Irreducible complexity is a concept popularized by Lehigh University biochemist Michael Behe in his 1996 book Darwin's Black Box. Behe rejects the generally accepted idea that life on Earth evolved through biological evolution, and that some intelligent designer (implied to be God, but never stated) must have designed life. As such, Behe's book supports what is known as intelligent design theory,

The idea of irreducible complexity is as old as the theory of evolution itself; examples of supposedly irreducibly complex systems are often described among those opposed to the theory of evolution.

Table of contents
1 What is irreducible complexity?
2 Forerunners
3 Can irreducible complexity be found in nature?
4 Opposition to irreducible complexity
5 Falsifiability and experimental evidence
6 Significance of irreducible complexity, if found
7 External links and further reading

What is irreducible complexity?

The term "irreducible complexity" is somewhat ambiguous. It is used loosely to describe any very complex organ or organism found in nature. More precisely it is used to describe systems that are not just very complex but:

"a single system which is composed of several interacting parts that contribute to the basic function, and where the removal of any one of the parts causes the system to effectively cease functioning" (Michael Behe, Molecular Machines: Experimental Support for the Design Inference)

Believers in the intelligent design theory use this term to refer to biological systems and organs that they believe could not have come about by a series of small changes. They hold that for such mechanisms or organs, anything less than their complete form would not work at all, or would in fact be a detriment to the organism, and would therefore never survive the process of natural selection. Proponents of intelligent design argue that while very complex systems and organs can be explained by evolution, organs and biological features which are irreducibly complex cannot be explained by current models, and that an intelligent designer (implied to be God) must thus have created or guided life.

The debate on irreducible complexity concerns two questions:

  1. Can irreducible complexity be found in nature?
  2. What would the significance be if irreducible complexity did exist in nature?


The argument from irreducible complexity is a descendant of the teleological argument for God (the argument from design or argument from complexity). This states that because certain things in nature are very complicated, they must have been designed, just as the existence of a watch implies the existence of a watchmaker. This argument is a very old one, and can be traced back as far as Cicero's De natura deorum, ii. 34 (see Hallam, Literature of Europe, ii. 385, note).

Charles Darwin's theory of evolution challenges the teleological argument by postulating an alternative explanation to that of an intelligent designer: namely evolution. The argument from irreducible complexity attempts to demonstrate that certain biological features cannot be purely the product of evolution.

Can irreducible complexity be found in nature?

Behe and others, including some evolutionists, have suggested a number of biological features that they believe may be irreducibly complex.

The bombardier beetle

The bombardier beetle (Genus Brachinus) is an organism that has become somewhat of a standard bearer for irreducible complexity. These beetles have three chambers in their abdomen, two of which contain liquids that are chemically inert, but when mixed they create an incendiary combination. The third chamber is a reaction chamber into which the two chemicals are squeezed when danger is near, and then expelled explosively towards the perceived danger.

The bird lung

The bird lung is quite different from other lungs, such as the reptile lung from which it is believed to have evolved. Transition from a reptile lung (bellows lung) to a bird lung (circulatory lung) seems unlikely since intermediate stages would be a detriment to the organism.

Recently, conventional wisdom has held that birds are direct descendants of theropod dinosaurs. However, the apparently steadfast maintenance of hepatic-piston diaphragmatic lung ventilation in theropods throughout the Mesozoic poses a fundamental problem for such a relationship. The earliest stages in the derivation of the avian abdominal airsac system from a diaphragmatic-ventilating ancestor would have necessitated selection for a diaphragmatic hernia [i.e. hole] in taxa transitional between theropods and birds. Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage. (Michael Denton)


The flagella of certain bacteria constitute a molecular motor requiring the interaction of about 40 complex protein parts, and the absence of any one of these proteins would make the flagella fail to function. Behe holds that the flagellum "engine" is irreducibly complex because if we try to reduce its complexity by positing an earlier and simpler stage of its evolutionary development, we get an organism which functions improperly.

This topic is discussed in the article on the Evolution of flagella.

Light detection

The biochemistry of light detection requires complex interactions among many different molecules, each performing a very specialized job. Eliminating even one component of the biological pathway can destroy the ability to detect light.

One evolutionary mechanism that may result in complex biological pathways such as this is "scaffolding", where a set of biochemical reactions are used to build up a pathway and then are discarded, in much the same way that a building is built from the bottom up even though removing any of the columns would cause the building to collapse.

Other examples

Opposition to irreducible complexity

It may be that irreducible complexity does not exist in nature: that the examples given by Behe and others are not in fact irreducibly complex, but can be explained in terms of simpler precursors. Thus they would either be merely very complex, or they would be misunderstood or misrepresented.

The precursors of complex systems, when they are not useful in themselves, may be useful to perform other, unrelated functions. Evolutionary biologists say that evolution often works in this kind of blind, haphazard manner in which the function of an early form is not necessarily the same as the function of the later form. The mammalian ear (derived from a jawbone) and the panda's thumb (derived from a wrist bone spur) are classic examples of this.

Evolution can act to simplify as well as to complicate. This raises the possibility that apparently irreducibly complex biological features may have been achieved with a period of increasing complexity, followed by a period of simplification. Consider the example of a brick wall, which is not irreducibly complex. If you remove bricks at random from the wall, one at a time, eventually you end up with something that will fall down if you remove one more brick. Such a structure could not have been created only by adding bricks one at a time.

Behe has been accused of using an argument by lack of imagination, or constructing a "God of the gaps". Behe acknowledges that simply because scientists cannot currently see how an "irreducibly complex" organism could evolve, it does not prove that there is no possible way for it to have occurred.

Falsifiability and experimental evidence

Some critics, such as Jerry Coyne (professor of evolutionary biology at the University of Chicago) and Eugenie Scott (pro-evolution activist, Executive Director at the National Center for Science Education) have argued that the concept of irreducible complexity, and more generally, the theory of Intelligent Design is not falsifiable, and therefore, not scientific.

Behe argues that the theory that irreducibly complex systems could not have been evolved can be falsified by an experiment where such systems were evolved. For example, he posits taking bacteria with no flagella and imposing a selective pressure for mobility. If, after a few thousand generations, the bacteria evolved the bacterial flagellum, then Behe believes that this would refute his theory.

Other critics validate this defence, by pointing to experimental evidence that they believe falsifies the argument for Intelligent Design from irreducible complexity. For example, Kenneth Miller cites the lab work of Barry Hall on E. coli, which he presents as conclusive evidence that "Behe is wrong".

Significance of irreducible complexity, if found

Behe argues that organs and biological features which are irreducibly complex cannot be wholly explained by current models of evolution. He argues 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. If irreducible complexity cannot be wholly explained by current models of evolution, then alternative models must be considered such as:

Another issue to consider is that even if there are no true irreducably complex systems in nature, the convoluted evolutionary paths that would be requried to produce them add orders of magnitude to the estimate of the time required for life to evolve.

External links and further reading