(excerpts from the book "Doing Biology" by Joel Hagen et al)
Modern biology inherited two great theories from the nineteenth century: evolutionary theory and cell theory. Surprisingly, these theories, so central to our understanding of the living world, have had a rather uneasy relationship. Until quite recently, most cell biologistsignored evolution, and most evolutionary biologists ignored cells. The exception to this historical generalization was the chromosomes, which both evolutionary biologists and cell biologists studied. But what about the cytoplasm, the contents of the cell outside the nucleus? Could knowing about other cellular structures (organelles) add anything to evolutionary theory? Could evolutionary theorysuggest interesting questions about the structure or function of organelles? For most biologists, the answer to these questions was no. The cytoplasm added little to understanding evolutionary theory, and vice versa. Occasionally, some biologists tried to bridge the theoretical gap, but they usually met with derision. For example, during the.1920s the microbiologist Ivan Wallin made the remarkableclaim that mitochondria had originated as free-living bacteria. According to Wallin, the former bacteria and their host cells evolved together to establish an inseparable symbiotic partnership. He even claimed to have removed mitochondria from cells and grown them in isolation.
Wallin's idea was almost universally rejected, and he was often ridiculed for his wild speculations. According to hiscritics, evolution by symbiosis was as improbable as that other great pseudoscientific idea of the 1920s: continental drift. Although intrigued by the possibility that mitochondria evolved from bacteria, America's leading cell biologist, E. B. Wilson, remarked that Wallin's ideas were "too fantastic for present mention in polite biological society".
With the benefit of hindsight it is easyto smile at the comparison between continental drift and endosymbiosis, two great scientific heresies that later revolutionized the way we look at the natural world. The criticisms were, however, justified. Wallin's theory was quite speculative. No one, then or now, has verified his claim that mitochondria can be grown outside of cells.
Assuming that mitochondria really did evolvefrom free-living bacteria, why might it be difficult or impossible to experimentally grow them outside of the host cell? How can you explain Wallin's unverified claim that he had isolated and grown mitochondria outside of cells?
Both the structure and the function of mitochondria were mysteries in 1920. The internal anatomy of bacteria was also almost totally unknown. The evidence Wallinneeded to support his theory required the electron microscope and other sophisticated laboratory techniques developed only after World War II. As in the case of continental drift, the theory of symbiosis in cellular evolution that was finally accepted during the 1970s was very different from the one suggested by Wallin in the 1920s.
LYNN MARGULIS: A REVOLUTlONARY SClENTlST
Like the eventualacceptance of continental drift, acceptance of a symbiotic theory of cell evolution has often been hailed as a scientific revolution The woman most responsible for bringing the idea to scientific respectability is Lynn Margulis. A prolific writer and dynamic speaker, Margulis captivates audiences and often irritates more traditional biologists with her unorthodox ideas. A profile in Science describedher as an unruly provocateur, but as one of the world's leading authorities on cellular evolution, she supports her claims with abundant evidence. Although many biologists continue to disagree with some of her ideas, everyone takes endosymbiosis seriously.
Margulis entered biology during a particularly exciting period. James Watson and Francis Crick were just discovering the structure of...