While Charles Darwin's vision of evolution was brilliant, natural selection ignores a crucial force that helps to explain the diversity and wonder of life: symbiosis. In Darwin's Blind Spot, Frank Ryan shows how the blending of life forms through symbiosis has resulted in gigantic leaps in evolution. The dependence of many flowering plants on insects and birds for pollination is an important instance of symbiosis. More surprising may be the fact that our cells have incorporated bacteria that allow us to breathe oxygen. And the equivalent of symbiosis within a species -- cooperation -- has been a vital, although largely ignored, force in human evolution. In Ryan's view, cooperation, not competition, lies at the heart of human society.
Ryan mixes stories of the many strange and beautiful results of symbiosis with accounts of the dramatic historic rivalries over the expansion of Darwin's theory. He also examines controversial research being done today, including studies suggesting that symbiosis among viruses led to the evolution of mammals and thus of humans. Too often Darwin's interpreters have put excessive emphasis on competition and struggle as the only forces in evolution. But the idea of "survival of the fittest" does not always reign. Symbiosis is critically important to the richness of Earth's life forms.
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· Introduction ·
A MYSTERY OF NATURE
Our universe is a sorry little affair unless it has in it something for every age to investigate . . . Nature does not reveal her mysteries once and for all.
— Seneca, Naturales Questiones, Book 7
The first-century Roman philosopher and statesman Lucius Annaeus Seneca had the misfortune of serving as the emperor Nero’s adviser. At sixty- nine, he was returning to Rome from retirement in Campania when he learned of the death sentence that Nero had imposed on him. Seneca met his fate calmly, embracing his wife and friends and asking them to moderate their grief through reflection on the lessons of philosophy. His contribution to civilization remains relevant today, especially his insight into the mysteries of nature.
Two millennia later, Kwang Jeon was privileged to investigate such a mystery. His investigation came about indirectly, as so many important discoveries do. Though Jeon would be much too modest to claim so, the mystery he unraveled lies at the very heart of the evolution of life on Earth.
Now a distinguished research scientist in the department of biochemistry at the University of Tennessee at Knoxville, Jeon one day in 1966, while studying the humble amoeba at the State University of New York at Buffalo, experienced an unexpected calamity: his cultures of amoebae had been struck down by a plague. When he investigated, he found that they had been infected with an unknown strain of a bacterium later called the X-bacterium. He had no more idea where the infection had come from than the inhabitants of Europe had of the origins of the Black Death in the Middle Ages. “It just arrived, out of the blue. And suddenly, all of my amoebae began to die.” To find out if this bacterium was really causing the plague, Jeon tried infecting a few amoebae with the bacteria; the amoebae died. In the everyday world of clinical microbiology, that observation would have been enough: the X-bacterium would have been seen as a parasite to be eliminated. Jeon would have eradicated all the infected amoebae and sterilized his laboratory before starting the weary process of rebuilding his cultures.
The great Louis Pasteur once made a perceptive statement about the role of serendipity in scientific discovery: “Chance,” he declared, “favors only the prepared mind.” In Jeon, a combination of personality and circumstance ensured that chance had favored such a mind.
Korean-born Kwang Jeon never intended to become a biologist. At junior high school he had wanted to become a doctor of medicine: “In my young mind, I felt that I wanted to help others and do research on illnesses.” The Korean War put an end to that ambition. But then he had a stroke of luck. In 1961, having just completed his master’s degree in Seoul, he was adopted by the British Council and sent to London, where he studied zoology at King’s College. After completing his doctorate, he became involved in research for the first time. Once again he ended up on a path he had not originally intended to follow. He was interested in embryology, but several people in the Department of Zoology happened to be studying the amoeba and he rather reluctantly joined their research program. His imagination was quickly captured. “When for the first time I actually saw amoebae moving under the microscope, I was fascinated.” Just so does the thrill of discovery often begin for a scientist.
Most of us are familiar with the amoeba from high school biology, though we soon forget about it, assuming it has no relevance to our lives. This single-celled creature that lives in the mud of freshwater streams and ponds is about a fiftieth of an inch across and consists of a nucleus, which contains the DNA, surrounded by cytoplasm. We might even recall from biology class that the amoeba moves by pushing out blunt, fingerlike processes known as pseudopods. The young and curious Kwang Jeon asked himself how a cell with no limbs or obvious skeletal structures could engage in purposeful locomotion. “I watched, in a perfect stillness, how they put out their pseudopods to move.” He put amoebae into an environment with another creature, the green hydra. From the human perspective, hydras are minuscule, but they are predatory giants compared with amoebae. They also have a distinctive manner of feeding, pouncing on smaller life forms that come up close, stinging them with poisonous tentacles prior to devouring them. “I was expecting them to make a meal of my amoebae. But what actually happened was the hydra was gobbled up by the amoebae.” Jeon’s love affair with the amoeba had begun.
One of the commonest amoebae in the world is Amoeba proteus. One strain of this species, known as the “DD” strain, was discovered in Scotland in the early 1950s and subsequently found its way into research laboratories. Biologists were interested innnnn the D strain because its tissues were known to contain some curious passengers: living particles of unknown origin with a striking resemblance to bacteria. While working in London, Jeon became very interested in the D strain of Amoeba proteus.
Like the hydra, the amoeba is a predator of even smaller creatures, such as Colpidium, enfolding them, together with a drop of water, within its pseudopodia. This process is known as phagocytosis. But the amoeba is prey to infection by certain bacteria, which it ingests in much the same way. The bacteria are resistant to the amoeba’s digestive processes, and they infect it, causing serious ill health and sometimes death.
Some years later, Kwang Jeon moved to Buffalo, taking his beloved amoebae with him, to study under Jim Danielli, a world-renowned theoretical biologist who was interested in the phenomenon of cytoplasmic inheritance. The cytoplasm is the outer zone of the cell, separated from the nucleus by a double membrane. In the 1940s, a small number of scientists, including Danielli, began to doubt that all the hereditary programming was confined to the nucleus. To conventional biologists such doubt was outrageous. Since the close of the nineteenth century, biologists had been convinced that the nucleus was the sole repository of hereditary factors, so that any notion of cytoplasmic heredity seemed almost blasphemous. But those who took the idea seriously were very interested in finding out what actually happened when a microbe, containing its own genetic information, entered the cytoplasm of a cell whose hereditary information was supposedly confined within the nucleus.
Jeon’s perspective on the plague that wiped out his amoebae was radically different from what might have been expected of a conventional microbiologist. Examining the lethal epidemic, he discovered that not all of the infected amoebae died. The precious few that survived did so even though their cytoplasm carried tens of thousands of living X-bacteria. It was clear that these few amoebae differed from all the others, possessing some inherited resistance to the plague bacillus. Intrigued, Jeon put aside his populations of uninfected amoebae, making sure they were protected from contamination, and began a new line of experiments, studying the interaction between the infected amoeba and the X-bacterium.
His experiments continued for many years, with some startling observations. After infecting an amoeba, the X-bacteria were resistant to the digestive enzymes that would normally devour them. Infection was followed by multiplication of the bacteria in such massive numbers that the host died, releasing large numbers of bacteria to infect others. This sequence explained both the lethality and means of spread of the plague.
But in the tiny minority of resistant amoebae, the process was very different. The bacteria took up permanent residence in the amoeba’s cytoplasm, as if they had found a new home. And then the amoebae began to change. Newly infected ones — called xD amoebae — grew faster than those that had not been infected. They also seemed more fragile, being more vulnerable to starvation, overfeeding, and even minor temperature changes. They were so exquisitely sensitive to overcrowding that in normal colony densities the infected strain simply curled up and died. The hybrid life form might not have survived in nature because of this vulnerability.
Other changes in the xD amoebae were stranger still. The “genome” is the name scientists have given to the sum total of the genes that make up the heredity of any given species. For example, our human genome is made up of about 40,000 genes, parceled out in 46 chromosomes. At the time of Jeon’s experiment, biologists thought that all of this genetic material was confined to the nucleus. Jeon wondered if the interaction between the amoeba and the bacteria was confined to the cytoplasm or whether there might be some nuclear component. Knowing that the amoeba’s nucleus is remarkably tough, he placed two amoebae side by side and used a blunt probe to transplant the nucleus from one into the other. Normally the recipient amoeba would tolerate this transplantation very well. But when he transplanted a nucleus from an xD amoeba into a normal one, the grafted nucleus killed the recipient amoeba. This told Jeon that the infection was not affecting just the cytoplasm. It had changed the nucleus of the xD amoeba in some way that made it lethal to others.
Interaction with the amoeba also changed the bacterium. While initially up to 160,000 bacteria were found infecting a single amoeba, now, as some kind of equilibrium became established, the numbers of bacteria fell to about 45,000. Stranger still, if he removed the bacteria from the amoeba, they were no longer able to grow and reproduce in a laboratory culture. The bacteria could not survive outside the cytoplasm of their partner. At the same time, the host amoeba had become dependent on the bacterium for its survival. From an evolutionary perspective, something remarkable was taking place. Two utterly different species had melded into one, creating a new life form that was a hybrid of amoeba and bacterium. And the time frame was also interesting: the union was virtually instantaneous, although some further honing of the relationship continued afterward.
Some thirty-five years after it began, Dr. Jeon’s experiment is still continuing, but already he has solved a little of the mystery. When I asked him if he had found any evidence for direct genome-to-genome interaction during the evolution of the hybrid, he replied: “We think, now, that we have a handle on some aspects of this question. For example, the bacteria somehow suppress a gene that would normally be essential for the amoeba.” Many genes work by coding for the manufacture of proteins that play an important role in the body’s inner chemistry. In the hybrid amoebae an important enzyme was no longer being coded by nuclear genes, yet the enzyme was still in place and played a vital role in the hybrid’s chemistry. In Jeon’s words, “The enzyme must be coming from somewhere. Our feeling is that the bacterium is now supplying the gene for the enzyme.” In evolutionary terms the implications of this experiment are iconoclastic, differing radically from the theory proposed by Charles Darwin. Darwin believed that evolution proceeds by the gradual accumulation of small changes within individuals, governed by natural selection. Competition between individuals within a single species was the driving force. But what Jeon has observed is the union of two dissimilar species in a permanent living interaction. Their genomes, comprising thousands of genes that had evolved over a billennium of separate existence, have, in the evolutionary equivalent of the blink of an eye, melded into one. This is an example of an evolutionary mechanism known as “symbiosis,” which is very different from Darwin’s idea of natural selection. Today the overwhelming evidence suggests that this interactive pattern has played a formative, if largely unacknowledged, role in the origins and subsequent diversification of life on Earth.
Symbiosis complicates the unitary viewpoint taught in biology classes, but it brings a wonderful new perspective on life in general and on human society in particular. From the very beginning, evolutionary theory has been applied to many fields of human affairs, such as sociology, psychology and even politics. Such interpretations, viewed from a Darwinian perspective alone, lead to an excessive emphasis on competition and struggle. Most damaging of all, the social Darwinism of the first half of the twentieth century led directly to the horrors of eugenics. The rise, once more, of social Darwinism is therefore a source of worry to many scientists, philosophers, and sociologists. Recently, some evolutionary psychologists have gone so far as to suggest that rape may be a natural behavior. A broader understanding of evolution, taking into account not only interactions between species but also cooperation within our human species, would introduce some sense of balance into our understanding of these highly controversial aspects of human societal and psychosexual behavior.
Copyright © 2002 by Frank Ryan. Reprinted by permission of Houghton Mifflin Company
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