Evolution is a theory with a bumpy history. That history brings with it much baggage, in the form of ideas, words, and controversies that continue to shape discussion of evolution today. So we begin by looking at that history, not just out of respect for the workers that have gone before but because understanding that history can illuminate important current concepts.
Introduction
Sometimes well-established ideas (like a spherical planet earth, or continental drift, or evolution by natural selection) can seem self-evidently true -- until we remember how strange and counterintuitive they once were. Often in the history of science, some familiar and obvious truths have had to be modified or overthown before deeper truths could be appreciated and accepted.
Similarly, some old ideas may seem quaint or even silly until we remember how rational they once were in an earlier era. To properly understand evolution's place in modern science, we need to consider what discoveries made such a theory necessary, and what prior ideas stood in the way.
Science really does progress, hopefully achieving an ever-closer approximation of reality. But some humility remains appropriate. Our c urrent understanding of biology may more closely approximate the underlying reality than did much of what has gone before. But mysteries do remain, and careful consideration of evolution can accentuate some of these mysteries. Science has experienced profound revolutions in the past; we should remain aware that ultimate reality (Truth with a capital "T") remains elusive.
The roots of many modern controversies extend back many decades. Many of the original problems in evolutionary biology remain unresolved.
- When and how (and where and how often) did life begin?
- Once evolvable life had begun, what events (mutations and conditions of selection) have been responsible for each of the major "innovations" in the life's subsequent history --
- eukaryotic cells?
- multicellularity?
- complex differentiated body plans?
- sexual reproduction?
- intelligence?
These are old questions that still remain largely unanswered today. Other deep questions in evolution have many tentative answers --
- How do populations diverge into distinct species?
- How is the apparent balance of ecosystems maintained?
- To what extent is biological organization shaped by selection? (Alternatively, to what extent are particular features of biological organization shaped by chance circumstance, by prebiotic physical and chemical laws, or by some intrinsic logic of self-organization?)
But fully satisfactory understanding remains elusive.
Prelude to evolution: Ideas related to evolution -- including the relationships among different life forms and the possibility of their transformation over time, the question of spontaneous generation, and the age of the Earth, have deep roots in human thought. During and subsequent to the Renaissance, new ideas and discoveries set the stage for Darwin's theory.
Mechanism.
The fundamental idea of mechanistic explanation (i.e., that "natural" events have "natural" causes and can be explained by cause-and-effect relationships that do not involve special action of supernatural agency) is fundamental to modern science.
The invention and proliferation of machines (e.g., clocks, pumps, windmills), culminating in the steam engines and the industrial revolution of Darwin's time, sustained a growing interest in mechanistic explanation.
By the 1700s, a mechanistic worldview (i.e., the "clockwork universe") had been conspicuously successful in astronomy, notably through the work of Kepler, Galileo and Newton (Galileo died in 1642, the same year that Newton was born.) Life also began to receive mechanistic explanation in the early 1600s, notably with William Harvey's theory that the heart works like a pump to cause the circulation of blood, dramatically confirmed later that century by the microscopist Malpighi's discovery of capillaries. Other discoveries in anatomy, physiology, and embryology followed.
Without a basic presumption of mechanism, "explanation" in the modern scientific sense has little meaning. But within a mechanistic worldview, the basic patterns of life could no longer be taken for granted.
- Where does life come from? How do organisms develop from formless eggs into complex structures?
- How do offspring acquire specific traits of their parents?
- What accounts for the great diversity of living creatures?
- How has each creature come to be so well suited for its way of life?
- What accounts for the difference between present life and past life revealed in fossils? If living things change over time, how do they do so?
A variety of mechanisms were proposed in answer to one or more of these questions. Many of the answers seem to beg the question, at least to modern sensibilities. But remember, at base all scientific explanation is nothing but assertion of pattern, with universal patterns seen as "laws" whose reliable occurrence serves to "explain" particular events. Whether such laws are simply inherent in matter and energy or are expressions of the divine, the goal of science is to discern the patterns.
Nevertheless, right up through the nineteenth century many scientists felt that an unbridgable explanatory gulf separated "living" from "nonliving" matter. This distinction continues to influence popular thinking into the 21st century, especially with regard to the matter of the human mind.
Geology: The emerging science of geology set the stage for appreciating both the great age of the Earth and the transformations of life that had occurred over Earth's history.
- For a very readable narrative about geology, including much on the history of the discipline, seee John McPhee (1998) Annals of the Former World. This is elegant, entertaining prose by a much-acclaimed writer. The volume is a compiliation of several earlier works: In Suspect Terrain, Basin and Range, Rising from the Plainsand Assembling California.
In the 17th century, Nicolaus Steno advocated recognition that that fossils represented remains of former living things, buried in the earth. He also established several basic principles describing the formation of sedimentary rocks. These ideas may seem quite simple, but they were basic to reconstructing the history of the earth.
James Hutton introduced in 1795 the concept of a very ancient earth which had undergone many great transformations. Theory of the Earth contained the famous phrase describing Earth's history, "No vestige of a beginning, no prospect of an end." (more)
William Smith, a mine surveyor and canal builder, employed on behalf of the emerging industrial revolution to supply coal for steam engines and factories, collected data demonstrating that different geological incorporated distinctive fossils. (more)
Meanwhile, Georges Cuvier, founder of comparative anatomy, earned a reputation for interpreting past life from fossil bones. Cuvier also mustered evidence for extinction, especially of giant animals. Cuvier believed in discontinuity (and catastrophism) rather than the scala naturae, and thought progressive evolutionism was inconsistent with the evidence. (more)
Finally, Charles Lyell is famous for the concepts of uniformitarian geology. What shaped the past is the same as what acts in the present. His Principles of Geology (1830-33) inspired Darwin, who read Lyell's books while on board the Beagle. But Lyell never accepted the transformation of species.
Comparative Anatomy and Systematics. Observers had noticed for centuries that organisms could be grouped according to similarities. Some similarities seemed more superficial, others much deeper. Linneaus' Systemae Naturae (1758) used such similarities to establish a basic scheme (still in use) which classified organisms into nested sets of taxa. Were such nested similarities just the way things were, or could they be explained by some more basic process, such as shared descent from common ancestry?
Fossils. Once fossils were recognized as a record of past life, it became evident that past life had been different from that known at present. (The science of geology was also emerging in the late 18th and early 19th centuries -- see above). Did such differences reflect distinct prior creations? Or was there continuity of life from past to present? If the latter, what could account for life's transformations?
Diversity and Biogeography. As the wide world beyond familiar Europe was explored ever more thoroughly, similar ecological circumstances were found to be occupied by differing species. Such differences between areas were commonly observed seen to be correlated with geographical isolation. Here again were many more observations in need of explanation. The patterns of biogeography, including the biogeography of fossils, that Darwin observed during the voyage of the Beagle moved him toward an evolutionary perspective.
Obstacles. Certain prior ideas served as powerful organizing principles that displaced or obscured alternative explanations.
Science grew out of a scholarly tradition that shared a long heritage with Christian theology. In that tradition scripture was widely accepted as authoritative on matters of objective fact.
- "Young Earth". The account of Earth's creation in Genesis and subsequent Biblical history (with 77 generations between Adam and Jesus) suggests only a few thousand years between "creation" and present time. In 1650, an Irish archbishop James Ussher used "cutting edge" biblical scholarship to date the year of creation at 4004 B.C. This date has become infamous as a target for ridicule, but at the time it represented a valiant effort to quantify the extent of history.
- Noah's Flood, as recorded in the biblical book of Genesis, provides a superficially satisfactory explanation for many dramatic features of the Earth's surface, including all sedimentary rocks as well as fossils of marine creatures high inland. By extension, multiple (nonbiblical) catastrophes could be used to develop a more detailed geological record for times predating the arrival of man.
Several widely accepted biological theories and empirical observations were also inconsistent with Darwinian evolution.
Fixity of species. "Like reproduces like." It is only common sense that lizards do not hatch from fish eggs, a field sown in wheat does not produce oats, and cats do not give birth to puppies. This simple, everyday observation reflects a universal orderliness of nature that would begin to unravel if species could transform one into another.
Scala naturae, the popular concept that all forms of life could be arranged into a continuous and progressive ladder of life, from primitive to complex, was readily amenable to evolutionary interpretation. However, such "progressive" ideas of life, including those of Lamarck, created considerable interference with Darwin's ideas. [The confusion and conflation of Darwinism and "progress" continues to the present day, reflected in such terms as "primitive" or "lower" vs. "advanced" or "higher" forms of life. Such terms have no place in a Darwinian view. (more)]
Catastrophism, the scientific theory that Earth has had several episodes of creation, each ending with a catastrophic destruction, was decidedly at odds with any direct, uninterrupted process of evolution (including both Lamarckianism and Darwinism). Noah's flood was understood as the archetypal (and most recent) catastrophe, but catastrophism was solid science, not myth or theology. Catastrophism was based on a rough but objective reading of fossil evidence, which truly records several catastrophes -- including those defining the boundaries between the major Paleozoic, Mesozoic and Cenozoic eras. These great Permian and Cretaceous extinctions were each followed by creation of a "new-and-improved" biota. Note that the evidence still stands -- these extinctions are currently understood as the result of asteroid impact or massive volcanism, which, although not extinguishing life and requiring a new creation, did significantly reset the stage for ongoing evolution.
Spontaneous generation. Louis Pasteur finally laid to rest the idea of spontaneous generation in the mid 1800s, an idea which motivated some early evolutionary ideas (e.g., life is continually emerging, then progressing toward perfection, with the scala naturae resulting from the contemporary existence of many species at various stages of progress). Thus the concepts of Omnis vivo e vivo and Omnis cellula e cellula seemed to refute evolutionary ideas.
Vitalism, the idea that life involves some special essence, or "vital principle", has always seemed obvious to many observers of nature. Vitalism renders mechanistic explanation inapplicable.
Vitalism has proven to be a moving target for refutation by science, and is still quite strong now at the beginning of the 21th century. After metabolism began to yield to chemistry (respiration and nutrition as chemical reaction), there remained the mysteries of heredity and development. Since basic genetics has yielded to chemistry (i.e., DNA), and development is beginning to crack, there is still the mind. Although most evolutionary biologists and neuroscientists believe that thought, memory, emotion and personality operate firmly within the domain of mechanistic explanation, that view does not prevail in the wider culture.
An abiding vitalist intuition may underlie the deep tension between evolutionism and creationism, even between science and religion: the idea that evolution cannot explain, and may appear to deny, the concept of "soul" (which is often equated in popular thinking with the idea of "mind"). Even Alfred Russell Wallace was convinced that evolution could not explain the human mind, and so parted ways with Darwin and dabbled in spiritualism (see Richard Milner, "Charles Darwin and Associates,Ghostbusters", Scientific American, October 1996.)
[Students desiring a deeper discussion of issues related to the historical and/or current "conflicts" between evolution and religion should contact the instructor or click here. Extra sessions may be arranged if there is sufficient interest. Alternatively, see Science and Religion (an essay recently published in Science) and then check out the many links available at National Center for Science Education.]
Recent items of relevance to this discussion:
Stephen Jay Gould, "A division of worms / The use and disuse of Lamarck", Natural History, February 1999 and March 1999. An extended essay on Lamarck's contributions, going well beyond the standard textbook caricature.
Richard Milner, "Charles Darwin and Associates,Ghostbusters", Scientific American, October 1996. An interesting article on the fad of spiritualism in Britain in the late 1800s, discussing Darwin's and Wallace's opposite roles.
Stephen Jay Gould, "Writing in the margins", Natural History, November 1998, and "Capturing the center", Natural History, December 1998. A two-part essay on Antoine-Laurent Lavoisier and geology in the late 1700s, shortly before the French Revolution in which he was executed. Lavoisier, you will recall from high school chemistry, was the great French chemist who discovered oxygen. Here we learn about Lavoisier's important contributions to geology as well.
Readable references: (also note end-of-chapter references in Ridley)
John A. Moore (1993) Science As a Way of Knowing. The Foundations of Modern Biology. Harvard University Press. An elegantly written book on the nature of science, with sections on ancient worldviews and evolution (relevant here) as well as genetics and development.
Loren Eiseley (1958) Darwin's Century: Evolution and the Men Who Discovered It. Doubleday. A scholarly but eloquent discussion of people and ideas associated with evolution in the 1800s.
Ernst Mayr (1982) The Growth of Biological Thought. Diversity, Evolution, and Inheritance. Harvard University Press. A thorough, insider's view of the history of some central ideas in biology. Discusses diversity, evolution and genetics both before and after Darwin.
John McPhee (1998) Annals of the Former World. About geology rather than evolution, including commentary on the history of ideas relating to the earth's history. Elegant, entertaining prose by a much-acclaimed writer. This volume is a compiliation of several earlier works, In Suspect Terrain, Basin and Range, Rising from the Plainsand Assembling California. This book is reviewed in Science 282:885 (30 October 1998).
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