A tribute to Lynn Margulis, evolutionary biologist, 1938-2011.
Why the devil is symbiosis so annoying? While other biologists believed that species only diverge from one another, she claimed that, no, species formed new composite entities by fusion and merger. She took rather a longer view.
The First Three Billion Years
“Evolutionists have been preoccupied with the history of animal life in the last 500 million years,” Dr. Margulis wrote in 1995. “But we now know that life itself evolved much earlier than that. The fossil record begins nearly 4,000 million years ago! Until the 1960s, scientists ignored fossil evidence for the evolution of life, because it was uninterpretable."
I know that my readers are will be champing at the bit for Zenobia's interpretation of the "uninterpretable". But before I am led into interpretation, let's review the background.
Darwin vs Mendel
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 re-assort, 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 white or pink as the original parent or grandparent. The genes are simply shuffled around to come out in different combinations, but those same combinations generate exactly the same types.
Neo-Darwinism attempts to reconcile Mendelian genetics -- which says that organisms do not change with time -- with Darwinism, which claims they do. The neo-Darwinists square this circle by saying that variation originates from random mutation, defining mutation as any genetic change. Mutation was thus touted as the source of variation -- that upon which natural selection acted .
Of course, inherited variants do appear spontaneously but they have nothing to do with whether or not they're good for the organism in which they appear. It is known from many experiments that, for example, even if fruit flies are isolated completely from X rays, solar radiation, and other environmental upsets, spontaneous mutations will still occur. But the result of such mutation is always sick or dead flies. No new species of fly appears — that is the rub. Everyone agrees that such mutagens produce inherited variation. Everyone agrees that natural selection acts on this variation.
The question is: From where comes the useful variation upon which selection acts? Does natural selection operate at the level of the gene, the organism, or the species, or all three?
In the beginning there were single cells and micro-organisms
Zoology, according to Lynn Margulis, is simply three billion years too late. Animals (including, of course, people) arrive very late on the evolutionary scene. Thus, they provide little real insight into the major sources of evolution's development.
In cell evolution, on the other hand, the great event was the appearance of the membrane-bound nucleated (eukaryotic) cell — the cell upon which all larger life-forms are based. Nearly forty-five years ago, Margulis argued for its symbiotic origin: that it arose by associations of different kinds of bacteria. Her ideas were generally either ignored or ridiculed when she first proposed them. Now, symbiosis in cell evolution is considered one of the great scientific breakthroughs.
From bacteria to bugs
For more than a billion years, the only life on this planet consisted of bacterial cells, which lack nuclei. They looked very much alike, and from the human vantage point seem boring. However, bacteria are the source of reproduction, photosynthesis, movement — indeed, almost all the interesting features of early life.
The criteria that we use for species of animals and plants and fungi simply do not apply to bacteria:
Bacteria are much more of a continuum. They drop their genes all the time. It's like going swimming in a swimming pool, going in blue-eyed and coming out brown-eyed, just because you've gulped the water. That's what bacteria do, all the time. They just pick up genes, they throw away genes, and they are very flexible about that.
Say you have a bacterium like Azotobacter. This is a nitrogen-fixing bacterium. It takes nitrogen out of the air and puts it into useable food. Nitrogen fixing is a big deal. It takes a lot of genes. If you put a little something like arsenium bromide in a test tube with these organisms, and put it in a refrigerator overnight, lo and behold, the next day the cells can't do this any more, they can't fix nitrogen. So by definition you have to change them from one genus to another.
I'll give you another example: E.coli. It's a normal inhabitant of the human gut. If you put a particular plasmid into E.coli, all of a sudden you have Klebsiella and not E. coli. You've changed not only the species, but the genus. It's like changing a person to a chimpanzee. Can you imagine doing that, putting a chimpanzee in the refrigerator, and getting him out the next morning, and now he's a person?
Mitochondria and More Stuff
Mitochondria are wriggly bodies that generate the energy required for metabolism. To Margulis, they looked remarkably like bacteria. There were parallel examples in all plant cells. Algae and plant cells have a second set of bodies (chloroplasts) that they use to capture incoming sunlight energy in photosynthesis. Chloroplasts, like mitochondria, bear a striking resemblance to bacteria. She became convinced that chloroplasts and mitochondria evolved from symbiotic bacteria — specifically, that they descended from cyanobacteria, the light-harnessing small organisms that abound in oceans and fresh water.
Margulis spent much of the rest of the 1960s honing her argument that symbiosis was an unrecognized major force in the evolution of cells. Needless to say, no one believed her. The manuscript in which she first presented her findings was rejected by 15 journals before being published in 1967 by the Journal of Theoretical Biology. After ten years of research, she produced a book called the Origin of Eukaryotic Cells, with additional evidence to support the theory. Even under contract, it was rejected by Academic Press. Finally, in 1970, the revised work was published by Yale University Press as Symbiosis in Cell Evolution.
Now, it is orthodox biology to argue that symbiotic events had a profound impact on the organization and complexity of many forms of life. Nucleated cells are more like tightly knit communities than single individuals. Evolution is more flexible than was once believed.
Symbiosis is a physical association between organisms, the living together of organisms of different species in the same place at the same time. From the beginning, I was curious about these unruly genes that weren't in the nucleus. The most famous of them was a cytoplasmic gene called "killer," which, in the protist Paramecium aurelia, followed certain rules of inheritance. The killer gene, after twenty years of intense work and shifting paradigmatic ideas, turns out to be in a virus inside a symbiotic bacterium. Nearly all extranuclear genes are derived from bacteria or other sorts of microbes. In the search for what genes outside the nucleus really are, I became more and more aware that they're cohabiting entities, live beings. Live small cells reside inside the larger cells.Her contention is that "symbiogenesis" — long-term symbioses that lead to new forms of life — has occurred and is still occurring. Symbiogenesis, as she proposed, is the result of long-term living together — staying together, especially involving microbes -- and that it's the major evolutionary innovator in all lineages of larger nonbacterial organisms.
Dr Margulis argued that the eukaryotic (nucleated) cell -- which includes all the cells in the human body -- appeared because of symbiogenesis, that is, though a transformation of what started out as a parasitic infestation of one cell by another.
"The long-lasting intimacy of strangers
This hypothesis was a direct challenge to the neo-Darwinist belief that the primary evolutionary mechanism was random mutation.
The theory undermined significant precepts of the study of evolution, underscoring the idea that evolution began at the level of micro-organisms long before it would be visible at the level of species. Symbiosis, she argued, was a more important mechanism; that is, evolution is a function of organisms that are mutually beneficial growing together to become one and reproducing.
"Gaia is a tough bitch"
Dr. Margulis was also, somewhat controversially, a supporter of James E. Lovelock, whose Gaia theory states that Earth itself — its atmosphere, the geology and the organisms that inhabit it — is a self-regulating system, maintaining the conditions that allow its perpetuation. In other words, it is something of a living organism in and of itself. She agreed with a weaker version of this theory:
In the early seventies, I was trying to align bacteria by their metabolic pathways. I noticed that all kinds of bacteria produced gases. Oxygen, hydrogen sulfide, carbon dioxide, nitrogen, ammonia — more than thirty different gases are given off by the bacteria whose evolutionary history I was keen to reconstruct. Why did every scientist I asked believe that atmospheric oxygen was a biological product but the other atmospheric gases — nitrogen, methane, sulfur, and so on — were not?
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| Lynn Margulis, wearing her National Medal of Science Award |
Cooperation or Competition
Lynn Margulis never made an issue of being a woman in science. Still, it can't be entirely coincidental that her work stressed cooperation over competition as a major factor in evolutionary history, something still difficult for men of science to handle. That may be why the "survival of the fittest" and "nature red in tooth and claw" crowds are still so dug in. The extraordinary thing, surely, is that what began as competition evolved into what is fundamentally a cooperative arrangement. That's its beauty. Of course, it doesn't show that cooperation is the norm or that cooperation is always good or that it's always possible:
The problem is NOT "competition versus cooperation". Those words are totally inappropriate for life. The language of life is metabolic chemistry. Even bankers and sports teams have to cooperate in order to compete. It's crucial to realize that it doesn't matter what team you're on, when you compete, even in sports where the term is valid, you still cooperate!The last word in the debate, as always, belongs to Prof. Margulis:
The Gaia hypothesis is a biological idea, but it's not human-centered. Those who want Gaia to be an Earth goddess for a cuddly, furry human environment find no solace in it. They tend to be critical or to misunderstand. They can buy into the theory only by misinterpreting it. Yes, Gaia will take care of itself; yes, environmental excesses will be ameliorated, but it's likely that such restoration of the environment will occur in a world devoid of people.Lynn Margulis was not shy about expressing her opinions. Her in-your-face, take-no-prisoners stance was pugnacious and tenacious. She was impossible. She was wonderful.***
She died last week after suffering a hemorrhagic stroke. More than just the world of science will miss her.
Updated 28 December 2011
From ScienceDaily news 22/12: Evidence of the single-celled ancestors of animals, dating from the interval in Earth's history just before multicellular animals appeared, has been discovered in 570 million-year-old rocks from South China.
Left: 570 million year old multicellular spore body undergoing vegetative nuclear and cell division (foreground) based on synchrotron x-ray tomographic microscopy of fossils recovered from rocks in South China. The background shows a cut surface through the rock - every grain (about 1 mm diameter) is an exceptionally preserved gooey ball of dividing cells turned to stone. (Credit: Image courtesy of University of Bristol)
Lynn Margulis would have loved this news.
* W. Daniel Hillis, The Third Culture; quoted by Susan Mazur, Scoop, 16 March 2009.
** Daniel C. Dennett, on Edge.Org.
*** John Brockman, on Edge.Org.
This post is dedicated to my room-mate from my Oxford days, the immunologist Dr Dr Susan Carson.
Major sources for this post: Edge obituary 11/23/2011; the History of Evolutionary Thought, Berkeley: Lynn Margulis; Bruce Weber, Obituary, New York Times, Nov. 24, 2011; Suzan Mazur, Interview on Scoop: Lynn Margulis: Intimacy Of Strangers & Natural Selection, 16 March 2009; Astrobiology Magazine, Part II: We are all microbes, and Part III: Bacteria don't have species.
Illustrations
Top left: via Edge.orghttp://edge.org/conversation/lynn-margulis1938-2011
Centre: Endosymbiosis: Lynn Margulis, Berkeley.edu
Middle left: via Scoop
Lower left: Paul Hosefos/The New York Time
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