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Evolution's Other Narrative

Why science would benefit from a symbiosis-driven history of speciation.

Bradford Harris

Lynn Margulis’s Legacy

According to Margulis, the evolving relationships between microscopic organisms and other micro- and macroscopic organisms are the essence of the history of life. Despite scientists’ mid-century focus on eukaryotic life (organisms with larger cells featuring a bounded nucleus and organelles), the most prolific type of organism on Earth, bacteria, is prokaryotic (an organism without a bounded nucleus). Virtually all eukaryotic forms of life have adapted symbiotic associations with prokaryotic bacteria. Margulis was among the first Western scientists to attempt to popularize this fact. She spent virtually her entire career laboring to bring this mostly microscopic form of evolution to the macroscopic focus of her readers.

Margulis’s research in microbiology equipped her to verify and expand on Wallin’s symbiosis-centered theory. In 1966 she attempted to publish a summary of her perspectives on the evolution of complex life forms in “The Origin of Mitosing Eukaryotic Cells,” only to be rejected by more than a dozen scientific journals. When her article was finally published by the Journal of Theoretical Biology, criticism ensued. Nonetheless, the further Margulis pushed her symbiotic evolutionary theory, the more convinced she became that the emergence of eukaryotic cells a billion and a half years ago—a major evolutionary transition in the history of life—was the result of symbiogenesis.

In Margulis’s view, out of prokaryotic–prokaryotic symbiosis emerged eukaryotes. Out of prokaryotic–eukaryotic symbiosis emerged more competitive eukaryotes. And out of eukaryotic–eukaryotic symbiosis emerged multicellular life. The classic image of evolution, the tree of life, almost always exclusively shows diverging branches; however, a banyan tree, with diverging and converging branches is best. To this day, many scientists and most laypeople remain ignorant of this way of imagining evolution, which profoundly constricts how they imagine themselves.

Most of the mass of our DNA is “ours” because there are many more cells making up our body than there are composing the microorganisms living in and on our body. Nonetheless, most of the genetic diversity in and on our body is not ours. Rather, it is found in the microscopic organisms with which our body interacts to maintain optimal health. Eyelash mites, skin fungi, gut bacteria, and more all work in harmony with us. “Each one of us is a massive colony of microorganisms,” Margulis explained. “Former protists are now eloquently orchestrated animals with fancy tissues and organs.”

Some of Margulis’s most vivid contributions to a richer understanding of evolution appear in her books Symbiotic Planet: A New Look at Evolution (1999) and Acquiring Genomes: A Theory of the Origins of Species (2002). In Acquiring Genomes Margulis describes, for example, the evolutionary integration of genetically distinct bacteria onto the surface of a genus of ciliated saltwater microorganisms called Euplotidium. All six species of Euplotidium bear surface bacteria that act like archers defending the royal Euplotidium castle. When Euplotidium species sense approaching predators, their surface bacteria react by shooting out ribbons of protein, like little micron-sized crossbows. The ciliated Euplotidium cannot survive without these protective bacteria, nor do these particular extracellular bacterial organelles appear to exist anywhere except on the surface of the six known Euplotidium species.

Laboratory experiments to cultivate these bacteria removed from Euplotidium have failed; without the association between the two entities, neither could exist. The association that evolved between the ancestors of Euplotidium and its surface bacteria catalyzed the origin of this genus. This kind of symbiogenetic speciation typifies the role that prokaryotes play in the evolutionary history of eukaryotic life. Wherever biologists now look in nature, they find examples of unicellular and multicellular eukaryotes that exist in association with bacteria and algae possessing distinct genomes.

Despite Margulis’s legacy, early 20th-century concepts of “survival of the fittest” continue to determine how evolution is taught and, therefore, how it is understood even by most scientists. Beyond the popular discourse, relatively advanced textbooks devoted entirely to the study of evolution omit the concept of symbiogenesis. The fourth edition of a leading undergraduate textbook of evolution, Evolutionary Analysis (2008), still devotes entire sections to “combat,” “competition,” and “conflict,” neglecting symbiosis. Most high school graduates are taught the term symbiosis, but it is typically presented to mean little more than mutually beneficial cooperation. Students learn how symbiosis benefits individuals, but not how symbiotic relationships themselves often constitute emergent organisms that display their own evolutionary histories. Only evolutionary specialists have universally adopted an appreciation for symbiogenesis.

Recovering the story of evolution goes a long way toward understanding how to maintain the integrity of a living organism. Wherever symbiotic ideas spread, they lead to important new practical insights. Some of the potentially most therapeutic applications of symbiogenetic science today are being directed toward problems of chronic disease. Many medical scientists are drawing inspiration from Margulis’s and others’ symbiogenetic research to reorient approaches to human health, as it becomes clearer that microbes that have evolved symbiotically with humans are integral to our well-being. The dramatic rise in rates of allergies, eczema, ulcerative colitis, and other chronic health problems are increasingly believed to involve a perturbation in the human–microbe relationship, leading in 2008 to the Human Microbiome Project.

Exploring the history of scientists’ attempts to understand evolution reveals neglected insights into the associations between individuals, associations at least as meaningful as the individuals themselves. Now, thanks to Margulis, these insights are finally beginning to percolate from the realm of the lone expert to that of the wider scientific community, the classroom, and even the dinner table. Supporting this trend will benefit our stewardship of human and global ecology. As we disentangle some of our political discursive traditions from our scientific ones, the story of evolution may itself evolve.


  • Huxley, T. H. 1888. The struggle for existence in human society. Nineteenth Century 23:195–236.
  • Khakhina, L. N. 1992. Concepts of Symbiogenesis: A Historical and Critical Study of the Research of Russian Botanists. New Haven, CT: Yale University Press.
  • Kropotkin, P. 1904. Mutual Aid: A Factor of Evolution. Reprint. Hong Kong: Forgotten Books.
  • Margulis, L. 2002. Acquiring Genomes: A Theory of the Origins of Species. New York: Basic Books.
  • Sapp, J. 2002. Symbiogenesis: The hidden face of Constantin Merezhkowsky. History and Philosophy of the Life Sciences 24:413–440.

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