Forerunners The argument from irreducible complexity is a descendant of the
teleological argument for God (the argument from design or from complexity). This states that complex functionality in the natural world which looks designed is evidence of an intelligent creator.
William Paley famously argued, in his 1802
watchmaker analogy, that complexity in nature implies a God for the same reason that the existence of a watch implies the existence of a watchmaker. written in 45 BC.
Up to the 18th century Galen (1st and 2nd centuries AD) wrote about the large number of parts of the body and their relationships, which observation was cited as evidence for creation. The idea that the interdependence between parts would have implications for the origins of living things was raised by writers starting with
Pierre Gassendi in the mid-17th century and by
John Wilkins (1614–1672), who wrote (citing Galen), "Now to imagine, that all these things, according to their several kinds, could be brought into this regular frame and order, to which such an infinite number of Intentions are required, without the contrivance of some wise Agent, must needs be irrational in the highest degree." In the late 17th-century,
Thomas Burnet referred to "a multitude of pieces aptly joyn'd" to argue against the
eternity of life. In the early 18th century,
Nicolas Malebranche wrote "An organized body contains an infinity of parts that mutually depend upon one another in relation to particular ends, all of which must be actually formed in order to work as a whole", arguing in favor of
preformation, rather than
epigenesis, of the individual; and a similar argument about the origins of the individual was made by other 18th-century students of natural history. In his 1790 book,
The Critique of Judgment,
Kant is said by Guyer to argue that "we cannot conceive how a whole that comes into being only gradually from its parts can nevertheless be the cause of the properties of those parts".
19th century Chapter XV of Paley's
Natural Theology discusses at length what he called "relations" of parts of living things as an indication of their design.
Georges Cuvier applied his principle of the
correlation of parts to describe an animal from fragmentary remains. For Cuvier, this related to another principle of his, the
conditions of existence, which excluded the possibility of
transmutation of species. While he did not originate the term,
Charles Darwin identified the argument as a possible way to falsify a prediction of the theory of evolution at the outset. In
The Origin of Species (1859), he wrote, "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case." Darwin's theory of evolution challenges the teleological argument by postulating an alternative explanation to that of an intelligent designer—namely, evolution by natural selection. By showing how simple unintelligent forces can ratchet up designs of extraordinary complexity without invoking outside design, Darwin showed that an intelligent designer was not the necessary conclusion to draw from complexity in nature. The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of Darwinian evolution. In the late 19th century, in a dispute between supporters of the adequacy of
natural selection and those who held for
inheritance of acquired characteristics, one of the arguments made repeatedly by
Herbert Spencer, and followed by others, depended on what Spencer referred to as
co-adaptation of
co-operative parts, as in: "We come now to Professor
Weismann's endeavour to disprove my second thesis—that it is impossible to explain by natural selection alone the co-adaptation of co-operative parts. It is thirty years since this was set forth in 'The Principles of Biology.' In § 166, I instanced the enormous horns of the extinct
Irish elk, and contended that in this and in kindred cases, where for the efficient use of some one enlarged part many other parts have to be simultaneously enlarged, it is out of the question to suppose that they can have all spontaneously varied in the required proportions." Darwin responded to Spencer's objections in chapter XXV of
The Variation of Animals and Plants Under Domestication (1868). The history of this concept in the dispute has been characterized: "An older and more religious tradition of idealist thinkers were committed to the explanation of complex adaptive contrivances by intelligent design. ... Another line of thinkers, unified by the recurrent publications of Herbert Spencer, also saw
co-adaptation as a composed, irreducible whole, but sought to explain it by the inheritance of acquired characteristics."
St. George Jackson Mivart raised the objection to natural selection that "Complex and simultaneous co-ordinations ... until so far developed as to effect the requisite junctions, are useless". In the 2012 book
Evolution and Belief, Confessions of a Religious Paleontologist, Robert J. Asher said this "amounts to the concept of 'irreducible complexity' as defined by ... Michael Behe".
20th century Hermann Muller, in the early 20th century, discussed a concept similar to irreducible complexity. However, far from seeing this as a problem for evolution, he described the "interlocking" of biological features as a consequence to be expected of evolution, which would lead to irreversibility of some evolutionary changes. He wrote, "Being thus finally woven, as it were, into the most intimate fabric of the organism, the once novel character can no longer be withdrawn with impunity, and may have become vitally necessary." In 1975
Thomas H. Frazzetta published a book-length study of a concept similar to irreducible complexity, explained by gradual, step-wise, non-teleological evolution. Frazzetta wrote: "A complex adaptation is one constructed of
several components that must blend together operationally to make the adaptation 'work'. It is analogous to a machine whose performance depends upon careful cooperation among its parts. In the case of the machine, no single part can greatly be altered without changing the performance of the entire machine." The machine that he chose as an analog is the
Peaucellier–Lipkin linkage, and one biological system given extended description was the jaw apparatus of a python. The conclusion of this investigation, rather than that evolution of a complex adaptation was impossible, "awed by the adaptations of living things, to be stunned by their complexity and suitability", was "to accept the inescapable but not humiliating fact that much of mankind can be seen in a tree or a lizard." In 1985
Cairns-Smith wrote of "interlocking": "How can a complex collaboration between components evolve in small steps?" and used the analogy of the scaffolding called
centering—used to
build an arch then removed afterwards: "Surely there was 'scaffolding'. Before the multitudinous components of present biochemistry could come to lean together
they had to lean on something else." However, neither Muller or Cairns-Smith claimed their ideas as evidence of something supernatural. An early concept of irreducibly complex systems comes from
Ludwig von Bertalanffy (1901–1972), an Austrian biologist. He believed that complex systems must be examined as complete,
irreducible systems in order to fully understand how they work. He extended his work on biological complexity into a general theory of systems in a book titled
General Systems Theory. After
James Watson and
Francis Crick published the structure of
DNA in the early 1950s, General Systems Theory lost many of its adherents in the physical and biological sciences. However,
systems theory remained popular in the social sciences long after its demise in the physical and biological sciences.
Creationism Versions of the irreducible complexity argument have been common in
young Earth creationist (YEC)
creation science journals. For example, in the July 1965 issue of
Creation Research Society Quarterly
Harold W. Clark described the complex interaction in which
yucca moths have an "inherited action pattern" or instinct to fertilize plants: "Before the pattern can be inherited, it must be formed. But how could yucca plants mature seeds while waiting for the moths to learn the process and set the pattern? The whole procedure points so strongly to intelligent design that it is difficult to escape the conclusion that the hand of a wise and beneficent Creator has been involved." Similarly, honeybees pollinate apple blossom: "Again we may well ask how such an arrangement could have come about by accident, or how either the flowers or the bees could have survived alone. Intelligent design is again evident." In 1974 the YEC
Henry M. Morris introduced an irreducible complexity concept in his creation science book
Scientific Creationism, in which he wrote; "The creationist maintains that the degree of complexity and order which science has discovered in the universe could never be generated by chance or accident." He continued; "This issue can actually be attacked quantitatively, using simple principles of mathematical probability. The problem is simply whether a complex system, in which many components function unitedly together, and in which each component is uniquely necessary to the efficient functioning of the whole, could ever arise by random processes." In 1975
Duane Gish wrote in
The Amazing Story of Creation from Science and the Bible; "The creationist maintains that the degree of complexity and order which science has discovered in the universe could never be generated by chance or accident." A 1980 article in the creation science magazine
Creation by the YEC
Ariel A. Roth said "Creation and various other views can be supported by the scientific data that reveal that the spontaneous origin of the
complex integrated biochemical systems of even the simplest organisms is, at best, a most improbable event". In 1985, countering the creationist claims that all the changes would be needed at once,
Cairns-Smith wrote of "interlocking": "How can a complex collaboration between components evolve in small steps?" and used the analogy of the scaffolding called
centering—used to
build an arch then removed afterwards: "Surely there was 'scaffolding'. Before the multitudinous components of present biochemistry could come to lean together
they had to lean on something else." Neither Muller or Cairns-Smith said their ideas were evidence of anything supernatural. An article in the
Creation Research Society Magazine for June 1994 called a flagellum a "bacterial nanomachine", forming the "bacterial rotor-flagellar complex" where "it is clear from the details of their operation that nothing about them works unless every one of their complexly fashioned and integrated components are in place", hard to explain by natural selection. The abstract said that in "terms of biophysical complexity, the bacterial rotor-flagellum is without precedent in the living world. ... To evolutionists, the system presents an enigma; to creationists, if offers clear and compelling evidence of purposeful intelligent design."
Intelligent design The biology supplementary textbook for schools
Of Pandas and People was drafted presenting
creation science arguments, but shortly after the
Edwards v. Aguillard ruling, that it was unconstitutional to teach creationism in public school science classes, the authors changed the wording to "intelligent design", introducing the new meaning of this term when the book was published in 1989. In a separate response to the same ruling, law professor
Phillip E. Johnson wrote
Darwin on Trial, published in 1991, and at a conference in March 1992 brought together key figures in what he later called the '
wedge movement', including biochemistry professor
Michael Behe. According to Johnson, around 1992 Behe developed his ideas of what he later called his "irreducible complexity" concept, and first presented these ideas in June 1993 when the "Johnson-Behe cadre of scholars" met at Pajaro Dunes in California. The second edition of
Of Pandas and People, published in 1993, had extensive revisions to Chapter 6
Biochemical Similarities with new sections on the complex mechanism of blood clotting and on the origin of proteins, written by Behe though he was not initially acknowledged as their author. He argued that "all of the proteins had to be present simultaneously for the blood clotting system to function", so it could not have evolved. In later publications, he named the argument "irreducibly complexity", but changed his definition of this specific system. In
Doubts About Darwin: A History of Intelligent Design (2003), historian Thomas Woodward wrote that "Michael Behe assisted in the rewriting of a chapter on biochemistry in a revised edition of Pandas. The book stands as one of the milestones in the infancy of Design." On
Access Research Network, Behe posted (on 3 February 1999) "Molecular Machines: Experimental Support for the Design Inference" with a note that "This paper was originally presented in the Summer of 1994 at the meeting of the
C. S. Lewis Society, Cambridge University." An "Irreducible Complexity" section quoted Darwin, then discussed "the humble mousetrap", and "Molecular Machines", going into detail about
cilia before saying "Other examples of irreducible complexity abound, including aspects of protein transport, blood clotting, closed circular DNA, electron transport, the bacterial flagellum, telomeres, photosynthesis, transcription regulation, and much more. Examples of irreducible complexity can be found on virtually every page of a biochemistry textbook." Suggesting "these things cannot be explained by Darwinian evolution," he said they had been neglected by the scientific community. Behe first published the term "irreducible complexity" in his 1996 book ''
Darwin's Black Box'', where he set out his ideas about theoretical properties of some complex biochemical
cellular systems, now including the bacterial flagellum. He posits that evolutionary mechanisms cannot explain the development of such "irreducibly complex" systems. Notably, Behe credits philosopher
William Paley for the original concept (alone among the predecessors). Intelligent design advocates argue that irreducibly complex systems must have been deliberately engineered by some form of
intelligence. In 2001, Behe wrote: "[T]here is an asymmetry between my current definition of irreducible complexity and the task facing natural selection. I hope to repair this defect in future work." Behe specifically explained that the "current definition puts the focus on removing a part from an already functioning system", but the "difficult task facing Darwinian evolution, however, would not be to remove parts from sophisticated pre-existing systems; it would be to bring together components to make a new system in the first place". In the 2005
Kitzmiller v. Dover Area School District trial, Behe testified under oath that he "did not judge [the asymmetry] serious enough to [have revised the book] yet." Behe additionally testified that the presence of irreducible complexity in organisms would not rule out the involvement of evolutionary mechanisms in the development of organic life. He further testified that he knew of no earlier "peer reviewed articles in scientific journals discussing the intelligent design of the blood clotting cascade," but that there were "probably a large number of peer reviewed articles in science journals that demonstrate that the blood clotting system is indeed a purposeful arrangement of parts of great complexity and sophistication." (The judge ruled that "intelligent design is not science and is essentially religious in nature".) The
scientific theory of
evolution incorporates evidence that genetic variations occur, but makes no assumptions of
purposeful design or intent. The environment "selects" the variants which have the highest fitness for conditions at the time, and these heritable variations are then passed on to the next generation of organisms. Change occurs by the gradual operation of natural forces over time, perhaps slowly, perhaps more quickly (see
punctuated equilibrium). This process is able to
adapt complex structures from simpler beginnings, or convert complex structures from one function to another (see
spandrel). Most intelligent design advocates accept that evolution occurs through mutation and natural selection at the "
micro level", such as changing the relative frequency of various beak lengths in finches, but assert that it cannot account for irreducible complexity, because none of the parts of an irreducible system would be functional or advantageous until the entire system is in place.
The mousetrap example believes that many aspects of life show evidence of design, using the
mousetrap in an analogy disputed by others. Behe uses the mousetrap as an illustrative example of this concept. A mousetrap consists of five interacting pieces: the base, the catch, the spring, the hammer, and the hold-down bar. All of these must be in place for the mousetrap to work, as the removal of any one piece destroys the function of the mousetrap. Likewise, he asserts that biological systems require multiple parts working together in order to function. Intelligent design advocates claim that natural selection could not create from scratch those systems for which science is currently unable to find a viable evolutionary pathway of successive, slight modifications, because the selectable function is only present when all parts are assembled. In his 2008 book
Only A Theory, biologist
Kenneth R. Miller challenges Behe's claim that the mousetrap is irreducibly complex. Other systems identified by Miller that include mousetrap components include the following: Behe's original examples of irreducibly complex mechanisms included the bacterial
flagellum of
E. coli,
the blood clotting cascade,
cilia, and the
adaptive immune system. Behe argues that organs and biological features which are irreducibly complex cannot be wholly explained by current models of
evolution. In explicating his definition of "irreducible complexity" he notes that: An irreducibly complex system cannot be produced directly (that is, by continuously improving the initial function, which continues to work by the same mechanism) by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional. Irreducible complexity is not an argument that evolution does not occur, but rather an argument that it is "incomplete". In the last chapter of ''
Darwin's Black Box'', Behe goes on to explain his view that irreducible complexity is evidence for
intelligent design. Mainstream critics, however, argue that irreducible complexity, as defined by Behe, can be generated by known evolutionary mechanisms. Behe's claim that no scientific literature adequately modeled the origins of biochemical systems through evolutionary mechanisms has been challenged by
TalkOrigins. The judge in the
Dover trial wrote "By defining irreducible complexity in the way that he has, Professor Behe attempts to exclude the phenomenon of
exaptation by definitional fiat, ignoring as he does so abundant evidence which refutes his argument. Notably, the
NAS has rejected Professor Behe's claim for irreducible complexity..." == Claimed examples ==