Saturday, June 09, 2012

The endosymbiotic theory of Lynn Margulis


Published by Basic Books
Lynn Margulis passed away last November, sadly. She was renowned for the endosymbiotic theory of evolution, which is now part of biology textbooks. She had a wonderful insight: the mitochondria and chloroplasts that are found in eukaryotic cells were, in distant past, free-living bacteria. Thanks to at least two distinct endosymbiosis events, they were incorporated—and not digested—in the eukaryotic ancestor. They became responsible of key functions within the new association, namely respiration and photosynthesis. These symbionts persisted until at some point they were indistinguishable from their host, and all merged to become one new organism, a eukaryotic cell. 

Recently I found a copy of her book Symbiotic Planet (1998) in my usual second-hand bookshop in Davis. It is a short book in which Margulis deals with the scientific idea that has occupied her during most of her career: the serial endosymbiosis theory (or SET). The author sums up the book as follows (p.33):
In short, I believe that most evolutionary novelty arose, and still arises, directly from symbiosis.

Margulis refers to this phenomenon as symbiogenesis. The scientific community now acknowledges the endosymbiosis of mitochondria and chloroplasts from respectively Proteobacteria and Cyanobacteria ancestors, thanks to a plethora of molecular evidences. However, it is still a far cry from acknowledging that all evolutionary novelty is based on the same principle. For instance, Margulis believes that a third symbiosis event contributed to the formation of eukaryotic cells; this event is the fusion of a swimming bacterium with the proto-eukaryotic cell, leading to cilia, sperm tails, and a variety of other appendages. Now this is found dubious by the scientific community, because evidence is lacking. 

The original SET paper, "Origin of Mitosing Cells", was published in 1967, after fifteen or so rejections according to Margulis… That's what I call perseverance! The paper was signed "Lynn Sagan", since at that time she was still married to the famous cosmologist Carl Sagan. I didn't read the details of her argument in favor of the symbiosis with a swimmer cell, but I feel a little bit uncomfortable when I read such a passage (Symbiotic Planet, p. 39):
Some colleagues label me combative; others, unfair. Some say I only collect relevant work and unfairly ignore contradictory data. These accusations may be correct.
Unsettling, I would say.

The word that keeps popping up about Margulis, is "controversy". Over evolution and AIDS, notably. To give you an idea it is worth to read this recent interview of Margulis in Discover Magazine. Despite her controversial side, she truly was an inspiring figure, and I'm very glad I had the chance to attend one of her talk in 2010 (during the general meeting of the American Society for Microbiology in San Diego). Well, not everyone can claim to be in biology textbooks!

One of the best obituaries I read so far was signed by Elio Schaechter, the host of the Small Things Considered blog, who wrote a moving, honest account of Lynn Margulis' career and personality. 

To conclude, one last quote from Symbiotic Planet, which put a wide grin on my face after I read it (p.91):
At public lectures I am often asked, "At what moment does human life begin?" Of course, it began, as all life did, at least 3.5 billion years ago! The question reflects a misconception.

Reference:
  • Margulis L., 1998. Symbiotic Planet A new look at evolution. Basic Books. 147 p.

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