The Britannica Guide to Theories and Ideas That Changed the Modern World

Britannica Educational Publishing, Kathleen Kuiper

• Editore: Britannica Educational Publishing
• Anno: 2009
• ISBN:
  9781615300631
• Argomento: History
• Categoria: World History

There was a time when people assumed that the world was flat. Once an alternate theory was proposed, however, that conceit was challenged and, eventually, disproved. In short, theories and ideas can be potent agents of change—none more so than those that are extensively detailed in this book.

Chapter 1:
THE BIOLOGICAL SCIENCES

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The probing of the origin of life, cell theory, germ theory, and genetic inheritance, as well as the ideas regarding evolution and ecology—the development of a sense of how living things interact and form a system of relationships—represent profound turning points in human history. This chapter addresses some of the discoveries that have affected the way we think about ourselves, other living things, and life’s vital processes.

THE ORIGIN OF LIFE

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If a species can develop only from a preexisting species, then how did life originate? Among the many philosophical and religious ideas advanced to answer this question, one of the most popular was the theory of spontaneous generation, according to which living organisms could originate from nonliving matter. With the increasing tempo of discovery during the 17th and 18th centuries, however, investigators began to examine more critically the Greek belief that flies and other small animals arose from the mud at the bottom of streams and ponds by spontaneous generation. Then, when the English physician William Harvey announced his biological theory ex ovo omnia (“everything comes from the egg”), it appeared that he had solved the problem, at least insofar as it pertained to flowering plants and the higher animals, all of which develop from an egg. But Antonie van Leeuwenhoek’s subsequent disquieting discovery of animalcules demonstrated the existence of a densely populated but previously invisible world of organisms that had to be explained.

A 17th-century Italian physician and poet, Francesco Redi, was one of the first to question the spontaneous origin of living things. Having observed the development of maggots and flies on decaying meat, Redi in 1668 devised a number of experiments, all pointing to the same conclusion: if flies are excluded from rotten meat, maggots do not develop. On meat exposed to air, however, eggs laid by flies develop into maggots. But renewed support for spontaneous generation came from the publication in 1745 of a book, An Account of Some New Microscopical Discoveries, by John Turberville Needham, an English Catholic priest; he found that large numbers of organisms subsequently developed in prepared infusions of many different substances that had been exposed to intense heat in sealed tubes for 30 minutes. Assuming that such heat treatment must have killed any previous organisms, Needham explained the presence of the new population on the grounds of spontaneous generation. The experiments appeared irrefutable until Lazzaro Spallanzani, an Italian biologist, repeated them and obtained conflicting results. He published his findings about 1775, claiming that Needham had not heated his tubes long enough nor had he sealed them in a satisfactory manner. Although Spallanzani’s results should have been convincing, Needham had the support of the influential French naturalist Buffon; hence the matter of spontaneous generation remained unresolved.

THE DEATH OF SPONTANEOUS GENERATION

After a number of further investigations had failed to solve the problem, the French Academy of Sciences, in January 1860, offered a prize for contributions that would “attempt, by means of well-devised experiments, to throw new light on the question of spontaneous generation.” In response to this challenge, Louis Pasteur, who at that time was a chemist, subjected flasks containing a sugared yeast solution to a variety of conditions. Pasteur was able to demonstrate conclusively that any microorganisms that developed in suitable media came from microorganisms in the air, not from the air itself, as Needham had suggested. Support for Pasteur’s findings came in 1876 from an English physicist, John Tyndall, who devised an apparatus to demonstrate that air had the ability to carry particulate matter. Because such matter in air reflects light when the air is illuminated under special conditions, Tyndall’s apparatus could be used to indicate when air was pure. Tyndall found that no organisms were produced when pure air was introduced into media capable of supporting the growth of microorganisms. It was these results, together with Pasteur’s findings, that put an end to the doctrine of spontaneous generation.

When Pasteur later showed that parent microorganisms generate only their own kind, he thereby established the study of microbiology. Moreover, he not only succeeded in convincing the scientific world that microbes are living creatures, which come from preexisting forms, but also showed them to be an immense and varied component of the organic world, a concept that was to have important implications for the science of ecology. Further, by isolating various species of bacteria and yeasts in different chemical media, Pasteur was able to demonstrate that they brought about chemical change in a characteristic and predictable way, thus making a unique contribution to the study of fermentation and to biochemistry.

THE ORIGIN OF PRIMORDIAL LIFE

In the 1920s a Soviet biochemist, A.I. Oparin, and other scientists suggested that life may have come from nonliving matter under conditions that existed on the primitive Earth, when the atmosphere consisted of the gases methane, ammonia, water vapour, and hydrogen. According to this concept, energy supplied by electrical storms and ultraviolet light may have broken down the atmospheric gases into their constituent elements, and organic molecules may have been formed when the elements recombined.

Some of these ideas have been verified by advances in geochemistry and molecular genetics; experimental efforts have succeeded in producing amino acids and proteinoids (primitive protein compounds) from gases that may have been present on the Earth at its inception, and amino acids have been detected in rocks that are more than three billion years old. With improved techniques it may be possible to produce precursors of or actual self-replicating living matter from nonliving substances. But whether it is possible to create the actual living heterotrophic forms from which autotrophs supposedly developed remains to be seen.

Although it may never be possible to determine experimentally how life originated or whether it originated only once or more than once, it would now seem—on the basis of the ubiquitous genetic code found in all living organisms on Earth—that life appeared only once and that all the diverse forms of plants and animals evolved from this primitive creation.

ANTONIE VAN LEEUWENHOEK

(b. Oct. 24, 1632, Delft, Neth.—d. Aug. 26, 1723, Delft)

The Dutch microscopist Antonie van Leeuwenhoek was the first to observe bacteria and protozoa. In his youth he was apprenticed to a draper; a later civil position allowed him to devote time to his hobby: grinding lenses and using them to study tiny objects. With his simple microscopes—skillfully ground, powerful single lenses of very short focal length, capable of high image quality—he observed what he called “very little animalcules,” protozoa in rainwater and pond and well water and bacteria in the human mouth and intestine. He also discovered blood corpuscles, capillaries, and the structure of muscles and nerves, and in 1677 he first described the spermatozoa of insects, dogs, and humans. His research on lower animals argued against the doctrine of spontaneous generation, and his observations helped lay the foundations for the sciences of bacteriology and protozoology.

CELL THEORY

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The history of cell theory is a history of the actual observation of cells, because early prediction and speculation about the nature of the cell were generally unsuccessful. The decisive event that allowed the observation of cells was the invention of the microscope in the 17th century, after which interest in the “invisible” world was stimulated. English physicist Robert Hooke, who described cork and other plant tissues in 1665, introduced the term cell because the cellulose walls of dead cork cells reminded him of the blocks of cells occupied by monks. Even after the publication in 1672 of excellent pictures of plant tissues, no significance was attached to the contents within the cell walls. The magnifying powers of the microscope and the inadequacy of techniques for preparing cells for observation precluded a study of the intimate details of the cell contents. Leeuwenhoek, beginning in 1673, discovered a number of things such as blood cells and spermatozoa. A new world of unicellular organisms was opened up. Such discoveries extended the known variety of living things but did not bring insight into their basic uniformity. Moreover, when Leeuwenhoek observed the swarming of his animalcules but failed to observe their division, he could only reinforce the idea that they arose spontaneously.

Cell theory was not formulated for nearly 200 years after the introduction of microscopy. Explanations for this delay range from the poor quality of the microscopes to the persistence of ancient ideas concerning the definition of a fundamental living unit. Many observations of cells were made, but apparently none of the observers was able to assert forcefully that cells are the units of biological structure and function.

Three critical discoveries made during the 1830s—when improved microscopes with suitable lenses, higher powers of magnification without aberration, and more satisfactory illumination became available—were decisive events in the early development of cell theory. First, the nucleus was observed by Scottish botanist Robert Brown in 1833 as a constant component of plant cells. Next, nuclei were also observed and recognized as such in some animal cells. Finally, a living substance called protoplasm was recognized within cells, its vitality made evident by its active streaming, or flowing, movements, especially in plant cells. After these three discoveries, cells, previously considered as mere pores in plant tissue, could no longer be thought of as empty, because they contained living material.

German physiologist Theodor Schwann and German biologist Matthias Schleiden clearly stated in 1839 that cells are the “elementary particles of organisms” in both plants and animals and recognized that some organisms are unicellular and others multicellular. This statement was made in Schwann’s famous Mikroskopische Untersuchungen über die Übereinstimmung in der Struktur und dem Wachstume der Tiere und Pflanzen (1839; Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants). Schleiden’s contributions on plants were acknowledged by Schwann as the basis for his comparison of animal and plant structure.

Robert Brown. Hulton Archive/Getty Images

Schleiden and Schwann’s descriptive statements concerning the cellular basis of biologic structure are straightforward and acceptable to modern thought. They recognized the common features of cells to be the membrane, nucleus, and cell body and described them in comparisons of various animal and plant tissues. A statement by Schleiden pointed toward the future direction of cell studies:

Each cell leads a double life: an independent one, pertaining to its own development alone; and another incidental, insofar as it has become an integral part of a plant. It is, however, easy to perceive that the vital process of the individual cells must form the first, absolutely indispensable fundamental basis, both as regards vegetable physiology and comparative physiology in general.

THE PROBLEM OF THE ORIGIN OF CELLS

Schwann and Schleiden were not alone in contributing to this great generalization of natural science, for strong intimations of the cell theory occur in the work of their predecessors. Recognizing that the basic problem was the origin of cells, these early investigators invented a hypothesis of “free cell formation,” according to which cells developed de novo out of an unformed substance, a “cytoblastema,” by a sequence of events in which first the nucleolus develops, followed by the nucleus, the cell body, and finally the cell membrane. The best physical model of the generation of formed bodies then available was crystallization, and their theory was inspired by that model. In retrospect, the hypothesis of free cell formation would not seem to have been justified, however, since cell division, a feature not characteristic of crystallization processes, had frequently been observed by earlier microscopists, especially among single-celled organisms. Even though cell division was observed repeatedly in the following decades, the theory of free cell formation lingered throughout most of the 19th century; however, it came to be thought of more and more as a possible exception to the general principle of the reproduction of cells by division. The correct general principle was affirmed in 1855 by a German pathologist and statesman, Rudolph Virchow, who asserted that “omnis cellula e cellula” (“all cells come from cells”).

The inherently complex events of cell division prevented a quick resolution of the complete sequence of changes that occur during the process. First, it was noted that a cell with a nucleus divides into two cells, each having a nucleus; hence, it was concluded that the nucleus must divide, and direct division of nuclei was duly described by some. Better techniques served to create perplexity, because it was found that during cell division the nucleus as such disappears. Moreover, at the time of division, dimly discerned masses, now recognized as chromosomes, were seen to appear temporarily. Observations in the 1870s culminated in the highly accurate description and interpretation of cell division by German anatomist Walther Flemming in 1882. His advanced techniques of fixing and staining cells enabled him to see that cell reproduction involves the transmission of chromosomes from the parent to daughter cells by the process of mitosis and that the division of the cell body is the terminal event of that reproduction.

Rudolf Virchow. Courtesy of Bildarchiv Preussischer Kulturbesitz BPK, Berlin

Chromosomes are inside the cells of every living thing. They are so small that they can be seen only through a powerful microscope. © Howard Sochurek/Corbis

Discovery that the number of chromosomes remains constant from one generation to the next resulted in the full description of the process of meiosis. The description of meiosis, combined with the observation that fertilization is fundamentally the union of maternal and paternal sets of chromosomes, culminated in the understanding of the physical basis of reproduction and heredity. Meiosis and fertilization therefore came to be understood as the complementary events in the life cycle of organisms: meiosis halves the number of chromosomes in the formation of spores (plants) or gametes (animals), while fertilization restores the number through the union of gametes. By the 1890s “life” in all of its manifestations could be thought of as an expression of cells.

THE PROTOPLASM CONCEPT

As the concept of the cell as the elementary particle of life developed during the 19th century, it was paralleled by the “protoplasm” concept—the idea that the protoplasm within the cell is responsible for life. Protoplasm had been defined in 1835 as the ground substance of living material and hence responsible for all living processes. That life is an activity of an elementary particle, the cell, can be contrasted with the view that it is the expression of a living complex substance—even a supermolecule—called a protoplasm. The protoplasm concept was supported by observations of the streaming movements of the apparently slimy contents of living cells.

Advocates of the protoplasm concept implied that cells were either fragments or containers of protoplasm. Suspicious and often contemptuous of information obtained from dead and stained cells, such researchers discovered most of the basic information on the physical properties—mechanical, optical, electrical, and contractile—of the living cell.

An assessment of the usefulness of the concept of protoplasm is difficult. It was not wholly false; on the one hand, it encouraged the study of the chemical and mechanical properties of cell contents, but it also generated a resistance, evident as late as the 1930s, to the development of biochemical techniques for cell fractionation and to the realization that very large molecules (macromolecules) are important cellular constituents. As the cell has become fractionated into its component parts, protoplasm, as a term, no longer has meaning. The word protoplasm is still used, however, in describing the phenomenon of protoplasmic streaming—the phenomenon from which the concept of protoplasm originally emerged.

THE CONTRIBUTION OF OTHER SCIENCES

Appreciation of the cell as the unit of life has accrued from important sources other than microscopy; perhaps the most important is microbiology. Even though the small size of microorganisms prohibited much observation of their detailed structure until the advent of electron microscopy, they could be grown easil...