastly consequential. Some bacteria have simple flagella, by which they propel themselves through a liquid environment, moving clumsily toward attractions, clumsily away from repellants. But the flagella (and cilia) of eukaryotic cells are entirely different from those bacterial versions, using a different kind of motive power to produce a different kind of motion—potentially faster and more efficiently directed. Adding spirochetes as flagella and cilia to the outside of eukaryotic cells, if that’s what happened, might have been the first big step toward greater mobility and complexity. Cilia also facilitated, among other things, the movement of fluids along the internal surfaces of multicellular creatures. And the addition of centrioles, somehow derived from those spirochetes, Margulis thought, would have enabled the development of two new capacities: mitosis and meiosis. Systematic duplication and division of chromosomes. The words “enabling meiosis” may sound dull, so let me rephrase that: what we’re talking about is the invention of sex.

  The structural feature I alluded to, which persuaded Margulis that these three improvements are related to one another and to some spirochete-like bacterial symbiont, is simple. Imagine a thick utility cable, a main line. You cut that cable using an industrial hacksaw (turn off the cable power first) and view it in cross section. What you see inside are the cut ends of nine smaller cables arranged in a neat ring. That’s what Margulis saw by electron microscopy: flagella, cilia, and centrioles all shared that arrangement of nine tiny tubules, distinct in cross section and ordered radially like numbers on a clock. She deduced that eukaryotic cells had inherited that feature, commonly, after acquiring some ancestral spirochete as a symbiont, which then became cilia, flagella, and centrioles. There seemed no other plausible reason—happenstance? no—that each of those three anatomical features would have nine inner tubules, just nine, arranged in a circle. It was their mark of Cain, ineradicable. Furthermore, the flagella and cilia of eukaryotic cells both showed two additional tubules inside the neat ring of nine. She called that the (9+2) structure. The centrioles of cells, with no such central pair, had a (9+0) structure. Still close enough to be persuasive, Margulis reckoned. The whole nine-tubules arrangement may have come straight from the spirochete, she suggested, or else evolved early from that common ancestor. And the difference between flagella in bacteria and flagella in eukaryotes was so basic, so telling, she decided later, that it was necessary to give the latter a different name. She revived an old one. From 1980 onward, she referred to eukaryote flagella, and cilia along with them, as undulipodia. From the Latin undula and the Greek podos, it meant: “little waving feet.”

  You can almost picture them, those teeny undulipodious eukaryotes, waving their little feet at us to indicate kinship.

  There are two points worth noting about this bit of the story, which may otherwise seem rather arcane. The first is that Lynn Margulis was a cell biologist and a microbiologist of the old school, of the classical methods, meaning that she worked primarily from visual evidence with whole organisms: growing bugs in her lab, collecting new forms from the wild, peering at them through a light microscope or inspecting the electron micrographs produced by her colleagues. The recent improvements in electron microscopy, she said herself, had made some of these insights possible. She was also deeply knowledgeable about paleobiology, biochemistry, and geochemistry. Her books for a scientific audience, such as Origin of Eukaryotic Cells in 1970 and Symbiosis in Cell Evolution in 1981, are filled with graphic illustrations presenting her evidence in schematic form, as well as photos revealing the microscopic structures of all manner of living things, from purple sulfur bacteria to chloroplasts in a tobacco leaf, and from spirochetes in the hind gut of a termite to a centriole taking shape within a human cell. Browsing those images, looking deep into cell structure and the primordial beginnings of complex life, is enough to make you dizzy. To a person who’s no microbiologist—to you or to me—it’s like abstract art painted in protoplasm. But at least you won’t be dizzied by any long strings of letters such as AAUUUUCAUUCG. Molecular sequencing was not her métier. RNA catalogs were not her kind of data. Some of her signature work preceded the Woesean revolution in molecular phylogenetics, but even amid the revolution and afterward, she showed little interest in that sort of evidence.

  The second notable point is that Margulis claimed originality for only a single element of her three-element theory of endosymbiosis. “Every major concept presented in this book,” she wrote in her 1981 tome, “has been developed by others”—except one. She acknowledged Merezhkowsky and Wallin and various other early speculators; she credited E. B. Wilson and her old teacher Hans Ris with having alerted her to such precursors; she listed the many aspects of her own thinking that had previously been articulated by others—from mitochondria as captured bacteria to the role of endosymbiosis in triggering major evolutionary transitions. Having granted all that, she asserted her proprietary pride in just one idea: the bacterial origin of undulipodia, those little waving feet. It was she who had first imagined their origin—that they were spirochetes, serpentine parasites, which had come aboard for no benign purpose and, having gotten stuck, stayed to help.

  All these ideas can now be tested by the newer methods, she added.

  And so they were. Science marched forward, into the age of Woese, bringing new forms of supporting evidence and broadened credibility to some of her most radical ideas—but not to the one part that was hers alone.

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  From the cool distance of Halifax, Nova Scotia, Ford Doolittle regarded the wild ideas of Margulis and got interested. He decided they were worth testing.

  In the early 1970s, Doolittle was an assistant professor, barely thirty years old and lately arrived in the Department of Biochemistry at Dalhousie University, his position funded by a scholarship from the Medical Research Council, Canada’s version of the British agency that had supported Fred Sanger and Francis Crick. Doolittle wasn’t doing medical research, not in any applied sense, but that didn’t matter. He studied ribosomal RNA and its transcription from DNA w
ithin cells—especially an aspect called RNA maturation, the cutting of long, raw RNA molecules into those 16S and 5S and other sections for assembling the ribosome, which he had explored earlier during a postdoctoral fellowship in Denver. That postdoc was in the lab of a smart, young scientist roughly his own age, Norman R. Pace, who will figure later in this story. Biochemistry was Doolittle’s field, but now his curiosity was shifting toward evolution. Big questions: What were the major events in the history of life, how did complexity arise, how did the eukaryotic cell originate? Three factors converged to reroute his path, one of which was the Margulis book. The other two were cyanobacteria (still sometimes called blue-green algae, in those days) and a skilled assistant.

  Serious thinker though he is, Ford Doolittle often shows a certain whimsical detachment from the scientific enterprise and his own career within it. He spoke about all this in our several conversations. “When I came to Dalhousie, I was supposed to keep working on ribosomal RNA maturation,” he told me one day as we sat in his office. “Which is what I had done with Norm.” It was technical biochemistry, involving the isolation of certain enzymes. “And I’m not a biochemist.” Not by disposition, he meant. “I hate doing that kind of stuff.” In the department at Dalhousie, he met another fellow, a true biochemist, who was working with the cyanobacteria. “I thought, ‘My, what lovely color.’ ” They grew in nice varied shades of blue and green, he recalled, and they were “fun.” Wouldn’t it be pleasant to work on them?