The Tangled Tree: A Radical New History of Life

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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 inside 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.

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