(Copyright © 1999 Piero Scaruffi | Legal restrictions - Termini d'uso )
Based on her theory of endosymbiosis, the American biologist Lynn Margulis reconstructs the evolution of life on Earth, from the earliest bacteria to multicellular organisms. Margulis' fundamental thesis is that our bodies are amalgams of several different strains of bacteria: endosymbiosis of bacteria is responsible for the creation of complex forms of life. Margulis follows the American biologist Ivan Wallin, who in 1927 was the first one to propose that bacteria may represent the fundamental cause of the "origin of species" (Darwin's unsolved mystery) and that the creation of a species may occur via endosymbiosis.
Margulis noted that not all the DNA is contained in the nucleus of the cell. As originally noted by Wallin, The "mitochondria" are organelles of the cell that function as its "power plants" and they contain their own DNA. While most DNA is organized as double sets of chromosomes in the nucleus, the DNA of mitochondria stands apart. Margulis believes that the presence of "extra" DNA in the cell is a fossil of an ancient evolutionary event: it testifies the fusion of at least two different kinds of organisms which together formed a "eukaryiotic" cell. Margulis found out the two bacteria involved are
Margulis believes that such symbiotic merger, or "symbiogenesis", has been common in the evolutionary history of life on Earth, and actually accounts for life as we know it today. The ancestors of all life are bacteria. They fused into "protists" (algae, amoebas, etc) which fused into multicelled organisms. Margulis tracks their evolution into plants, animals and fungi.
Margulis emphasizes that the Earth is still dominated by bacteria, which not only account for the vast majority of life, but also maintain the conditions for life on the planet. All life is either bacteria or descends from bacteria: life "is" bacteria. Bacteria are also closer to immortality than animals with bodies: cell division generates identical bacterial copies of a bacterial cell; bacteria can be killed but they do not naturally die. The life of a multicellular creature is far more fragile. Bacteria can also reproduce at amazing rates, compared with "higher" forms of life. Life can even be viewed as a plan for bacteria to exist forever: bodies are desirable food sources for bacteria, so one could view the evolution of bacteria into such bodies as a plan by bacteria to create food for themselves. The biosphere is controlled mostly by bacteria, it is, in a sense, "their" environment, not ours. Margulis emphasizes that not only the atmosphere but even the geology of our planet is due to the work of bacteria (mineral deposits have been shaped by the work of bacteria over million of years, or by the reaction with the waste gas of bacteria). We are allowed to live in it, thanks to the work of bacteria, which maintain the proper balance of chemicals in the air. If all bacteria died, everything would die. It is their world. Everything other form of life exists because they exist. (On a smaller scale, if you "fumigated" your body and destroyed all bacteria that live in it, your body would not be able to perform vital functions such as synthesizing vitamins and would die).
The mitochondria which dot all cells of all living beings are former bacteria, using oxygen to generate energy.
The property of bacteria that intrigues Margulis is that they trade genes, rapidly and easily. DNA is loose inside bacteria's "bodies". Bacteria reproduce by simply splitting their DNA in two. This yields two offspring identical to the parent (same genes). The change of genes occurs only when genes are traded among bacteria. Bacterial sex ("conjugation") is about making a new bacterium out of an existing bacterium by adding genes donated by another bacterium. The new bacterium resulting from the "engrafting" can even change sex, if the "sex" gene is received from the other bacterium (the "sex" gene specifies whether a bacterium is a donor or a receiver). This process is not really related to our sex: sex is about two beings making a new being which partially inherits genes from each parent. When bacteria "create" a new being, they do so by splitting (there is only one parent and the new being is identical to the parent). When bacteria trade genes, a being is changed into another being. We don't have either of these processes. I cannot split myself into identical copies of me, and I cannot mutate into another being by absorbing somebody else's genes. (Incidentally, bacteria can also trade genes as plasmids and viruses). This process of "recombination" occurs even among bacteria of different species. It is as if I could absorb genes from an eagle and turn into a human with wings, and making children who are also humans with wings. The genetic material of bacteria is extremely flexible and versatile.
Margulis thinks that this is the process that enabled life to evolve rapidly. Scale is crucial: what Margulis realized is the extent to which bacteria rule the planet. They account for a vast portion of the atmosphere and the geology of the planet. They spread in ancient times and are still spreading today at fantastic speed. Any phenomenon that involves bacteria is involving billions of rapidly moving and mutating beings. Once life was created, once the first bacteria appeared, things happened quickly and on a massive scale. Bacteria spread quickly, thanks to their reproductive efficiency and to their ability to feed on ubiquitous organic compounds. The first bacteria were "fermenters", feeding on the sugars available on the surface of the planet. They were followed by photosynthesizers: photosynthesis enabled these bacteria to feed on light. Then came bacteria ("cyanobacteria") that could tolerate oxygen and could therefore feed on water (extract hydrogen atoms from water). Each new type of bacteria was "polluting" the Earth and therefore changing the environmental conditions for future generations of bacteria. Pollution is an integral part of the evolution of life. The power of bacteria is that their "gene trading" habits made it relatively easy to adapt to whatever new conditions climate and their own doing were producing. The history of life is the history of a planet blanketed with rapidly reproducing and rapidly changing beings: the bacteria.
Not surprisingly, Margulis subscribes to the vision that the Earth as a whole is a living being.
Protoctists were born about 2 billion years ago from the fusion of bacterial cells. "Endosymbiosis" is the process by which a being lives inside another being. Eukaryotes (living beings who cells have a nucleus and whose DNA is confined in that nucleus) evolved from those protoctists. Mitochondria are visible remnants of this process of endosymbiosis. Experiments by the Korean biologist Kwang Jeon showed that even virulent pathogens can become organelles. Margulis deduces that predators can become symbionts, that a deadly infection can become a bodily part.
Then Margulis leaps to bodies made of several organs, and suggests that those organs also were accumulated the same way, that they are also due to the fusion with independent organisms by endosymbiosis.
If this is the essentials of Margulis' thesis, a mystery still has to be solved. Bacteria reproduce by replication and mutate by conjugation. Mitosis ("the dance of chromosomes") is the process by which eukaryotic cells reproduce: the DNA of the new being is a combination of the DNA of the two parents. In eukaryotes the DNA is not just a string: genes are organized in chromosomes (a minimum of two, humans have 46). How did this striking difference between bacteria and their descendents came to be?
Mitosis is truly responsible for the origin of species. Before mitosis, bacteria were freely exchanging genes: the concept of "species" as we know them today did not exist, as any bacterium could mutate into a novel "species" at any time. Bacteria do not have true species. Multicellular beings cannot trade genes. Therefore they cannot mutate into anything else, and their offspring belongs to the same species (because both parents must be of the same species in order to interbreed) and it inherits their genes. Genes stay in the same family, the "species". Any multicellular being is a member of a species: it is an obvious fact, but a quite striking one. It is one of nature's whims. In the beginning, a new bacterium could be just about any combination of available DNA. Later, a new individual had to be a member of a species.
It cannot be a coincidence that death was invented with multicellular sexual beings. They age and die, whereas bacteria didn't.
Both sex and death are a mystery: why did sexless and immortal bacteria evolve into beings that have sex and die?
Bacteria have only one sex, they can mutate (change their DNA), they can interbreed with any bacteria, they don't make children, and they never age or die. Animals that evolved from them have two sexes, they cannot mutate (cannot change their DNA), the can only inbreed within their species, they make children and they age and die. The differences are more and more striking. (And let us not forget that the DNA of animals is organized and inherited in units called chromosomes, a detail that may turn to be crucial to explain all of the above discrepancies).
Margulis says that "death was the first sexually transmitted disease". Once animals started aging and dying (once death had been programmed in their DNA), their offspring inherited the same disease.
Margulis' idea is that, once upon a time, "eating and mating were the same". Cannibal unicellular beings may have merged into multicellular beings. The evolutionary advantages of this behavior may have led to sexual beings. But this is far fetched: the genders are exactly two, and each member of a gender has the same sexual organs. How did that happen?
Notwithstanding Margulis' cunning speculations on the subject, sexually reproducing species are still a mystery, and so is death, that came with sex.
While Darwin was emphasizing competition as the driving process of evolution, Margulis is emphasizing cooperation.
Margulis' fascination with the wonders of the bacterial world led her to believe that no other single force has shaped evolution in a more important way. Everything the Earth is today and everything we and other living forms do today is due to conditions that have been created and maintained by bacteria.
Margulis also introduces the novel idea that life has "free will" and has used it to influence its own evolution. It is not only humans who can affect their environment to direct their own evolution: the whole environment is doing the same. Living beings make decisions all the time and are thus responsible in part for their own evolution, as first speculated by Samuel Butler, a contemporary of Darwin.
The book is a good introduction to Margulis' ideas and, while being readable, provides a detailed account of the chemical processes required to make life evolve.