Wallin's idea was almost universally rejected, and he was often ridiculed for his wild speculations. According to his critics, 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
easy to 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 evolve from 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 Wallin needed 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 eventual acceptance 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 described her 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 DNA when Margulis was in college. A few years later, when she was a graduate student, two of her professors discovered DNA in chloroplasts. Other scientists reported finding DNA in mitochondria. Because these early reports were hotly disputed, searching for DNA outside the nucleus was not the sort of research project that most graduate students would have chosen. Despite warnings, Margulis plunged into the controversial problem for her Ph.D. dissertation. Using radioactively labeled nucleotides, she convincingly demonstrated the presence of DNA in the chloroplasts of Euglena gracilis, one of the curious unicellular organisms that shares both plant and animal characteristics.
Margulis wrote her first article on the endosymbiotic theory in 1967, two years after she completed her Ph.D. At the time, she was a single mother without a permanent teaching position. She was also writing her first book on endosymbiosis, which sparked a lively controversy when it was published in 1970. Although it initially brought Margulis notoriety, the controversy over cellular evolution was rather short lived. By the time she published a second book on endosymbiosis in 1981, most biologists accepted important parts of her theory. As a result, Margulis became a scientific celebrity whose success was publicized in both popular and professional magazines.
BACKGROUND TO A CONTROVERSY
In 1970, when Margulis's first book was published, most biologists had never heard of endosymbiosis. Those who knew about it usually dismissed it. In order to succeed, Margulis had to carefully distinguish her ideas from the discredited theory proposed by Ivan Wallin half a century earlier. She also had to overcome a basic assumption about evolution held by nearly all biologists at the time. According to the traditional view, evolution usually occurs gradually; endosymbiosis, however, is based on the idea of rather sudden evolutionary changes. Finally, Margulis had to convince biologists to take DNA in the cytoplasm seriously. Although evidence for DNA in chloroplasts and mitochondria was growing stronger, the idea that some genes reside outside the nucleus remained unorthodox.
Despite these biases against endosymbiosis, Margulis's book was widely read. Even those who strongly disagreed with her did not ridicule her theory the way biologists had belittled Ivan Wallin's theory about the evolution of mitochondria. Indeed, the book convinced many biologists that cellular evolution was an exciting, if controversial, field. How had cell biology changed during the 50 years after Wallin proposed his unsuccessful theory?
Much more was known about the internal structure
of cells in 1970 than in 1920. Unlike Wallin, who knew little about the
internal structure or function of mitochondria, Margulis had access to
a great deal of information about the intemal structure of cells when she
wrote her book. Powerful electron microscopes, perfected after World War
II, allowed scientists to study the previously hidden parts of organelles.
Using new biochemical techniques, scientists were able to discover many
details of cellular activities. Mitochondria, long an enigma, were now
known to be important sites of adenosine triphosphate (ATP) production,
and for the first time scientists were beginning to under stand how this
critical process occurred on mitochondrial membranes. By 1970 biologists
also became aware of major differences between prokaryotic bacteria, which
lack nuclei and most other organelles, and eukaryotic cells, which have
both. The sharp discontinuity between prokaryotes and eukaryotes, which
previously had not been fully recognized, was highlighted by Robert Whittaker's
new system of classification, which used the two cell types to distinguish
kingdom Monera from four eukaryotic kingdoms (Animalia, Plantae, Fungi,
and Protista). The prokaryotic/eukaryotic distinction was now at the forefront
of biological attention. What other similarities and differences might
be found between the two types of cells? How had eukaryotic cells evolved?
What was the evolutionary significance of the DNA found in some organelles?
These were the questions that Margulis set out to answer in 1970.
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THE SERlAL ENDOSYMBlOTlC THEORY (SET)
According to Margulis, eukaryotic cells evolved through a series of symbiotic partnerships involving several different kinds of prokaryotic cells. The smaller partners invaded larger host cells and eventually evolved into three different kinds of organelles: mitochondria, chloroplasts, and flagella. Because these evolutionary steps supposedly occurred as a series of discrete events, Margulis's theory is often referred to as the SET: serial endosymbiotic theory.
Like other evolutionary biologists, Margulis believes that life first appeared on the earth about four billion years ago. The first organisms were extremely simple--microscopic droplets of water containing a few genes and enzymes surrounded by a membrane. They fed on abundant organic molecules that had been produced earlier in the earth's history by various nonliving chemical processes. Like some modern bacteria, early prokaryotic cells extracted energy from these molecules by fermentation, using various forms of metabolism that do not require oxygen.
Luckily for the fermenters, there was almost no oxygen in the atmosphere. If there had been, the primitive cells would have been poisoned by this highly reactive gas. Later, as the supply of energy-rich molecules in the watery environment began to be depleted, other types of bacteria evolved which used solar energy to synthesize their own supplies of large, organic molecules. These early photosynthetic bacteria were also anaerobic. In other words, they did not use oxygen and their primitive photosynthetic reactions did not produce oxygen as a by-product. For over a billion years, primitive ecosystems included only two types of prokaryotic organisms: simple photosynthetic bacteria and fermenting bacteria.
Perhaps 2.5 billion years ago, a new group of photosynthetic bacteria evolved, the ancestors of today's cyanobacteria. These advanced photosynthesizers split water to produce the hydrogen ions (H+) needed to build sugar molecules. A byproduct of this water-splitting reaction was oxygen gas. This was a catastrophic event in the history of life. Oxygen is such a reactive element that it easily destroys delicate biological structures. As the amount of oxygen in the atmosphere increased, most species of anaerobic bacteria were driven to extinction, victims of the earth's first case of air pollution. Some survivors retreated to areas of brackish water or other oxygen-depleted habitats, where their anaerobic descendants still flourish today. A few prokaryotes became aerobic by evolving various mechanisms to detoxify oxygen. The most successful of these processes was respiration, which not only converted toxic oxygen back into harmless water molecules, but also generated large quantities of ATP.
According to the SET, the photosynthetic production of oxygen gas and the subsequent evolution of respiration set the stage for the evolution of all eukaryotic cells. This evolutionary process occurred in several separate symbiotic events. The first eukaryotic organelles to evolve were mitochondria--structures found in almost all eukaryotic cells. In Margulis's theory, small respiring bacteria parasitized larger, anaerobic prokaryotes. Like some bacteria today (Bdellovibrio), these early parasites burrowed through the cell walls of their prey and invaded their cytoplasm. Either the host or the parasite was often killed in the process, but in a few cases the two cells established an uneasy coexistence. The mutual benefits to the partners are obvious. The respiring parasite, which actually required oxygen, would allow its host to survive in previously uninhabitable, oxygen-rich environments. Perhaps the parasite also shared with its host some of the ATP that it produced using oxygen. In exchange, the host provided sugar or other organic molecules to serve as fuel for aerobic respiration. Eventually, as often occurs with parasites, the protomitochondria lost many metabolic functions provided by the host cell. Similarly, as oxygen in the atmosphere continued to increase, the host became more and more dependent upon its pro-tomitochondria to detoxify the gas. What began as a case of opportunistic parasitism evolved into an obligatory partnership. The small respiratory bacteria eventually evolved into the mitochondria of eukaryotic cells.
Although virtually all eukaryotic cells contain mitochondria, only those of plants and certain protists contain chloroplasts. Therefore, it seems likely that chloroplasts evolved in only a few lines of eukaryotic cells, and this event occurred after mitochondria were already well established. How did this new evolutionary partnership evolve? With higher metabolic rates, cells containing mitochondria were more efficient than anaerobic cells. Some of these newer, unicellular organisms grew larger and evolved into predators capable of eating smaller cells. Their prey undoubtedly included cyanobacteria. In rare cases, these small photosynthetic cells may have resisted digestion after being engulfed. Inside the predator, they set up a semi-independent existence and eventually evolved into chloroplasts.
Although such a scenario may seem far-fetched, we know that similar partnerships exist today. For example, the unusual ciliate Paramecium bursaria is host to many unicellular green algae in the genus Chlorella. These "pseudochloroplasts" produce sugar molecules that are shared with the host. If the Chlorella are experimentally removed, both partners continue to exist independently. Without its photosynthetic partners, however, the Paramecium becomes totally dependent , upon external sources of food. Provided the opportunity, the Paramecium will eat Chlorella but will not digest them, thus reestablishing the symbiotic partnership. Paramecium bursam'a is not a unique case of modern endosymbiosis. Many other organisms, including several multicellular animals, also play host to photosynthetic algae or cyanobacteria.
The most controversial claim made by Margulis is that eukaryotic flagella evolved from small, corkscrew-shaped bacteria called spirochetes. Many spirochetes are parasites (the best known, Treponema pallidurn, causes syphilis). Others are free-living, found in such exotic environments as the intestines of termites. Regardless of how they live, these unusual bacteria swim with an undulating motion reminiscent of the whiplike movement of eukaryotic flagella. Is this similariti evidence for Margulis's evolutionary claim, or is it simply a coincidence? Why not accept the more orthodox explanation that eukaryotic flagella gradually evolved from the simpler flagella found on many bacteria?
Margulis points out that although both types
of flagella are used for locomotion, prokaryotic and eukaryotic structures
are very different. Prokaryotic flagella consist of a single, hollow filament
of protein that spins on its axis like a tiny propeller. Eukaryotic flagella
are much larger; they contain a complex arrangement of 11 microtubules,
and the entire structure is surrounded by an extension of the cell membrane.
In contrast to the spinning prokaryotic flagellum, the eukaryotic structure
propels the cell by lashing back and forth in a whiplike fashion. Because
they are so different in structure, function, and perhaps evolutionary
origin, Margulis proposes that the eukaryotic flagellum should be referred
to by a different term: undullpodium.
Assuming that mitochondria really did evolve from free-living bacteria, what evidence can prove it? Can you think of other scientists who have 'crazy' ideas that later became strongly supported or may soon come to be accepted?