What Is Endosymbiosis?

Three biologists stand out in the history of the theory of endosymbiosis—the relationship in which a member of one species lives not just near or even permanently ON a member of another species, but INSIDE IT. The three biologists are Konstantin Merezhkovsky, Ivan Wallin, and Lynn Margulis.

Konstantin Merezhkovsky (1855-1921) was a prominent Russian biologist whose research on lichens led him to theorize that larger, more complex cells evolved from a symbiotic relationship between less complex ones. In 1905 he coined the term “symbiogenesis” to describe “the living together of different kinds of organisms”. He adamantly rejected British naturalist Charles Darwin’s theory of evolution, which suggested that new species arose from natural selection and that the source of genetic variation on which natural selection acted came from “random mutations”. Rubbish, said Konstantin! The acquisition and inheritance of microbes best explains “biological novelty”. He was criticized by his colleagues, but wrote “Symbiogenesis and the Origin of Species” (published in 1926), which preserved his ideas for future generations.  

In the 1920s American biologist Ivan Wallin (1883-1969) again proposed that organelles such as chloroplasts (plastids) and mitochondria (energy producing organelles) originated as symbiotic bacteria. In his 1927 book (in English) titled “Symbionticism and the Origin of Species”, he described how new species form by the permanent acquisition of symbiotic bacteria. Wallin posited that mitochondria were once independent bacteria that took up permanent residence inside of existing cells to become what we today know as cell “organelles”. Cells evolved, he concluded, by symbionticism—the formation of microsymbiotic complexes. Wallin’s colleagues, like Merezhkovsky’s, roundly castigated his theory for two reasons. First, no one could repeat his claim to have actually cultivated mitochondria outside of cells. Second, Charles Darwin argued that the source of inherited variation that gives rise to new species comes from “random mutations”, not symbiosis. Wallin abandoned his idea.

American evolutionary biologist Lynn Margulis resuscitated the idea of symbiosis as the source of inherited variation in 1967 after graduating from University of California, Berkeley, in 1963. She too proposed that organelles such as mitochondria living happily inside of nucleated cells evolved from ancient independent-living bacteria. Furthermore, these organelles were the source of cytoplasmic DNA genes living outside of the cell’s nucleus that were confounding geneticists at the time.

 “The crucial piece of evidence unavailable to Wallin until just before he died,” writes Margulis, “was the discovery that mitochondria and plastids possess their very own DNA. Wallin knew though that mitochondria and plastids tend to reproduce at different times than do the cells in which they reside—as if demonstrating a residual impulse of their earlier, wilder days.” (1) Wallin, it turns out, correctly proposed that bacteria, the organisms which are popularly associated with disease, may represent the fundamental causative factor in the origin of species.” (1)

Lynn Margulis and her son Dorion Sagan continue to champion endosymbiosis theory today. They argue that random mutation, long believed (but never demonstrated) to be the main source of genetic variation, is of only marginal importance. Much more significant is the acquisition of new genomes by symbiotic merger. (2)

Bacteria: Masters of Biosphere

Bacteria have been perceived by most people as disease-causing microbes since the germ theory of contagion caught on. Otherwise they have been largely ignored. Yet “[h]ad [bacteria] been discovered on Mars, their description would have been much more dramatic and the bizarre quality of their natural history, which often seems like science fiction, would not have been missed,” notes Sorin Sonea (a physician) and Maurice Panisset (a veterinarian) (3)

The greatest division in the kingdom of the living is NOT between plants and animals, but between “bacteria” and “organisms made of nucleated cells”. (4) Bacteria are called “prokaryotes” and “organisms made of nucleated cells” are named “eukaryotes”.

A third category of tough bacteria living still today are called “archaebacteria”. They are considered to be the direct descendants of the earliest life on earth. Archaebacteria include salt-loving “halophiles”, heat-loving “thermophiles”, and methane-producing “methanogens”. The most important thing to remember about archaebacteria is that they despise oxygen. They prefer to live in anaerobic (oxygen-less) environments, such as on the bottom of the ocean, in sewer water, in the hot springs of Yellowstone National Park, and even in the stomachs of cows. (5) It is easy to understand how these ancient bacteria thrived on earth when there was no or very little oxygen, but a lot of carbon dioxide.

So, how did oxygen get into the atmosphere? Oxygen was only released into the atmosphere when “blue-green bacteria” evolved a way to use energy from sunlight to break apart water molecules to grab their precious hydrogen, explains Margulis. (5) “Combining the hydrogen with carbon atoms drawn from then-abundant carbon dioxide, blue-green bacteria were able to manufacture DNA, proteins, sugars, and all their other cell components. These light-needy bacteria quickly expanded to sunny waters everywhere on the Archean Earth. In so doing, they released vast amounts of molecular oxygen left over from their hydrogen mining of water.” (5) These earliest of bacteria were true innovators! They are the predecessors of plastids in plants, which remove carbon dioxide from the atmosphere, using carbon for their bodies and eliminating as waste the oxygen we breathe in fresh air. 

Bacteria, to repeat, do NOT have a nucleus, which is why they are prokaryotes (which means “before nuclei”) and NOT eukaryotes. Instead, their DNA is loose within their bodies. As a result of this situation, bacteria NEVER reproduce by mitosis, which evolved AFTER the Archean time (in the Proterozoic). “A parent bacterium simply elongates its DNA, dragged by growing membrane to which it is attached, until the full-grown cell splits to form two offspring identical to it,” explains Margulis (p. 94)

Bacteria may not reproduce by mitosis, but they DO trade their DNA very easily. Margulis imagines the Archean Earth as a “promiscuous place of prodigious growth and rapid genetic transfer that led, by and by, to the genetic restrictions of the Proterozoic descendants known as “protists”. (p. 93) Bacteria will sometimes trade naked pieces of DNA called “plasmids” or as protein-coated pieces called “viruses”. [Hmmmm] The way that bacteria trade their DNA is to grow a cell bridge through which the genes pass. This is called “conjugation”. The offspring is a unique genetic recombinant. Bacterial recombination is the rage among biotechnologists who force the colon bacterium Escherichia coli to produce, for example, human insulin by getting the bacteria to take up a particular human gene.

Prokaryotic bacterial cells NEVER fuse (like an egg and sperm). Their genes instead FLOW. Margulis paints a compelling picture of a world in which human genes behaved like prokaryotes: “Imagine you are a blue-eyed person (perhaps with newly acquired green hair) who, in a swimming pool, gulps the more common gene for brown eyes. Toweling off, you pick up genes from sunflowers and pigeons. Soon the brown-eyed you is sprouting petals and flying—eventually reproducing into gliding brown-eyed, green-haired quintuplets. This fantasy is mundane reality in the world of bacteria, except that most genes traded there are for metabolic and subvisible traits.” (p. 96)

Permanent Mergers

Two billion years ago, the interactions of bacteria created a new kind of cell. These new cells were the first “proctotists”, or nucleated cells. They formed when bacteria MERGED. Proctocists include about 250,000 species today, such as the amebas and diatoms, and giant kelps and red seaweed. Proctocists that are single cells (say, an ameba) are called “protists” (small proctocists). How then did the evolution of a cell with a nucleus occur—what is this “merging” all about?

The quick answer is by the merging of different kinds of bacteria, says Margulis. “Protoctists evolved through symbiosis; twigs and limbs on the tree of life not only branched out but grew together and fused. Symbiosis refers to an ecological and physical relationship between two kinds of organisms that is far more intimate than most associations…Symbiosis, like marriage, means living together for better or worse; but whereas marriage is between two different people, symbiosis is between two or more different types of live beings.” (p. 119)

For example, humans have a symbiotic relationship with the bacteria that live in the spaces between their teeth and in their intestines. The bacteria in human intestines produce vitamins such as B and K, without which humans would perish. Margulis delicately moves to her thesis: Organisms form “many kinds of symbioses, but the most awe-inspiring is the exceedingly close association known as endosymbiosis. This is a relationship in which one being—microbe or larger—lives not just near (nor even permanently on) another, but INSIDE IT. In endosymbiosis, organic beings merge. Endosymbiosis is like a long-lasting sexual encounter except that the participants are members of different species. Indeed, some endosymbiotic linkages have become permanent.” (p. 120) 

Evidence in modern organisms of earlier permanent mergers fascinates Margulis who likes to describe evidence of three permanent mergers in particular: mitochondria, plastids (e.g., chloroplasts), and “squirmers”.

Precursor Bacteria to Modern Mitochondria Organelles

Recall the blue-green photosynthetic bacteria discussed earlier that put oxygen as a waste product into the atmosphere. The new oxygen rapidly reacted with minerals in the environment, such as iron and sulphur, to form new minerals like sulfates, magnetite and hematite. But excess oxygen accumulated. One type of early bacteria began to take advantage of this excess oxygen. They employed oxygen to improve cell processes of energy transformation. They did this by developing pathways that produced 36 ATP molecules of energy when fermenting 1 sugar molecule, compared with a meager 2 ATP molecules produced without oxygen.

These smart new oxygen-respiring bacteria were purple! Margulis believes that they are similar to modern rod-shaped bacteria like Paracoccus denitrificans, which have more than 40 enzymes in common with human mitochondria. But more probably, says Margulis, the purple bacteria were similar to the respiring Bdellovibrio or Daptoacter—modern predatory prokaryotes that attack, multiply, and then explode out of larger bacteria. But something must have happened to turn attackers into permanent needed mitochondrial organelles that did not explode the cells they victimized. This happens, although it is obscure HOW it happens. Kwang Jeon at the University of Tennessee was able to demonstrate how this happens in experiments described elsewhere. (pp. 131-132)

Precursor Bacteria to Modern Plastids

The cells of plants and algae proctoctists (with nuclei) possess colorful bodies called plastids. Plastids are photosynthetic organelles that are bounded by double membranes and that contain the enzymes and pigments for photosynthesis, ribosomes, nucleoids, and other structures. The DNA in the plastids of the cells of the red seaweed Porphyridium is closer in its nucleotide sequence to that of certain oxygen-respiring green bacteria (cyanobacteria) than to DNA in the nucleus of the red seaweed itself, declares Margulis (p. 121)

Precursor Bacteria to Swimmers

Margulis presents convincing evidence that the ancestors of today’s spirochetes were “proton-powered bacteria that ferment carbohydrates and whip about like possessed corkscrews.” (p. 122) The most rapid swimmers of the entire bacterial kingdom, spirochetes literally “screw their way through mud, tissue, and slime.” (p. 122) Spirochetes are motile heterotrophic bacterial cells in which flagella inserted at the ends of the cell are wound beneath the inner and outer membrane in the flexible cell wall. (p. 263)

Among the most successful life forms on earth, spirochetes form alliances sometimes by

docking reversibly onto receptors of larger organisms and propelling them along. One modern organelle related to the spirochete is a simple shaft (kinetsome) that in cross section shows a distinctive “9(2) + 2 pattern: nine sets of two tubules arranged near the perimeter of the circular axis, with one set of two tubules at the center.” These squirming shafts are found in plants, animals, fungi, and protists, which is strong evidence of a common origin. For example, the 9(2) + 2 symmetry is found in the cell extension of the balance organ of human inner ears, and in tails propelling sperm cells of men. Margulis refers to these shafts as “undulipodia”, which is a very good name for them. “It may be that the spirochetes that symbiotically became undulipodia have become so integrated with their partners that they have dissolved away to mere traces and genetic shadows of their former selves.” (p. 127) This is particularly true in the case of mitotic spindles, which also show the 9(2) + 2 symmetry. In Margulis’ view, motility was the first endosymbiotic acquisition of the nascent eukaryote; losing parts of themselves as they evolved, squirming spirochetes invaded and animated what were to become nucleated cells. (p. 127)

Summary

Random mutation is not the only source of the genetic variation on which natural selection acts to produce evolutionary change in organisms. Rather endosymbiosis is probably the main source of genetic variation, according to Konstantin Merezhkovsky, Ivan Wallin, and Lynn Margulis. Mitochondria and plastids inside of, and tails on, nucleated cells exist because billions of years ago, bacteria of different kinds permanently merged.

Sources:

1. Lynn Margulis, Dorion Sagan: “What Is Life?” University of California Press, 1995, pp. 132-133.

2. Lynn Margulis has published a number of best-selling books over the years, including:  “Acquiring Genomes: The Theory of the Origins of the Species” (2003), “Microcosmos: Four Billion Years of Microbial Evolution” (1997); “What Is Life?” (2000); “Symbiotic Planet: A New Look at Evolution” (2000); “Early Life” (2002); “Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth” (1998); “Five Kingdoms: A Multimedia Guide to the Phyla of Life on Earth” (1995); “The Ice Chronicles: The Quest to Understand Global Climate Change” (2002); “Slanted Truths: Essays on Gaia, Symbiosis, and Evolution” (1997); “The Nature of Life” (2001); and “Environmental Evolution” (2000).

3. Ibid, p. 113.

4. See also: Sorin Sonea, Leo Mathieu: “Prokaryotology: A Coherent View.” Les Presses De L'universite De Montreal. 2001; and Sorin Sonea: “A New Bacteriology.” Jones & Bartlett, 1983.

4. Lynn Margulis, Dorion Sagan: “What Is Life?” University of California Press, 1995, p. 113.

5. Ibid, p. 89.