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The History of Evolutionary Thought   

Endosymbiosis: Lynn Margulis

Chloroplasts may have evolved from cyanobacteria
Margulis and others hypothesized that chloroplasts (bottom) evolved from cyanobacteria (top).

The Modern Synthesis established that over time, natural selection acting on mutations could generate new adaptations and new species. But did that mean that new lineages and adaptations only form by branching off of old ones and inheriting the genes of the old lineage? Some researchers answered no. Evolutionist Lynn Margulis showed that a major organizational event in the history of life probably involved the merging of two or more lineages through symbiosis.

Lynn Margulis
Symbiotic microbes = eukaryote cells?
In the late 1960s Margulis (left) studied the structure of cells. Mitochondria, for example, are wriggly bodies that generate the energy required for metabolism. To Margulis, they looked remarkably like bacteria. She knew that scientists had been struck by the similarity ever since the discovery of mitochondria at the end of the 1800s. Some even suggested that mitochondria began from bacteria that lived in a permanent symbiosis within the cells of animals and plants. There were parallel examples in all plant cells. Algae and plant cells have a second set of bodies that they use to carry out photosynthesis. Known as chloroplasts, they capture incoming sunlight energy. The energy drives biochemical reactions including the combination of water and carbon dioxide to make organic matter. Chloroplasts, like mitochondria, bear a striking resemblance to bacteria. Scientists became convinced that chloroplasts (below right), like mitochondria, evolved from symbiotic bacteria — specifically, that they descended from cyanobacteria (above right), the light-harnessing small organisms that abound in oceans and fresh water.

When one of her professors saw DNA inside chloroplasts, Margulis was not surprised. After all, that's just what you'd expect from a symbiotic partner. Margulis spent much of the rest of the 1960s honing her argument that symbiosis (see figure, below) was an unrecognized but major force in the evolution of cells. In 1970 she published her argument in The Origin of Eukaryotic Cells.

Hypothesized origin of mitochondria and chloroplasts

		    may be descended from relatives of a typhus-causing bacteria
Mitochondria are thought to have descended from close relatives of typhus-causing bacteria.

The genetic evidence
In the 1970s scientists developed new tools and methods for comparing genes from different species. Two teams of microbiologists — one headed by Carl Woese, and the other by W. Ford Doolittle at Dalhousie University in Nova Scotia — studied the genes inside chloroplasts of some species of algae. They found that the chloroplast genes bore little resemblance to the genes in the algae's nuclei. Chloroplast DNA, it turns out, was cyanobacterial DNA. The DNA in mitochondria, meanwhile, resembles that within a group of bacteria that includes the type of bacteria that causes typhus (see photos, right). Margulis has maintained that earlier symbioses helped to build nucleated cells. For example, spiral-shaped bacteria called spirochetes were incorporated into all organisms that divide by mitosis. Tails on cells such as sperm eventually resulted. Most researchers remain skeptical about this claim.

It has become clear that symbiotic events have had a profound impact on the organization and complexity of many forms of life. Algae have swallowed up bacterial partners, and have themselves been included within other single cells. Nucleated cells are more like tightly knit communities than single individuals. Evolution is more flexible than was once believed.

Cladogram showing relationships between typhus/mitochondria and cyanobacteria/chloroplasts
Phylogenetic analyses based on genetic sequences support the endosymbiosis hypothesis.

Margulis image by Jerry Bauer; Chloroplast image courtesy of New Mexico State University Electron Microscopy Laboratory; Cyanobacterium image courtesy of the University of Tsukuba Institute of Biological Sciences; Mitochondria image courtesy of the CDC, Public Health Image Library; Typhus-causing bacteria (Rickettsia) image © David H. Walker and Vsevolod Popov, authors. Licensed for use, ASM MicrobeLibrary

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