Symbiotic Planet: A New Look at Evolution, by Margulis, Lynn
- ISBN: 9780465072729 | 0465072720
- Cover: Paperback
- Copyright: 10/1/1999
|About the Author||147|
A Bee his burnished Carriage
Drove boldly to a Rose--
Symbiosis, the system in which members of different species live in physical contact, strikes us as an arcane concept and a specialized biological term. This is because of our lack of awareness of its prevalence. Not only are our guts and eyelashes festooned with bacterial and animal symbionts, but if you look at your backyard or community park, symbionts are not obvious but they are omnipresent. Clover and vetch, common weeds, have little balls on their roots. These are the nitrogen-fixing bacteria that are essential for healthy growth in nitrogen-poor soil. Then take the trees, the maple, oak, and hickory. As many as three hundred different fungal symbionts, the mycorrhizae we notice as mushrooms, are entwined in their roots. Or look at a dog, who usually fails to notice the symbiotic worms in his gut. We are symbionts on a symbiotic planet, and if we care to, we can find symbiosis everywhere. Physical contact is a nonnegotiable requisite for many differing kinds of life.
Practically everything I work on now was anticipated by unknown scholars or naturalists. One of my most important scientific predecessors thoroughly understood and explained the role of symbiosis in evolution. The University of Colorado anatomist Ivan E. Wallin (1883-1969) wrote a fine book arguing that new species originate through symbiosis. Symbiogenesis , an evolutionary term, refers to the origin of new tissues, organs, organisms--even species--by establishment of long-term permanent symbiosis. Wallin never used the word symbiogenesis , but he entirely understood the idea. He especially emphasized animal symbiosis with bacteria, a process he called "the establishment of microsymbiotic complexes" or "symbionticism." This is important. Although Darwin entitled his magnum opus On the Origin of Species , the appearance of new species is scarcely even discussed in his book.
Symbiosis, and here I fully agree with Wallin, is crucial to an understanding of evolutionary novelty and the origin of species. Indeed, I believe the idea of species itself requires symbiosis. Bacteria do not have species. No species existed before bacteria merged to form larger cells including ancestors to both plants and animals. In this book I will explain how long-standing symbiosis led first to the evolution of complex cells with nuclei and from there to other organisms such as fungi, plants, and animals.
That animal and plant cells originated through symbiosis is no longer controversial. Molecular biology, including gene sequencing, has vindicated this aspect of my theory of cell symbiosis. The permanent incorporation of bacteria inside plant and animal cells as plastids and mitochondria is the part of my serial endosymbiosis theory that now appears even in high school textbooks. But the full impact of the symbiotic view of evolution has yet to be felt. And the idea that new species arise from symbiotic mergers among members of old ones is still not even discussed in polite scientific society.
Here is an example. I once asked the eloquent and personable paleontologist Niles Eldredge whether he knew of any case in which the formation of a new species had been documented. I told him I'd be satisfied if his example were drawn from the laboratory, from the field, or from observations from the fossil record. He could muster only one good example: Theodosius Dobzhansky's experiments with Drosophila , the fruit fly. In this fascinating experiment, populations of fruit flies, bred at progressively hotter temperatures, became genetically separated. After two years or so the hot-bred ones could no longer produce fertile offspring with their cold-breeding brethren. "But," Eldredge quickly added, "that turned out to have something to do with a parasite!" Indeed, it was later discovered that the hot-breeding flies lacked an intracellular symbiotic bacterium found in the cold breeders. Eldredge dismissed this case as an observation of speciation because it entailed a microbial symbiosis! He had been taught, as we all have, that microbes are germs, and when you have germs, you have a disease, not a new species. And he had been taught that evolution through natural selection occurs by the gradual accumulation, over eons, of single gene mutations.
Ironically, Niles Eldredge is author with Stephen Jay Gould of the theory of "punctuated equilibrium." Eldredge and Gould argue that the fossil record shows evolution to be static most of the time and to proceed suddenly: rapid change in fossil populations occurs over brief time spans; stasis then prevails for extended periods. From the long view of geological time, symbioses are like flashes of evolutionary lightning. To me symbiosis as a source of evolutionary novelty helps explain the observation of "punctuated equilibrium," of discontinuities in the fossil record.
Among the only other organisms besides fruit flies in which species have been seen to originate in the laboratory are members of the genus Amoeba and symbiosis was involved. Symbiosis is a kind, but not the notorious kind, of Lamarckianism. "Lamarckianism," named for Jean Baptiste Lamarck, who the French claim was the first evolutionist, is often dismissed as "inheritance of acquired characteristics." In simple Lamarckianism, organisms inherit traits induced in their parents by environmental conditions, whereas through symbiogenesis, organisms acquire not traits but entire other organisms, and of course, their entire sets of genes! I could say, as my French colleagues often have, that symbiogenesis is a form of neo-Larmarckianism. Symbiogenesis is evolutionary change by the inheritance of acquired gene sets.
Living beings defy neat definition. They fight, they feed, they dance, they mate, they die. At the base of the creativity of all large familiar forms of life, symbiosis generates novelty. It brings together different life-forms, always for a reason. Often, hunger unites the predator with the prey or the mouth with the photosynthetic bacterium or algal victim. Symbiogenesis brings together unlike individuals to make large, more complex entities. Symbiogenetic life-forms are even more unlike than their unlikely "parents." "Individuals" permanently merge and regulate their reproduction. They generate new populations that become multiunit symbiotic new individuals. These become "new individuals" at larger, more inclusive levels of integration. Symbiosis is not a marginal or rare phenomenon. It is natural and common. We abide in a symbiotic world.
In Brittany, on the northwest coast of France, and along beaches bordering the English Channel is found a strange sort of "seaweed" that is not seaweed at all. From a distance it is a bright green patch on the sand. The patches slosh around, shimmering in shallow puddles. When you pick up the green water and let it slip through your fingers you notice gooey ribbons much like seaweed. A small hand lens or low-power microscope reveals that what looked like seaweed are really green worms. These masses of sunbathing green worms, unlike any seaweed, burrow into the sand and effectively disappear. They were first described in the 1920s by an Englishman, J. Keeble, who spent his summers at Roscoff. Keeble called them "plant-animals" and diagrammed them splendidly in the color frontispiece of his book, Plant-Animals . The flatworms of the species Convoluta roscoffensis are all green because their tissues are packed with Platymonas cells; as the worms are translucent, the green color of Platymonas , photosynthesizing algae, shows through. Although lovely, the green algae are not merely decorative: they live and grow, die and reproduce, inside the bodies of the worms. Indeed they produce the food that the worms "eat". The mouths of the worms become superfluous and do not function after the worm larvae hatch. Sunlight reaches the algae inside their mobile greenhouses and allows them to grow and feed themselves as they leak photosynthestic products and feed their hosts from the inside. The symbiotic algae even do the worm a waste management favor: they recycle the worm's uric acid waste into nutrients for themselves. Algae and worm make a miniature ecosystem swimming in the sun. Indeed, these two beings are so intimate that it is difficult, without very high-power microscopy, to say where the animal ends and the algae begin.
Such partnerships abound. Bodies of Plachobranchus , snails, harbor green symbionts growing in such even rows they appear to have been planted. Giant clams act as living gardens, in which their bodies hold algae toward the light. Mastigias is a man-of-war type of medusoid that swims in the Pacific Ocean. Like myriad small green umbrellas, Mastigias medusoids float through the light beams near the water's surface by the thousands.
Similarly, freshwater tentacled hydras may be white or green, depending on whether or not their bodies are packed with green photosynthetic partners. Are hydras animals or plants? When a green hydra is permanently inhabited by its food producing partners (called Chlorella ), it is hard to tell. Hydras, if green, are symbionts. They are capable of photosynthesizing, of swimming, of moving, and of staying put. They have remained in the game of life because they become individuals by incorporation.
We animals, all thirty million species of us, emanate from the microcosm. The microbial world, the source and wellspring of soil and air, informs our own survival. A major theme of the microbial drama is the emergence of individuality from the community interactions of once-independent actors.
I love to gaze on the daily life struggles of our nonhuman planetmates. For many years Lorraine Olendzenski, my former student, now at the University of Connecticut, and I have videographed life in the microcosm. More recently we have worked with Lois Byrnes, the vivacious former associate director of the New England Science Center in Worcester, Massachusetts. Together we and a fine group of U MASS students make films and videos that introduce people to our microbial acquaintances.
Ophrydium , a pond water scum that, upon close inspection, seems to be countable green "jelly ball" bodies is an example of emergent individuality that we recently discovered in Massachusetts and redescribed. Our films show these water balls with exquisite clarity. The larger "individual" green jelly ball is composed of smaller cone-shaped actively contractile "individuals." These in turn are composite: green Chlorella dwell inside ciliates, all packed into rows. Inside each upside-down cone are hundreds of spherical symbionts, cells of Chlorella. Chlorella is a common green alga; those of Ophrydium are trapped into service for the jelly ball community. Each "individual organism" in this "species" is really a group, a membrane-bounded packet of microbes that looks like and acts as a single individual.
A nutritious drink called kefir consumed in the Caucasus Mountains is also a symbiotic complex. Kefir contains grainy curds the Georgians call "Mohammed pellets." The curd is an integrated packet of more than twenty-five different kinds of yeast and bacteria. Millions of individuals make up each curd. From such interactive bodies of fused organisms new beings sometimes emerge. The tendency of "independent" life is to bind together and reemerge in a new wholeness at a higher, larger level of organization. I suspect that the near future of Homo sapiens as a species requires our reorientation toward the fusions and mergers of the planetmates that have preceded us in the microcosm. One of my ambitions is to coax some great director into producing evolutionary history as the microcosmic image in IMAX or OMNIMAX, showing spectacular living relationships as they form and dissolve.
Now and throughout Earth's history, symbioses, both stable and ephemeral, have prevailed. Stories about this kind of evolution deserve broadcasting.
Copyright © 1998 Sciencewriters, Amherst, Massachusetts. All rights reserved.