a radical new theory of evolution -- 6/5/19
Today's selection -- from The Tangled Tree by David Quammen. In 1967, a young, little-known scientist named Lynn Margulis made a daring scientific proposal that was immediately and widely rejected: namely that the small organelles inside of cells had originally been separate bacteria that had taken up residence inside larger cells, which then became useful, symbiotic partners with that cell, and eventually simply part of the cell. It was a theory that radically rewrote evolutionary theory. Today, that theory is widely accepted.
"[Lynn] 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 [Dividing] Cells.' ...
"'This paper presents a theory,' [Lynn] Sagan [ne Margulis] wrote -- a theory proposing that 'the eukaryotic cell [cells that contain a nucleus and organelles] 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 chloroplasts of glaucophytes like this Glaucocystis have a peptidoglycan layer, evidence of their endosymbiotic origin from cyanobacteria.|
"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.
"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."