Lynn Margulis

Lynn Margulis

Lynn Margulis (March 15, 1938 – November 22, 2011) was a biologist and university professor who pioneered important concepts in the fields of cell biology and microbial evolution. She perhaps is best known for her contributions to the endosymbiotic theory, which is now generally accepted for how certain eukaryotic organelles were formed.

The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts). According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside another cell as endosymbionts. Both the host cells and the symbionts would have received advantages from the symbiotic relationship, and this would have eventually led to their integration. The fact that mitochondria and plastids have their own DNA and ribosomes is one of many supports for this theory. Mitochondria are considered to have developed from proteobacteria and chloroplasts from cyanobacteria.

Margulis sees symbiogenesis—the development of new organisms, organelles, and so forth from the merging of two separate organisms—as a fundamental factor in creating genetic variation (more so than mutation) and as a primary force in evolution. In general, and in contrast to neo-Darwinism, Margulis holds that "Life did not take over the globe by combat, but by networking" (Margulis and Sagan 1986)—in other words, more by cooperation than via Darwinian competition.

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Margulis was also an important collaborator with James Lovelock in developing the concepts related to the Gaia hypothesis. The Gaia hypothesis is a class of scientific models of the geo-biosphere in which life as a whole fosters and maintains suitable conditions for itself by helping to create a favorable environment on Earth for its continuity. The Gaia hypothesis addresses the remarkable harmony seen between biotic and abiotic elements on Earth; similarly, the endosymbiotic theory touches on the harmony among biotic elements.

Biography

Lynn Margulis attended the University of Chicago as an undergraduate, graduating with an A.B. in Liberal Arts in 1957, when she was only 19 years old. In 1960, she received her M. S. degree in genetics and zoology from the University of Wisconsin. Margulis received her Ph.D. in Genetics in 1963, from the University of California, Berkeley.

In 1966, Margulis took a position in the Biology Department at Boston University, where she was working when she did her pioneering work on cellular evolution and the endosymbiotic theory. She also collaborated there with Dr. James Lovelock on the Gaia hypothesis.

In 1988, Margulis took a position at the University of Massachusetts.

Margulis also has served as Chair of the National Academy of Science's Space Science Board Committee on Planetary Biology and Chemistry Evolution (1977-1980). In 1983, she was elected a member of the U.S. National Academy of Sciences (Sehi 2001).

Margulis held two fellowships after completing her doctoral degree, the Sherman Fairchild Fellowship in the Geological and Planetary Sciences Department at California Institute of Technology (1977) and the Guggenheim Fellowship for her work on microbial mats (Sehi 2001). Among the many awards Margulis has received are eight honorary doctorates (by 2001), and induction into the World Academy of Art and Science (1995), the Russian Academy of Natural Sciences (1997), and the American Academy of Arts and Sciences (1998) (Sehi 2001). In 1999, she was awarded the National Medal of Science. Margulis is profiled in a book published in 2006, by Resurgence Magazine in the United Kingdom, called Visionaries: The 20th Century's 100 Most Important Inspirational Leaders.

Margulis is the author or co-author of numerous articles and books, including the books, Symbiotic Planet: A New Look at Evolution (1998), Acquiring Genomes: A Theory of the Origins of Species (2002), What is Sex? (1997), What is Life? (1995), and Microcosmos: Four Billion Years of Evolution From our Microbial Ancestors (1986).

Margulis was the first wife of astronomer Carl Sagan and is the mother of Dorion Sagan, popular science writer and co-author; Jeremy Sagan, software developer and founder of Sagan Technology; Zachary Margulis-Ohnuma, New York City Criminal Defense lawyer; and Jennifer Margulis, teacher and author. In 2006, with her son Dorion, Margulis founded Sciencewriters Books, an imprint of Chelsea Green Publishing for science books.

Margulis died on November 22, 2011 at home in Amherst, Massachusetts.

Theoretical contributions

In 1966, as a young faculty member at Boston University, Margulis wrote a theoretical paper titled The Origin of Mitosing Eukaryotic Cells (Sagan 1967). The paper, however, was "rejected by about fifteen scientific journals," Margulis recalled (Brockman 1995). It was finally accepted by The Journal of Theoretical Biology and is considered today a landmark in modern endosymbiotic theory.

Although this article draws heavily on symbiosis ideas first put forward by scientists in the mid-nineteenth century, as well as the early twentieth century work of Merezhkovsky (1905) and Wallin (1920), Margulis's endosymbiotic theory formulation is the first to rely on direct microbiological observations (as opposed to paleontological or zoological observations, which were previously the norm for new works in evolutionary biology). Weathering constant criticism of her ideas for decades, Margulis is known for her tenacity in pushing her theory forward, despite the opposition she faced at the time.

The underlying theme of endosymbiotic theory, as formulated in 1966, was interdependence and cooperative existence of multiple prokaryotic organisms; one organism engulfed another, yet both survived and eventually evolved over millions of years into eukaryotic cells. Her 1970 book, Origin of Eukaryotic Cells, discusses her early work pertaining to this organelle genesis theory in detail. Currently, her endosymbiotic theory is recognized as the key method by which some organelles have arisen and is widely accepted by mainstream scientists. The endosymbiotic theory of organogenesis gained strong support in the 1980s, when the genetic material of mitochondria and chloroplasts was found to be different from that of the cell's nuclear DNA (Sehi 2001).

Symbiogenesis is the general term used for the merging of two separate organisms to form a single new organism. In Acquiring Genomes: A Theory of the Origins of Species, published in 2002, Margulis argues that symbiogenesis is a primary force in evolution; that is, symbiotic relationships between organisms of often different phyla or kingdoms are the driving force of evolution.

This concept challenges a central tenet of neodarwinism that inherited variation mainly comes from random mutations. According to Margulis' theory, acquisition and accumulation of random mutations are not sufficient to explain how inherited variations occur. Rather, Margulis argues that genetic variation occurs mainly as the result of the transfer of nuclear information between organisms. New organelles, bodies, organs, and species arise from symbiogenesis, evolving primarily through relationships between organisms, involving the fusion of genomes.

Whereas the classical interpretation of evolution, (neo-Darwinism), emphasizes competition as the main force behind evolution, Margulis emphasizes cooperation as the most important factor in the development of life.

While Margulis' organelle genesis ideas are widely accepted, her further hypothesis that symbiotic relationships are a current method of introducing genetic variation is not considered to be mainstream in evolutionary theory. Nonetheless, examination of the results from the Human Genome Project lends credence to an endosymbiotic theory of evolution—or at the very least it positions Margulis's endosymbiotic theory to serve as catalyst for generating ideas about the origins of the current composition of the human genome. From the perspective of the endosymbiotic theory, significant portions of the human genome are apparently either bacterial or viral in origin—with some clearly being ancient insertions, while others are more recent in origin. This strongly supports the idea of the close association of organisms, with symbiotic, or more likely parasitic relationships, being a driving force for genetic change in humans, and likely all organisms.

Overall, while many ecologists agree with Margulis's emphasis on symbiosis as a driving force of evolution, this idea has little support from other evolutionary biologists. They see little evidence that symbiogenesis has had a major impact on eukaryotic life, or that much of its diversification can be attributed to it, other than the two examples of mitochondria and chloroplasts. It is a fundamental principle of classical neo-Darwinism, or population genetics theory, that mutations arise one at a time and either spread through the population or not, depending on whether they offer an individual fitness advantage. Nevertheless, the neo-Darwinist perspective remains vulnerable to challenges like that of Margulis because its experimental support comes overwhelmingly from the laboratory, not from the wild. It is understood clearly how artificial selection works in the laboratory, but there is legitimate controversy over whether nature's laboratory works in just this way. Indeed, genome mapping techniques have revealed that family trees of the major taxa appear to be extensively cross-linked—possibly due to lateral transfer of genes carried by bacteria, as Margulis predicted.

It should be noted that while the endosymbiotic theory has often been presented as being fundamentally opposed to the neo-Darwinian model, the two theories are not incompatible. Nonetheless, Margulis holds a generally negative view of neo-Darwinism, as she believes that history will ultimately judge the theory as "a minor twentieth century religious sect within the sprawling religious persuasion of Anglo-Saxon Biology" (Mann 1991). She also believes that proponents of the standard theory, "wallow in their zoological, capitalistic, competitive, cost-benefit interpretation of Darwin—having mistaken him… Neo-Darwinism, which insists on (the slow accrual of mutations), is a complete funk" (Mann 1991).

Margulis' present day efforts, in the form of books and lectures, strongly stress a symbiotic—and cooperative—relationship between all organisms and a strong leaning toward Gaia theory. Her advocacy outside the realm of biology and toward more sociopolitical ends has been criticized by more mainstream scientists—somewhat similar to criticisms aimed toward Carl Sagan's latter day ideas.

Margulis remains a leading figure in cell biology and evolutionary theory. In 1995, prominent neo-Darwinist evolutionary biologist Richard Dawkins had this to say about Lynn Margulis and her work:

I greatly admire Lynn Margulis's sheer courage and stamina in sticking by the endosymbiosis theory, and carrying it through from being an unorthodoxy to an orthodoxy. I'm referring to the theory that the eukaryotic cell is a symbiotic union of primitive prokaryotic cells. This is one of the great achievements of twentieth-century evolutionary biology, and I greatly admire her for it (Brockman 1995).

Endosymbiotic theory

The endosymbiotic theory holds that the origin of mitochondria and plastids (e.g. chloroplasts)—which are organelles of eukaryotic cells—traces to separate prokaryotic organisms that were taken inside the cell as endosymbionts.

The endosymbiotic hypothesis suggests that mitochondria descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that had relied on glycolysis and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiotic bacteria capable of photosynthesis would also have had an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

The fact that mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, supports this view. Studies of mitochondrial DNA, which is circular and employs a variant genetic code, suggest their ancestor was a member of the Proteobacteria (Futuyma 2005) and probably related to the Rickettsiales.

Although this symbiotic merger is thought to have occurred at least two billion years ago, mitochondria still show some signs of their ancient origin. Mitochondrial ribosomes are the 70S (bacterial) type, in contrast to the 80S type ribosomes found elsewhere in the cell. (The unit "S," Svedberg unit, is a measure of size based on sedimentation rate in a centrifuge. The units are not additive because sedimentation rate is a function of mass in relation to surface area.) The mitochondrial DNA, like the DNA in prokaryotes, comprises a high proportion of coding DNA and an absence of repeats, unlike the nuclear DNA of eukaryotic cells, which tends to include a lot of noncoding DNA as well as repeating DNA along with the coding segments. Mitochondrial genes are transcribed as multigenic transcripts that are cleaved and polyadenylated to yield mature mRNAs. Unlike their nuclear cousins, mitochondrial genes are small, generally lacking introns (sections of DNA that will be spliced out after transcription, but before the RNA is used), and the chromosomes are circular, conforming to the bacterial pattern. Similar analyses are made of chloroplasts, which have their own DNA and ribosomes.

The endosymbiotic theory was first articulated by the Russian botanist Konstantin Mereschkowsky in 1905 (Mereschkowsky 1905). Mereschkowsky was familiar with work by the German botanist Andreas Schimper, who had observed in 1883 that the division of chloroplasts in green plants closely resembled that of free-living cyanobacteria, and who had himself tentatively proposed (in a footnote) that green plants had arisen from a symbiotic union of two organisms (Schimper 1883). Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s (Wallin 1923). These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris (Ris and Siggh 1961), combined with the discovery that plastids and mitochondria contain their own DNA (Stocking and Gifford 1959) led to a resurrection of the idea in the 1960s.

It was Lynn Margulis who fleshed out and popularized the endosymbiotic hypothesis, beginning in 1966. In her 1981 work, Symbiosis in Cell Evolution, Margulis argued that eukaryotic cells originated as communities of interacting entities, including endosymbiotic spirochaetes that developed into eukaryotic flagella and cilia. This last idea has not received much acceptance, since flagella lack DNA and do not show ultrastructural similarities to prokaryotes.

Important publications

  • Margulis, Lynn and Dorion Sagan. 2007. Dazzle Gradually: Reflections on the Nature of Nature. Sciencewriters Books. ISBN 9781933392318
  • Margulis, Lynn and Eduardo Punset, eds. 2007. Mind, Life and Universe: Conversations with Great Scientists of Our Time. Sciencewriters Books. ISBN 9781933392615
  • Margulis, Lynn. 2007. Luminous Fish: Tales of Science and Love. Sciencewriters Books. ISBN 9781933392332
  • Margulis, Lynn and Dorion Sagan. 2002. Acquiring Genomes: A Theory of the Origins of Species. Perseus Books Group, ISBN 0465043917
  • Margulis, Lynn, et al. 2002. The Ice Chronicles: The Quest to Understand Global Climate Change. University of New Hampshire. ISBN 1584650621
  • Margulis, Lynn. 1998. Symbiotic Planet: A New Look at Evolution, Basic Books, ISBN 0465072712
  • Margulis, Lynn and Karlene V. Schwartz. 1997. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. W.H. Freeman & Company. ISBN 0613923383
  • Margulis, Lynn and Dorian Sagan. 1997. What Is Sex? Simon and Shuster. ISBN 0684826917
  • Margulis, Lynn and Dorion Sagan. 1997. Slanted Truths: Essays on Gaia, Symbiosis, and Evolution. Copernicus Books. ISBN 0387949275
  • Margulis, Lynn. 1992. Symbiosis in Cell Evolution: Microbial Communities in the Archean and Proterozoic Eons. W.H. Freeman. ISBN 0716770288
  • Margulis, Lynn, ed. 1991. Symbiosis as a Source of Evolutionary Innovation: Speciation and Morphogenesis. The MIT Press. ISBN 0262132699
  • Margulis, Lynn and Dorion Sagan. 1991. Mystery Dance: On the Evolution of Human Sexuality. Summit Books. ISBN 0671633414
  • Margulis, Lynn and Dorion Sagan. 1987. Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors. HarperCollins. ISBN 004570015X
  • Margulis, Lynn and Dorion Sagan. 1986. Origins of Sex : Three Billion Years of Genetic Recombination. Yale University Press. ISBN 0300033400
  • Margulis, Lynn. 1982. Early Life. Science Books International. ISBN 0867200057
  • Margulis, Lynn. 1970. Origin of Eukaryotic Cells. Yale University Press. ISBN 0300013531

References

  • Brockman, J. 1995. The Third Culture. New York: Simon and Schuster. ISBN 0684803593
  • Mann, C. 1991. Lynn Margulis: Science's Unruly Earth Mother. Science 252: 378-381.
  • Margulis L., and D. Sagan. 1986. Microcosmos. New York: Summit Books. ISBN 0671441698
  • Mereschkowsky, C. 1905. Über Natur und Ursprung der Chromatophoren im Pflanzenreiche. Biol Centralbl. 25: 593-604.
  • Ris, H., and R. N. Singh. 1961. Electron microscope studies on blue-green algae. J Biophys Biochem Cytol. 9: 63-80.
  • Sagan, L. 1967. On the origin of mitosing cells. J. Theoretical Biology 14(3): 255-274.
  • Schimper, A. F. W. 1883. Über die Entwicklung der Chlorophyllkörner und Farbkörper. Bot. Zeitung 41: 105-114, 121-131, 137-146, 153-162.
  • Sehi, L. 2001. Acceptance doesn't come easy. Immaculata. Retrieved July 15, 2006.
  • Stocking, C. and E. Gifford. 1959. Incorporation of thymidine into chloroplasts of Spirogyra. Biochem. Biophys. Res. Comm. 1: 159-164.
  • Wallin, I. E. 1923. The mitochondria problem. The American Naturalist 57(650): 255-261.

External links

All links retrieved September 17, 2014.


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