Web Bit 12.1: Evolution of Chloroplasts
By Mary K. Miller
One of the fundamental questions in biology is how did complex,
multicelled organisms evolve? From the fossil record, we know
that the originators of life on Earth were bacteria and unique microbes called
Archaeans. These original life forms, known as prokaryotes (which means "before
a nucleus"), first appeared nearly four billion years ago. Prokaryotes
lack a cell nucleus and reproduce by simple division. Bacteria and Archaea are
the most prolific and diverse organisms on the planet, thriving in such extreme
environments as boiling hot springs in Yellowstone, snow drifts in Antarctica,
and hydrothermal vents on the ocean floor.
Somewhere in evolutionary history, a line of Archaean-type
prokaryotes developed a cell nucleus. These were the first eukaryotes (which
means "with a nucleus"). The nucleus may have protected the cell’s
DNA from damage from increasing amounts of oxygen in the atmosphere. These early
eukaryotes also acquired specialized cell activity centers called organelles.
Two of the most important organelles are mitochondria, which allows
the cell to metabolize oxygen, and chloroplasts, which allow a photosynthetic
plant cell to convert sunlight and carbon dioxide into an energy source.
How did eukaryotes acquire organelles? According to traditional evolutionary biology, these useful little cellular factories were created from a series of random mutations. From those early beginnings, according to theory, more mutations would result in the complex, multicellular organisms that populate the plant, animal, and other kingdoms of life.
But in the late 1960s, biologist Lynn Margulis rejected such traditional theories of cell evolution and began to champion something completely different. Picking up an idea first proposed by Russian biologist K.S. Mereschkovsky at the turn of the last century, Margulis wrote a paper theorizing that the original eukaryote was actually a merger between two prokaryotes. This merger, which she dubbed "endosymbiosis," benefitted both cells and created a brand-new life form. As evidence, Margulis cited data showing that DNA resided in the chloroplasts of plant cells, separate from the DNA in the cells' nuclei. According to Margulis, the chloroplast DNA physically and chemically resembled photosynthetic bacteria and represented the remnants of the original bacterial symbionts.
Here’s how it worked: About 1.4 billion years ago, a big prokaryotic cell ingested, or was invaded by, a smaller aerobic ("oxygen-loving") bacterium. The bacterium survived, nourished by the host cell, and eventually both host and invader lost their ability to survive without each other. This represented the first microbial merger under Margulis’ theory.
Plants evolved from the pioneering endosymbionts by acquiring photosynthetic bacteria sometime later. The chlorophyll-containing bacteria, resembling modern-day cyanobacteria, provided food for both themselves and their hosts and became the plant cell’s chloroplasts. The big mobile host cells helped the little symbionts (now evolved into mitochondria and chloroplasts) navigate to sources of oxygen and sunlight. The organelles manufactured food and provided the energy for cellular function. This cooperation, Margulis contends, was the driving force in evolution, creating entire new species and new branches in the tree of life.
It is now an accepted theory, but for years Margulis’
ideas were ridiculed. Her first paper was rejected by more than a dozen journals
before finally being accepted for publication by The Journal of Theoretical
Biology. Many in the scientific community protested that there was no evidence
for of endosymbiosis in the fossil record. But over
the years, compelling evidence started accumulating.
Molecular DNA studies in the late 1970s showed that sequences of genetic material in mitochondria were nearly identical to that of free-living purple bacteria commonly found in ponds, and quite unlike that of the nuclear DNA in the "host" cell. The same proved true when comparing the chloroplast and nuclear DNA of plant cells. "It’s undeniable that chloroplast DNA is most closely related to a group of photosynthesizing cyanobacteria," says Lynda Goff, a biologist specializing in the evolution of single-celled algae.
In another study conducted in the 1970s, a researcher at the University of Tennessee documented endosymbiosis in action. Kwang Jeon had noticed that some amoebae in his lab had developed dark spots inside their cells. The spots were parasitic bacteria, which had taken up residence in the amoeba cells and killed off most of Jeon’s collection. A few of the hardier amoebae survived and the biologist noticed that they soon returned to normal even with thousands of bacterial cells still living in them. Jeon continued monitoring his microbes and found that in just 200 generations, the amoebae not only tolerated the invader, but could no longer reproduce and survive without the once-pathogenic bacteria.
The implications of the endosymbiotic theory are far reaching. It means that cooperation and the swapping of genetic material is a fundamental mechanism of evolution. It also means that all complex living creatures, including humans, are made up of cells that are really conglomerations of cooperating microbes organized into a giant super-organism. Each cell in the human body really has two ancestors that merged long ago: a mitochondrial ancestor and a nuclear ancestor. Plants cells have three ancestral lines: nuclear, mitochondrial and chloroplast.
"It was arrogant of us to think we could go back and reconstruct a family tree, with one organism evolving neatly from one predecessor," Goff admits.
In fact, Goff says, we’re probably not dealing with an evolutionary tree at all. Instead, we should be thinking of a complex web of organisms whose collective genetic material is being swapped and absorbed across different species. "When one organism incorporates the genome of another, you’ve introduced a complex genetic event that has the possibility of bypassing a billion years of evolution in a single step."