The Tangled Tree

by David Quammen

  That’s the notable takeaway from his 1981 Munich talk: it reflects Carl Woese’s compulsion to dig ever deeper into the narrative of life. He was a man possessed by the most deep-diving curiosity. This work he was doing, this door he had opened, this journey he was on—it wasn’t just about the Archaea, a third kingdom. It was about the origins and history of the other two kingdoms also. How did they arise? How did they diverge from one another? How were each of the three related to the two others? Which came first? Why did just one of the three lineages lead onward to all visible, multicellular organisms—all animals, all plants, all fungi, ourselves—while the other two remained unicellular and microscopic, though still vastly abundant, diverse, and consequential? And what kind of creature, or process, or circumstance preceded them all? Where was the tree of life rooted?

  Woese wasn’t interested just in this separate form of life he had chanced upon. He was interested in the whole story.

  Immediately after the workshop, which had gone well and given its participants a sense of momentum for the archaea concept, Kandler and his wife took Woese and Wolfe on a larkish field trip. They drove south from Munich into the Bavarian Alps and climbed a modest but picturesque mountain, the Hohe Hiss, along a graded path. “Woese and especially Wolfe were not in top physical shape, but with some huffing and puffing, they reached the top,” according to Ralph Wolfe’s own self-mocking account. At the summit, Kandler’s wife took a photo of the three men, all of them sunlit and contented on a clear day. Wolfe and Kandler appear as what they are: middle-aged scientists, balding, amiable, savoring a day outdoors. To their right sits Woese, with a full beard, leonine hair, a sweater tied jauntily over his neck, a cup of champagne in his left hand, smiling an easy, full smile of triumph. He was fifty-two years old, at the height of his powers and fame, and looked like a man on his way to a Nobel Prize.

  PART  III

  Mergers and Acquisitions

  28

  The entrance of Lynn Margulis into this story occurred abruptly, with some fanfare, at a time when Carl Woese still labored in obscurity. Margulis was a forceful young woman from Chicago. Her role proved important because it brought new attention and credibility to a very strange old idea: the idea that living ghosts of other life-forms exist and perform functions inside our very own cells. Margulis, adopting an earlier term, called that idea endosymbiosis. It was the first recognized version of horizontal gene transfer. In these cases, rare but consequential, whole genomes of living organisms—not just individual genes or small clusters—had gone sideways and been captured within other organisms.

  Margulis made her debut in March 1967 with a long paper in the Journal of Theoretical Biology, the same journal that had carried Zuckerkandl and Pauling’s influential 1965 article on the molecular clock. This paper was much different. Its author was no canonized scientist like Pauling, and its assertions were peculiar, to say the least. Put more bluntly: it was radical, startling, and ambitious, proposing to rewrite two billion years of evolutionary history. It included some cartoonish illustrative figures, funny little pencil-line drawings of cellular shapes, and virtually no quantitative data. According to one account, it had been rejected by “fifteen or so” other journals before a daring editor at JTB accepted it. Once published, though, the Margulis paper provoked a robust response. Requests for reprints (a measure of interest, back in those slow-moving days before online access to journals, when scientists mailed one another their articles) poured in. It was titled “On the Origin of Mitosing Cells.”

  That was a quiet phrase for a huge subject, though the title’s echoes of Darwin’s On the Origin of Species suggest the loud aspirations of the paper’s author. Never short of confidence, she was twenty-nine years old at that time, an adjunct assistant professor at Boston University, and a single mother raising two boys. She had been married as a teenager to a flashy young astronomer and, for the moment, was still keeping his surname. Her authorship on the paper read: Lynn Sagan. Later, she would be famous—venerated by some, dismissed and disparaged by others, including Carl Woese—under the surname of her second husband, Thomas N. Margulis. But to many of those who knew her, she was always and informally: Lynn.

  The phrase “mitosing cells” is another way of saying eukaryotic cells, the ones with nuclei and other complex internal structures, the ones that compose all animals and plants and fungi (as well as some other intricate life-forms, less familiar because they’re microscopic). “Mitosing” refers to mitosis, of course, the phase in eukaryotic cell replication at which the chromosomes of the nucleus duplicate, then split apart into two bundles within two new nuclei, as a prelude to the cell fissioning into two complete new cells, each with an identical set of chromosomes. You learned about it in high school biology, not long before you dissected the poor frog. Mitosis is taught along with meiosis, the yang to its yin. Mitosis occurs during ordinary cell division, whereas meiosis constitutes “reduction division,” yielding the specialized sex cells known as gametes (eggs and sperm in an animal, eggs and pollen in a flowering plant). Meiosis in an animal yields four new cells, not two, after two divisions, not one, each resulting cell reduced to a half share of chromosomes. Later, sperm will meet egg, and, bingo, the full measure will be restored. It’s a little hard to remember which of those terms is which, I concede, but here’s my mnemonic: meiosis is reduction division because its spelling is reduced by the loss of the t in mitosis. Helpful? Granted, that leaves the inconvenient fact of meiosis containing the addition, not reduction, of an e. So, okay, never mind. But it works for me.

  Mitosis defines all the cell divisions by which a single fertilized egg grows into a multicellular embryo and then an adult, and also by which worn-out cells are replaced with new cells. Your skin cells, for instance. The cells of a scar when a wound heals. The cells that replace your worn-out colon lining. Mitosis occurs everywhere in a body. Meiosis, by contrast, occurs only in the gonads. Lynn Sagan’s paper, though, wasn’t focused on mitosis as an ongoing process. The key word in her title was origin.

  Her interest was the deep history, to the beginning, of eukaryotic cells. She quoted the statement from Roger Stanier and his textbook coauthors, declaring that the prokaryote-eukaryote distinction “probably represents the greatest single evolutionary discontinuity to be found in the present-day living world.” It was the biggest leap in the history of life—an Olympic long jump, a high jump, a backward slam dunk—forever reflected in the differences between bacteria and more complex organisms. She proposed to explain how that leap happened.

  “This paper presents a theory,” Sagan wrote—a theory proposing that “the eukaryotic cell is the result of the evolution of ancient symbioses.” Symbiosis: the living together of two dissimilar organisms. She gave her theory the more specific name endosymbiosis, connoting one organism resident inside the cells of another and having become, over generations, a requisite part of the larger whole. Single-celled creatures had entered into other single-celled creatures, like food within stomachs, or like infections within hosts, and by happenstance and overlapping interests, at least a few such pairings had achieved lasting compatibility. So she proposed, anyway. The nested partners had grown to be mutually dependent, staying together as compound individuals and supplying each other with certain necessities. They had replicated—independently but still conjoined—passing that compoundment down as a hereditary condition. Eventually they were more than partners. They were a single new being. A new kind of cell.

  No one could say, not in 1967, how many times such a fateful combining had occurred during the early eras of life, but it must have been very rare that the resultant amalgams survived for the long term. Later, there would be ways of addressing that question. Sagan left it open. Microscopy, which was her primary observational mode of research, couldn’t answer it.

  The little entities on the inside of such cells had begun as bacteria, she argued. They had become organelles—working components of a new, composite whole, like the liver or spleen insi
de a human—with fancy names and distinct functions: mitochondria, chloroplasts, centrioles. Mitochondria are tiny bodies, of various shapes and sizes but found in all complex cells, that use oxygen and nutrients to produce the energy packets (molecules known as adenosine triphosphate, or ATP) for fueling metabolism. ATP molecules are carriers of usable energy, like rechargeable AA batteries; when the ATP breaks into smaller pieces, that energy is released for use. Mitochondria are factories that build (or recharge) ATP molecules. To drive the production, mitochondria respire, like aerobic bacteria. Chloroplasts are little particles—green, brown, or red—found in plant cells and some algae, that absorb solar energy and package it as sugars. They photosynthesize, like cyanobacteria. Centrioles are crucial too, but for now, I’ll skip the matter of how. All these components, Sagan wrote, resemble bacteria by no coincidence but rather for a very good reason: because they evolved from bacteria.

  The bigger cells, within which the littler cells were subsumed, had been bacteria too (or possibly archaea, though that distinction didn’t exist at the time). They were the hosts for these endosymbioses. They had done the swallowing, the getting infected, the encompassing, and had offered their innards as habitat. The littler cells, instead of being digested or disgorged, took up residence and made themselves useful. The resulting compound individuals were eukaryotic cells.

  Never mind that “compound individuals” is oxymoronic. The whole process, as Sagan described it, was oxymoron brought to life—paradoxical and counterintuitive, though supported throughout the paper by her detailed arguments.

  Paradox is enticing, but was it real? Was it right? Had this adjunct assistant professor presented not just an astonishing cluster of possibilities but also a persuasive new vision of the origins of all complex life? The scientific consensus at first, and for some years afterward, was no. The early read on Lynn Sagan, soon to be Lynn Margulis, held that she was smart, knowledgeable, insistent, charming, and in thrall of a loony idea.

  29

  She was born and raised in Chicago, the eldest daughter of Morris and Leone Alexander, her father an attorney who also owned a paint company, her mother a homemaker who also ran a travel agency—enterprising and versatile people. Lynn Alexander was precocious but also a “bad student,” by her own account, at least bad in behavior, enough so that she stood in the corner a lot. (Hard to know whether to take that literally. Later in life, she stood in remote corners of the scientific community often, proudly, and by choice.) Brilliant and impatient, she switched schools, worked through a little adolescent revolt, and was good enough to get early admission to the University of Chicago as a young teenager. She loved her experience there—more particularly her experience at the College, as the undergrad school was known, a haven of broad learning within the university, its system of pedagogy shaped by the educational visionary Robert Maynard Hutchins. She thrived in an introductory course, Natural Science 2, that had her reading not textbooks but the seminal writings of great scientists themselves: Darwin, Weismann, Gregor Mendel, and J. B. S. Haldane, among others. One day in her freshman year, as she bounded up the steps of the mathematics building, she literally ran into Carl Sagan, then a nineteen-year-old graduate student in physics. He was tall, handsome, articulate, and polished, already something of a public figure on campus. “I was a scientific ignoramus,” she would recall. “Carl, and especially his gift of gab, fascinated me.” Three years later, one week after her graduation, she married him and became Lynn Sagan. Photos from the event show her as a small, pretty young woman, bare shouldered in a white gown and pearls, with a dangerous smile.

  She accompanied Sagan to Wisconsin, where he continued his grad work at an observatory while she started a master’s degree at the University of Wisconsin. That’s where she met Hans Ris, a professor in the Zoology Department, who taught her microscopy.

  Ris was “a fine teacher—the best of my whole career,” she wrote later. She took his class on cell biology, probably in 1959, while she was pregnant with her first son (who would become the writer Dorion Sagan). In addition to microscopy, Ris seems to have given her more: a pathway, from obscure earlier sources and his own research and thinking, to her theory of endosymbiosis. Some testimony to his influence remains, like fossil fragments among the rocks of a dusty gorge, in the references at the end of her 1967 paper. There she cited a paper coauthored by Ris a few years before, and among many other citations, she included work by two unconventional scientists from the early twentieth century, a Russian and an American: Constantin Merezhkowsky (whom Ris had also cited) and Ivan E. Wallin. These men anticipated some of the ideas that, pulled together later by Lynn Margulis and affirmed still later with molecular evidence, would radically change the understanding of how complex organisms arose.

  Hans Ris was a Swiss-born cell biologist and biochemist who, coming to the University of Wisconsin in 1949, had reinvented himself as an electron microscopist. In the early 1960s, with his colleague Walter Plaut from the Botany Department, he used microscopy and biochemical methods to investigate chloroplasts, the tiny cell organelles found in plant cells and some algae, enabling them to harvest solar energy by photosynthesis. What are these chloroplast things, Ris and Plaut wondered, and what’s their origin? The two men looked closely at the chloroplasts of a certain green alga. With biochemical staining, they found evidence of DNA. They could see it with their electron microscope.

  This was important because it suggested the possibility that genes can exist in the cytoplasm (the stew of liquids and solids inside a eukaryotic cell that includes everything but the nucleus) and not just in the nucleus itself. Genes in the cytoplasm were previously considered improbable if not impossible—except by a few earlier researchers. Chromosomes resided in the nucleus, protected there within the nuclear membrane, and that was thought to be that. Cytoplasmic inheritance, if it was real, represented an exception to the reliable rules of Mendelian inheritance, as articulated by Mendel, the Moravian monk who discovered those rules by crossbreeding peas. In Mendelian inheritance, determined by a sex cell from each parent, merging at fertilization, both parents contribute equally to the genetic makeup of the offspring. Cytoplasmic inheritance (also called maternal inheritance) was a very different proposition. If it existed, it wouldn’t be so neatly Mendelian. It wouldn’t be so binary. If genes were afloat in cytoplasm, that would tilt inherited genetic identity toward the female parent, in any sexual reproduction, because eggs carry a lot of cytoplasm, and sperm or pollen carry little.

  But that wasn’t the half of it. Ris and Plaut’s detection of DNA in chloroplasts, within the cytoplasm of green algae, had even larger implications than challenging Mendel. Those implications pointed toward endosymbiosis, a wholly unorthodox vision of the origins of complex life.

  By electron microscopy, Ris and Plaut detected aspects of their algal chloroplasts that closely resembled what was seen in certain bacteria: DNA fibrils, a double membrane, and other structural features. Specifically, these chloroplast traits seemed to match with the microbe group soon to be renamed cyanobacteria. This suggested that the chloroplasts, in a sense, were bacteria; or, at least, that they once had been. It suggested that cyanobacteria had been swallowed or otherwise internalized somehow in the deep past, that some or at least one of those captures had resisted digestion or ejection, that it had replicated inside a host cell, that those replicates were inherited through the lineage of algal cells, and that they gradually transmogrified from undigested prey, infectious bug, or neutral passengers to internal organelles. They survived and proliferated (by Darwinian selection) because they served a function: allowing the algae to derive energy from sunlight. Their role, as organelles, was photosynthesis. All this agreed with an old hypothesis, Ris and Plaut noted, that Constantin Merezhkowsky had proposed back in 1905. Merezhkowsky had been considered half crazy in his time. (In fact, half crazy and worse, as you’ll see.) But now, Ris and Plaut wrote, “endosymbiosis must again be considered seriously as a possible evolutionary step in the o
rigin of complex cell systems.”

  The paper by Ris and Plaut appeared in 1962, soon after young Lynn Sagan left Wisconsin to continue her life and studies elsewhere. She not only saw the paper and read it but also, from her personal contact with Ris, had presumably known it was coming. According to one account, she was introduced to these radical notions even earlier, and more directly, by Hans Ris himself. A classmate of hers in the Ris course on cell biology, back in 1959, remembers Ris laying out a very full treatment of endosymbiosis, along with supporting literature on its various aspects from obscure German and Russian sources, which Ris had pulled together. This classmate, Jonathan Gressel, now an emeritus professor of plant genetics at the Weizmann Institute of Science in Israel, says that “the theory was completely Ris’s idea, well set out in his course. She did a great job of promulgating it.” Gressel was friendly with Lynn Sagan at the time, and he recalls her “struggling to reach the microscope,” game but slightly impeded, when she was heavily pregnant with Dorion. Later, he was “aghast” that she hadn’t given Ris fuller acknowledgment for assembling the theory.