Lynn Margulis

Lynn Margulis

Lynn Margulis (born March 15, 1938) is a biologist who has pioneered several important concepts in the fields of cell biology and microbial evolution. She perhaps is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory—which is now generally accepted for how certain organelles were formed.

The endosymbiotic theory concerns the origins of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) and chloroplasts from cyanobacteria

Mitochondria have their own DNA, and, according to the generally accepted endosymbiotic theory, they were originally derived from external organisms. This theory, which was popularized by Lynn Margulis, fits her view that "Life did not take over the globe by combat, but by networking" (Margulis and Sagan 1986)—in other words, by cooperation rather than Darwinian competition.

The Endosymbiotic Theory, proposed in 1967 by Lynn Margulis, is now widely accepted and states among other things that mitochondria are the result of cells' endocytosis of aerobic (oxygen-dependent) bacteria.

The Gaia hypothesis addresses the remarkable harmony seen between biotic and abiotic elements on Earth. Noted microbiologist Lynn Margulis, who has been an important collaborator with Lovelock in developing Gaian concepts (Turney 2003), sees the history of life as having developed more by cooperation than by competition (see Endosymbiotic theory). Another way of viewing this harmony, as exemplified by the Gaia hypothesis, is as a reflection of the concept of dual purposes, whereby each living organism's individual purpose (oriented toward the self-preservation, development, and multiplication of the self) is integrated with its purpose for the whole (contributing to the development and preservation of the larger whole).

and university professor in the Department of Geosciences at the University of Massachusetts Amherst.

Research

Lynn Margulis attended the University of Chicago as an undergraduate, graduating with an A.B. in Liberal Arts in 1957. In 1960, she graduated with an M. S. degree 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, 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 it draws heavily on symbiosis ideas first put forward by scientists in the mid-19th century scientists, as well as the early 20th 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 famous 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 symbiont's nuclear DNA (Sehi 2001).

Margulis later formulated a theory to explain how symbiotic relationships between organisms of often different phyla or kingdoms are the driving force of evolution. Genetic variation is proposed to occur mainly as a result of transfer of nuclear information between bacterial cells or viruses and eukaryotic cells. While her organelle genesis ideas are widely accepted, symbiotic relationships as a current method of introducing genetic variation is somewhat of a fringe idea. However, examination of the results from the Human Genome Project lends credence toward an endosymbiotic theory of evolution—or at the very least Margulis's endosymbiotic theory is the catalyst for current ideas about the composition of the human genome. Significant portions of the human genome are either bacterial or viral in origin—some clearly ancient insertions, while others are more recent in origin. This strongly supports the idea of symbiotic—and more likely parasitic—relationships being a driving force for genetic change in humans, and likely all organisms. It should be noted that while the endosymbiotic theory has historically been juxtaposed with Neo-Darwinism, the two theories are not incompatible.

In 1995, prominent Neo-Darwinist evolutionary biologist Richard Dawkins had this to say about Lynn Margulis and her work: {{cquote|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).

She holds a 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).

Her 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.

Origin

As mitochondria contain ribosomes and DNA, and are only formed by the division of other mitochondria, it is generally accepted that they were originally derived from endosymbiotic prokaryotes. 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.

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 an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

This happened at least two billion years ago and mitochondria still show some signs of their ancient origin. Mitochondrial ribosomes are the 70S (bacterial) type, in contrast to the 80S ribosomes found elsewhere in the cell. As in prokaryotes, there is a very high proportion of coding DNA, and an absence of repeats. 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.

The endosymbiotic theory was first articulated by the Russian botanist Konstantin Mereschkowsky in 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 . Ivan Wallin extended the idea of an endosymbiotic origin to mitochondria in the 1920s. These theories were initially dismissed or ignored. More detailed electron microscopic comparisons between cyanobacteria and chloroplasts (for example studies by Hans Ris), combined with the discovery that plastids and mitochondria contain their own DNA (which by that stage was recognized to be the hereditary material of organisms) led to a resurrection of the idea in the 1960s.

The endosymbiotic hypothesis was fleshed out and popularized by Lynn Margulis. In her 1981 work Symbiosis in Cell Evolution she 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. See also Evolution of flagella.

Symbiogenesis is the merging of two separate organisms to form a single new organism. The idea originated with Konstantin Mereschkowsky in his 1926 book Symbiogenesis and the Origin of Species, which proposed that chloroplasts originate from cyanobacteria captured by a protozoan. Today both chloroplasts and mitochondria are believed to have such an origin; this is the endosymbiotic theory.

In Acquiring Genomes: A Theory of the Origins of Species, biologist Dr. Lynn Margulis argued that symbiogenesis is a primary force in evolution. According to her theory, acquisition and accumulation of random mutations are not sufficient to explain how inherited variations occur; rather, new organelles, bodies, organs, and species arise from symbiogenesis. Whereas the classical interpretation of evolution (neo-Darwinism) emphasizes competition as the main force behind evolution, Margulis emphasizes cooperation.

Many ecologists agree, but 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, there is no clear evidence of other major traits or transitions that can be attributed to symbiogenesis.

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. There is a major body of scientific work, both theoretical and experimental, based on this paradigm. Those who have worked in the field tend to regard its foundation as unassailable.

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. We understand 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.

Symbiogenesis was mentioned in the Star Trek Voyager episode "Tuvix".

Important publications

• Margulis, Lynn and Dorion Sagan. "Acquiring Genomes: A Theory of the Origins of Species", Perseus Books Group, 2002 ISBN 0-465-04391-7

Other

• Margulis was inducted into the World Academy of Art and Science, the Russian Academy of Natural Sciences, and the American Academy of Arts and Sciences between 1995 and 1998.
• In 1999, she was awarded the National Medal of Science.
• She is also a proponent and co-developer of the modern version of Gaia hypothesis, based on an idea developed by the English atmospheric scientist James Lovelock.
• She is profiled in a book published in 2006 by Resurgence Magazine in the UK, called Visionaries: The 20th Century's 100 Most Important Inspirational Leaders.
• In 2006 with her son Dorion, she founded Sciencewriters Books, an imprint of Chelsea Green Publishing for science books.

Personal

She 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.

See also

• Symbiogenesis

Publications and bibliography

• Margulis, Lynn and Dorion Sagan, 2007, Dazzle Gradually: Reflections on the Nature of Nature, Sciencewriters Books, ISBN 978-1-933392-31-8
• Margulis, Lynn and Eduardo Punset, eds., 2007 Mind, Life and Universe: Conversations with Great Scientists of Our Time, Sciencewriters Books, ISBN 978-1-933392-61-5
• Margulis, Lynn, 2007, Luminous Fish: Tales of Science and Love, Sciencewriters Books, ISBN 978-1-933392-33-2
• Margulis, Lynn and Dorion Sagan, 2002, Acquiring Genomes: A Theory of the Origins of Species, Perseus Books Group, ISBN 0-465-04391-7
• Margulis, Lynn, et al., 2002, The Ice Chronicles: The Quest to Understand Global Climate Change, University of New Hampshire, ISBN 1-58465-062-1
• Margulis, Lynn, 1998, Symbiotic Planet : A New Look at Evolution, Basic Books, ISBN 0-465-07271-2
• Margulis, Lynn and Karlene V. Schwartz, 1997, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth, W.H. Freeman & Company, ISBN 0-613-92338-3
• Margulis, Lynn and Dorian Sagan, 1997, What Is Sex?, Simon and Shuster, ISBN 0-684-82691-7
• Margulis, Lynn and Dorion Sagan, 1997, Slanted Truths: Essays on Gaia, Symbiosis, and Evolution, Copernicus Books, ISBN 0-387-94927-5
• Margulis, Lynn, 1992, Symbiosis in Cell Evolution: Microbial Communities in the Archean and Proterozoic Eons, W.H. Freeman, ISBN 0-7167-7028-8
• Margulis, Lynn, ed, 1991, Symbiosis as a Source of Evolutionary Innovation: Speciation and Morphogenesis, The MIT Press, ISBN 0-262-13269-9
• Margulis, Lynn and Dorion Sagan, 1991, Mystery Dance: On the Evolution of Human Sexuality, Summit Books, ISBN 0-671-63341-4
• Margulis, Lynn and Dorion Sagan, 1987, Microcosmos: Four Billion Years of Evolution from Our Microbial Ancestors, HarperCollins, ISBN 0-04-570015-X
• Margulis, Lynn and Dorion Sagan, 1986, Origins of Sex : Three Billion Years of Genetic Recombination, Yale University Press, ISBN 0-300-03340-0
• Margulis, Lynn, 1982, Early Life, Science Books International, ISBN 0-86720-005-7
• Margulis, Lynn, 1970, Origin of Eukaryotic Cells, Yale University Press, ISBN 0-300-01353-1

References

ISBN links support NWE through referral fees

• Margulis L., and D. Sagan. 1986. Microcosmos. New York: Summit Books. ISBN 0671441698.

L. Sehi. 2001.

External links

• UMass Bio Dept. (includes a partial list of technical publications) (Accessed March 3, 2005)
• UMass Geo Dept. (Accessed March 3, 2005)
• www.immaculata.edu (Accessed March 3, 2005)
• San Jose Science, Technology and Society, 2004-2005 Linus Pauling Memorial Lectures (Accessed March 3, 2005)
• The Endosymbiotic Theory (Accessed March 3, 2005)
• Gaia Is a Tough Bitch
• Interview with Lynn Margulis on Gaia 5 minute MP3 from October 2005

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