Endosymbiosis: Lynn Margulis
|Margulis and others hypothesized that chloroplasts (bottom) evolved from cyanobacteria (top).
The Modern Synthesis established that over
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.
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.
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.
|Phylogenetic analyses based on genetic sequences support the endosymbiosis hypothesis.