In his highly influential book "The Structure of Scientific Revolutions," science philosopher Thomas Kuhn presented the idea that science is not a gradual progression toward truth, but a series of insurgencies, with scientific theories constantly usurping one another.
That is sometimes true. And proponents of intelligent design love Kuhn's argument.
They see intelligent design (often called ID) as a revolutionary new science and themselves as revolutionaries. They envision toppling Darwinian evolution – once a revolutionary idea itself – and erecting in its place a theory about life that allows for supernatural explanations, a theory that makes God, or some entity very much like him, not just possible but necessary.
But in order to attract converts and win over critics, a new scientific theory must be enticing. It must offer something that its competitors lack. That something may be simplicity, which was one of the main reasons the Sun-centered model of the solar system was adopted over the Earth-centered one centuries. Or it could be sheer explanatory power, which was what allowed evolution to become a widely accepted theory with no serious detractors among reputable scientists.
So what does ID offer? What can it explain that evolution can't?
To answer this, it is necessary to examine the two main arguments — irreducible complexity and specified complexity — that ID proponents use to support their claim that a Supreme Being is responsible for many or all aspects of life.
Irreducible complexity asserts that certain biochemical systems in nature contain parts that are too well matched to be products of evolution.
Every part of an irreducibly complex system is necessary: take away even one, and the entire system will no longer work. Because their parts are so intricate and so interdependent, such systems could not possibly have been the result of evolution, ID supporters argue.
Irreducible complexity's main proponent is Michael Behe, a biochemist at Lehigh University in Pennsylvania. Among the systems that Behe claims are irreducibly complex are the bacterial flagellum, a microscopic whip-like structure that some bacteria use to swim, and the cascade of proteins that make up the human blood-clotting system.
Darwin himself admitted that if an example of irreducible complexity were ever found, his theory of natural selection would crumble.
"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," Darwin wrote.
Yet no true examples of irreducible complexity have ever been found. The concept is rejected by the majority of the scientific community.
To understand why, it is important to remember that Behe's main argument is that in an irreducibly complex system, every part is vital to the system's overall operation.
A necessary — and often unstated — flipside to this is that if an irreducibly complex system contains within it a smaller set of parts that could be used for some other function, then the system was never really irreducibly complex to begin with.
It's like saying in physics that atoms are the fundamental building blocks of matter only to discover, as physicists have, that atoms are themselves made up of even smaller and more fundamental components.
This flipside makes the concept of irreducible complexity testable, giving it a scientific virtue that other aspects of ID lack.
"The logic of their argument is you have these multipart systems, and that the parts within them are useless on their own," said Kenneth Miller, a biologist at Brown University in Rhode Island. "The instant that I or anybody else finds a subset of parts that has a function, that argument is destroyed."
Viewed this way, all of the systems that Behe claims to be irreducibly complex really aren't.
A subset of the bacterial flagellum proteins, for example, are used by other bacteria to inject toxins into other cells and several of the proteins in the human blood-clotting system are believed to be modified forms of proteins found in the digestive system.
Evolution takes pieces and parts and re-uses them.
The second major argument for intelligent design comes from William Dembski, a mathematician and philosopher affiliated with the Discovery Institute, a Seattle-based Christian think tank that serves as the nerve center for the ID movement.
Dembski argues that nature is rife with examples of non-random patterns of information that he calls "complex specified information," or CSI for short.
To qualify as CSI, the information must be both complex and specified. The letter "A," for example, is specific but not complex. A string of random letters such as "slfkjwer," on the other hand, is complex but not necessarily specific. A Shakespearean sonnet, however, is both complex and specific.
An example of CSI from nature is DNA, the molecule found in all cells that contains the genetic instructions for life. DNA is made up of four repeating chemical bases arranged into complimentary pairs. The bases can be thought of as "letters" in a four-letter alphabet and can be strung together to form genes, which can be thought of as the "words" that tell the cell what proteins to make.
The human genome is made up of some 3 billion DNA base pairs and contains about 25,000 genes. DNA is obviously complex. The fact that humans always give birth to humans and not chimpanzees or naked mole rats shows that DNA is also specific.
The fact that CSI exists in nature is evidence for design because intelligence is necessary to produce CSI, Dembski says. This is the part of Dembski's argument that many scientists have trouble with.
The nylon problem
There is a way to settle this, however, because like Behe's irreducible complexity, the concept of specified complexity can also be tested.
"If Dembski were right, then a new gene with new information conferring a brand new function on an organism could never come into existence without a designer because a new function requires complex specified information," Miller said.
In 1975, Japanese scientists reported the discovery of bacteria that could break down nylon, the material used to make pantyhose and parachutes. Bacteria are known to ingest all sorts of things, everything from crude oil to sulfur, so the discovery of one that could eat nylon would not have been very remarkable if not for one small detail: nylon is synthetic; it didn't exist anywhere in nature until 1935, when it was invented by an organic chemist at the chemical company Dupont.
The discovery of nylon-eating bacteria poses a problem for ID proponents. Where did the CSI for nylonase—the actual protein that the bacteria use to break down the nylon—come from?
There are three possibilities:
- The nylonase gene was present in the bacterial genome all along.
- The CSI for nylonase was inserted into the bacteria by a Supreme Being.
- The ability to digest nylon arose spontaneously as a result of mutation. Because it allowed the bacteria to take advantage of a new resource, the ability stuck and was eventually passed on to future generations.
Apart from simply being the most reasonable explanation, there are two other reasons that most scientists prefer the last option, which is an example of Darwinian natural selection.
First, hauling around a nylonase gene before the invention of nylon is at best useless to the bacteria; at worst, it could be harmful or lethal. Secondly, the nylonase enzyme is less efficient than the precursor protein it's believed to have developed from. Thus, if nylonase really was designed by a Supreme Being, it wasn't done very intelligently.
‘Death of science'
After examining ID's two main arguments, the answers to the original questions — what does ID offer? And what can ID explain that evolution can't? — is not much and nothing, leading scientists say.
"The most basic problem [with ID] is that it's utterly boring," said William Provine, a science historian at Cornell University in New York. "Everything that's complicated or interesting about biology has a very simple explanation: ID did it."
Evolution was and still is the only scientific theory for life that can explain how we get complexity from simplicity and diversity from uniformity.
ID offers nothing comparable. It begins with complexity — a Supreme Being — and also ends there. The explanations offered by ID are not really explanations at all, scientists say. They're more like last resorts. And, scientists argue, there is a danger in pretending that ID belongs next to evolution in textbooks.
"It doesn't add anything to science to introduce the idea that God did it," Provine told LiveScience. Intelligent design "would become the death of science if it became a part of science."