Gene Trading and Cell Symbiosis

Although the evolution of bacteria contains, as previously suggested, the seeds of all biological diversity, it cannot, of course, be detailed here. However, one fact especially stands out. That is the role of symbiosis—living together—in producing all the other life forms. Without immune systems or reproduction dependent upon mate recognition, bacteria are supremely promiscuous beings— indeed, beings in which infection and sex is virtually the same thing. Unlike animals, which must recognize and breed with mates of the opposite sex by donating an equal apportionment of genes from each of two parents, new bacteria can be produced directly, whenever one injects genes in the other. And such injections need not be 50 percent; they range from 2 or 3 percent of the parent's genetic endowment to nearly all of it. The result is that genetic engineering, although for us new, is an ancient bacterial "technology." The sexual proclivities of bac teria include a rampant exchange of genes next to which our species' most Bacchanalian orgies look like rather subdued affairs. Characteristic bacterial genes can be found in plants, animals, and fungi.

Yet symbiosis dwarfs even this bacterial sexuality or genetic engineering in its ability to bring together different genomes. Feeding on, with, and within other cells, invading and destroying or entering and living with other cells, bacteria propagated major transitions in evolution—the transitions that were to lead to protoctists (such as amoebae and parame-cia), fungi, animals, and plants. The most striking of these symbioses, for which there is much evidence (morphological, behavioral, and genetic), are those of the green parts of plant cells and algae—the plastids—and the previously mentioned respiring bacteria that evolved to become the oxygen-using parts of all familiar life from mushrooms to elephants— the mitochondria. In the case of plastids, the best evolutionary conjecture is that green-colored cyanobacteria (the first bacteria to use water for hydrogen) were eaten by other cells—but not digested. As time went by, they continued to photosynthesize: in return for a warm, sometimes mobile environment, they produced food from little more than sunlight, water, and air, feeding the cells in which they found themselves along the way. The ancestors to mitochondria, DNA-containing parts found outside the nucleus in all animal and plant cells, were probably bacteria that invaded larger cells. Again, however, instead of killing the cells in which they found themselves, or being killed by them, a partnership evolved. Over time, the genomes merged and multiple beings became one. That the host cells in these alliances came from the same stock is suggested by the genetic likeness of cells with nuclei, eukaryotic cells, to the presumably ancestral archaebacteria. In summary, then, the biodiversity we see today depended for its existence on metabolically diverse bacteria coming together in sex and symbiosis, and evolving in new, long-living unions. And, whatever we do to ourselves, we may take philosophical solace, at least, in the knowledge that the diverse bacterial underlayer that gave rise to all plants and animals, as well as us, was around for billions of years before we got here, and will likely survive for billions of years more after we are gone.

—Lynn Margulis and Dorion Sagan

See also: Benthos; Coevolution; Communities; Ecosystems; Oxygen, History of Presence in the Atmosphere; Protoctists; Speciation; Viruses

Bibliography

Madigan, Michael T., John M. Martinko, and Jack Parker. 1997. "Prokaryotic Diversity: Bacteria." In Biology of Microorganisms, edited by Thomas D. Brock, ch. 16. Englewood Cliffs, NJ: Prentice Hall; Margulis, Lynn, Clifford Matthews, and Aaron Haselton. 2000. "Five-kingdom Classification Scheme: Superkingdom Prokaryota." In Environmental Evolution. Appendix B. Cambridge: MIT Press; Margulis, Lynn, and Karlene Schwartz. 1998. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. New York: W. H. Freeman; Margulis, Lynn, Karlene V. Schwartz, and Michael Dolan. 1999. Diversity of Life: The Illustrated Guide to the Five Kingdoms. Sud-bury, MA: Jones and Bartlett; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt.; Sonea, Sorin, and Léo G. Mathieu. 2001. Prokaryotology: A Coherent View. Montreal: McGill-Queen's University Press.

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