Anthony B. Wight
Writers of mythologies, free to roam the breadth of human imagination, are “limited” by the extent of their linguistic expression. Modern scientists—even armed with vast new technologies—are limited by the constraints of a discipline which requires inquiry to meet rational, quantitative standard. Yet math and science have in common a searching, investigative outlook toward life. As paleontologist Stephen Jay Gould recently stated:
. . . there are about half a dozen scientific subjects that are immensely intriguing to people because they deal with fundamental issues that disturb us and cause us to wonder. . .[scientific study of] evolution is one of those subjects. It attempts, insofar as
science
can, to answer the questions of
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what our life means,
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why we are here,
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where we come from,
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who we are related to,
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what has happened through time, and
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what has been the history of this planet.
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These are the questions that all thinking people have to ponder.
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II. Theories of Evolution
Objectives: Students should be able
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1. To outline the history of thinking about evolution.
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2. To give examples of the evidence used to develop and defend theories of evolution.
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3. To identify several key thinkers in the history of evolution and their contributions to the debate.
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4. To explain the differences between Darwin’s and Lamarck’s theories.
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5. To explain the meaning of “natural selection” and give an example of its operation as a mechanism of evolution.
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6. To apply Darwin’s theory to the history of human evolution, with particular attention to differences between Neanderthals and Cro-
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Magnons.
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7. To draw a timeline of the hominid family tree.
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8. To explain the scope of modern molecular biology.
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9. To compare and contrast the two strands of scientific study of human origins—morphology and molecular biology.
Strategies:
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1. Watch Smithsonian video, “Tales of the Human Dawn,” and construct a timeline of the hominids as a small group exercise.
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2. Research and construct a timeline of the early ideas of evolution.
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3. Read selections from
BSCS
, Chapter 1, and draw a concept map to compare and contrast Darwin’s and Lamarck’s theories.
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4. Develop a “Who’s Who” of Evolution chart—individual student research and brief reports to the class.
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5. Read the “Lucy” article by Johanson on the discovery of the most complete
Australopithecine
fossil.
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6. Oral reports on key scientists and discoveries in development of molecular biology.
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7. Watch the movie “Race for the Double Helix” and discuss the significance of discovery of DNA molecular structure for evolution.
Discussion: Evidence for evolution has become so pervasive that to inveigh against it is similar to King Canute requesting the retreat of the tide.
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In 1977, William V. Mayer, director of the Biological Sciences Curriculum Study, could make the above claim with no dispute from the scientific community and strong reaction only from religious special interest groups within the larger population. This was not always the case, however.
The publication in 1859 of Charles Darwin’s
Origin of Species
touched off such broad debate and publicity about evolution that it still echoes in the public press and popular culture today—overshadowing, unfortunately, the great scientific strides in understanding and collection of evidence in the 100 years since Darwin. As one entertaining illustration of this point a class session might be spent analyzing, discussing, and sketching cartoons from a variety of popular artists (Gary Larson, Jim Davis, Burke Breathed) who make their bread and butter from humorous depictions of “Darwinian” evolutionary ideas. “Darwin” and “evolution” have become so inextricably linked in common lore that it may come as a great surprise to students to learn that evolution was a concept established long, long before Darwin and that “the current status of evolution bears about the same relationship to Darwin [and his finches] as today’s quantum physics holds to Newton [and his apple].”
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Attempts to explain the origin of life and the diversity of living things are as old as human history itself. In ancient myths can be found the strands of searching for answers to evolutionary questions. But not only myths reflect the roots of this quest. Students will possibly be interested in tracing the line of scientific theorizing about life as sketched in the time line below.
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The earliest written records of the Greeks reveal formulation of hypotheses about evolution:
636-546 B.C.
Thales
, an early philosopher, theorized in writings about the origin of life.
611-547 B.C.
Anaximander
conceived the idea of gradual evolution from a formless chaotic condition to ordered, organic life. He even held view of adaption and transformation of aquatic species to land.
495-435 B.C.
Empedocles
outlined a concept of gradual evolution—plant species preceding animals and better adapted forms replacing others. (William Mayer makes a case for Empedocles as a more appropriate choice than Darwin as the founder of the evolutionary idea.)
The emergence of the Christian Church with its doctrinal control promoted the dogma of “special creation”—essentially a literal interpretation of the biblical Genesis story. This did not, however, completely stifle attempts by some of the early and later church theologians to reconcile the idea of evolution with scripture:
331-396 A.D.
Gregory of Nyssa
, although believing that God created the fundamental properties and laws of nature, believed that present existence developed gradually out of chaotic material, a viewpoint similar to that of Anaximander.
335-430 A.D.
Augustine
, among his many writings, developed an interpretation of the biblical account of creation as allegoric.
1225-1274 A.D.
Thomas Aquinas
, an Augustinian scholar, supported his views and suggested that the earth had received the power to produce organisms, further questioning the Genesis creation ordering and time frame.
By the late 16th century, scientific and philosophical thinking in the west was no longer under total control of religious authorities. Alongside a rising movement for reform within the Christian Church came an upsurge of early enlightenment philosophy aggressively seeking to stretch the boundaries of human understanding:
1561-1626 A.D.
Francis Bacon
, the English philosopher, revived the idea of evolution during this time of challenging the dominant religious world view. With spreading enthusiasm,
Descartes
(1596-1650),
Leibniz
(1646-1716),
Kant
(1724-1804), and others pushed open the doors of inquiry which led the great naturalists of the 18th and early 19th centuries to explain how evolution had occurred.
1707-1778
Carl Linneaus
, while not specifically examining evolution, developed the system of classification of plant and animal kingdoms which is the basis of modern understanding of relationships and diversity.
1707-1788
Leclerc de Buffon
contributed the idea that environments can directly modify plant and animal structure, and that these changes may be conserved through heredity.
1731-1802
Erasmus Darwin
, the grandfather of Charles, raised questions about organisms’ internal source of adaptations, rather than the impact of the environment. He recognized the importance of a struggle for existence, but did not carry this idea far enough to propose “survival of the fittest” (leaving that as a legacy to his grandson!). Erasmus did, however, challenge the concept of a “young earth” and argued, along with the Scottish geologist James Hutton (1726-1797) that millions of years would be required for rock formation and evolutionary processes.
1744-1829
Jean Baptiste Lamarck
extended Buffon’s ideas to propose a theory that a change in environment produces a need for change in animals and that acquired characteristics in one generation will be passed on to the next. (This well-developed theory should be carefully compared with the Darwin/Wallace theory of evolution by natural selection.)
1797-1875
Charles Lyell
, another British geologist, developed the theory of “uniformitarianism” regarding natural phenomena. “The present,” he said, “is key to the past”—natural forces that created the world are still at work and change is a slow, unending process. Although speaking directly in reference to development of physical features of the earth, Lyell had a great influence on Charles Darwin’s thoughts about plant and animal evolution.
1809-1882
Charles Darwin
did not originate the concept of evolution, as should now be obvious. However, along with Alfred Russel Wallace (1823-1913), he developed the theory of evolution “by natural selection.” Darwin’s theory was based almost entirely on inferences rather than verification of hypotheses by experiment. “It stands as a unique triumph of this scientific method and has become essential for comprehension of biology as the atomic theory is for chemistry and physics.”
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Two decades before publishing The Origin of Species,
Charles Darwin
wrote in his notebook: “Man in his arrogance thinks himself a great work, worthy the interposition of a deity. More humble and I believe true to consider him created from animals.”
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If any one aspect of Darwin’s outlook touched off the Victorian maelstrom of reaction to his theory, repercussions of which are still felt today, it was this suggestion of the relationship of humans to the rest of nature.
Two main strands of scientific inquiry have developed over the century since Darwin’s publication: paleontology and genetics.
The first, which students are most likely to be familiar with, is the search for fossil evidence linking present day human morphology to earlier, less evolved hominids. The Smithsonian video, “Tales of the Human Dawn,” and recent articles from weekly news magazines
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will provide ample material for student discussion of human ancestors and the current thinking regarding the human family tree.
The discovery of “Lucy,” the most complete Australopithecine fossil, as described by Johanson
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, makes the field work of paleoanthropologists appear as lively as any successful treasure hunt. For students who wish to pursue further this area of science, the work of Louis and Mary Leaky or their son, Richard Leaky, will provide interesting topics to research. Historically, worth studying is Thomas Henry Huxley (1825-1895), the British zoologist who defended Darwin’s theory of natural selection, who asserted early on that he believed humans evolved from apes. Of interest in American science is George Gaylord Simpson (1902-1984), a paleontologist who classified the evolution of mammals and showed in his work that the fossil record is compatible with Darwin’s theory of natural selection.
As mentioned previously, the idea of human evolution is deeply rooted in popular culture and is reflected in stories, movies, and humor of all types. It will be particularly helpful to students who may range from ambivalence to firm beliefs to help them understand that thorough scientific inquiry is ongoing regarding human ancestry, and that theories will serve only as long as they can stand the test of further observations and research.
The second stream of scientific inquiry regarding evolution has given rise to fields of study and disciplines unknown to Darwin. Darwin would have been quite at home with the paleontologists, biogeographers, anthropologists, and comparative anatomists of the first stream of study. He would be astounded, perhaps, to see the evidence accumulated in the 20th century by cytologists, molecular geneticists, biochemists, and molecular biologists. A brief history of the development of this scientific strand will set a context for student study of modern genetics:
1882
Walter Flemming
published his results on the study of cell mitosis, detailing the role of chromosomes in cell division.
1884
Gregor Mendel
, within five years of the publication of Darwin’s world-shaking treatise, The Origin of Species, developed a theory of inheritance based on carefully controlled experiments with pea plants. He discovered that parents can pass on characteristics to their offspring through the action of discrete units of inheritance (named “genes” by the Dutch geneticist Wilhelm Johannsen in 1909), each controlling a specific trait.
1900
Hugo De Vreis
, a Dutch botanist, concluded that evolution was the result of the sudden appearance of new varieties (which he called mutants) and not the natural selection of shifting variations proposed by Darwin.
1903
Walter Sutton
observed that in cell division producing sperm or egg cells, each gamete receives only one chromosome of each original pair. He recognized that chromosomes must be the carriers of the Mendelian heredity units and hypothesized that parental sperm and egg each contribute one chromosome to each new individual.
1910
Thomas Hunt
Morgan developed studies on the chromosomes of the fruit fly (Drosophila melanogaster), and by 1920 he and other researchers firmly established the chromosome theory of heredity. Further work showed that chromosomes are regular linear arrangements of genes.
1931
Barbara McClintock
demonstrated that gene order in chromosomes can change by rearrangements and that specific traits in strains of corn are tied to their genetic distribution.
The natural selection ideas of Darwin and the De Vries mutation theory could now be seen as complimentary—”natural selection was found to be picking and choosing among variations in the genotype to produce effects for the whole organism.”
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Genetics and evolutionary theory merged in the 1930s, contributing to the formation of new fields of study (e.g, population biology, population genetics,
molecular genetics
, biochemical genetics,
molecular biology
). The final important key in this development was the merging of genetics and biochemistry with focus on the molecular basis of life: molecular genetics explains the mechanisms behind Mendelian genetics while molecular biology concentrates on the structure of cell components to uncover the “code” that determines the characteristics of an organism.
1944
Oswald Avery
, Colin MacLeod, and MacLyn McCarty discovered DNA (deoxyribonucleic acid) as the material of the gene. DNA is a long chain molecule made up of four different kind of molecular groups (nucleotides).
The leaps in scientific understanding that occurred between 1859, when Darwin published
Origin of Species
, and 1952, when a group of research scientists knew that DNA was the very controlling molecule of life, are of tremendous significance. (For students seeking to understand the role and importance of scientific inquiry in explaining life processes, a brief study of any of the key investigators named might prove informative.)
Immediately after the biological importance of DNA was recognized, its physical structure was discovered:
1953
James Watson
and
Francis Cric
k determined the structure of DNA as a “double helix”—a sort of twisted ladder shape—with spines made of sugar and phosphate and rungs made of pairs of the four bases adenine, guanine, thymine, and cytosine.
1958
Meselson and Stahl
, investigating how the DNA molecule manages to reproduce itself so exactly as cells in a developing organism divide and multiply, confirmed that the double spiral “unzips” along its length and nucleotides then link up with each half of the chain to form two duplicates of the original model.
The modern age of molecular biology, which has been chiefly concerned with how genes control cell activity and how proteins carry out tasks such as DNA and RNA formation, began with Watson and Crick’s determination of the helical structure of DNA.
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The technical and conceptual developments over the past three decades deserve far more treatment than can be included in this unit. This unit does, however, seek to establish the background and context from which students may launch into future study of genetics and biology (hopefully at undergraduate and graduate levels.)
It is not unreasonable, once a common vocabulary and language of discourse has been established, to introduce very recent discoveries, techniques, and possibilities. Today the study of DNA has been revolutionized by procedures collectively referred to as “gene cloning” and “recombinant DNA technology.” DNA from any organism can be cut into reproducible pieces, joined to plasmid DNA, and introduced into bacterial hosts for culturing and reproduction on a large scale.
The breakthroughs and benefits for medical science are constantly in the news and will be familiar to many students and certainly deserve study and discussion to relate to basic knowledge and familiar models. Researchers at Yale University and in several local companies are available to meet with students and share with them their excitement about developing potential cures for everything from the common cold to deadly cancers and AIDS.
Have molecular biologists displaced Darwinian ideas about evolution? Hardly. It might be more accurate to say that the focus has shifted with advances in concepts and technology, but the fundamental theory remains much the same. Whereas Darwin studied animal morphologies (e.g, the famous finches’ beaks), today’s theorists study minute structures within cells thanks to electron microscopy, gene splicing, and electrophoresis techniques, among others.
The discovery of DNA provides at least a new round of tentative answers to questions concerning heredity, origins of diversity in life, and animal and plant development. In the argument of one recent text, the discovery that DNA is the universal genetic material of cells suggests that
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1. Early evolution must have depended on development of a cell carrying sufficient
instructions in its
DNA to grow;
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2. The random variation and selection proposed by Darwin and Wallace that led to changes in species must have resulted from
random changes in the DNA
;
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3. Faithful
reproduction of DNA
from generation to generation causes “like to beget like;”
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4. The programmed instructions in the genetic endowment in
DNA underlie the development
of every new plant or animal.
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