A century and a half after Charles Darwin announced to a wary world that man's closest relations were apes and that all life evolved from a common ancestor, his theory of evolution is once again on trial, in a federal court in Harris-burg, Pennsylvania.

The courtroom drama is a skirmish in a larger battle, part of the "culture wars" that have divided the United States into liberal and conservative camps since the 1960s. Yet the trial is more than this: it is evidence of a wider malaise, the emergence of a new Age of Unreason which glorifies the irrational and dignifies ignorance.

The trial is a proxy battle between left and right about how best to interpret the constitution through the Supreme Court, with opposing forces funding each side. Inevitably, after a number of appeals, the matter will eventually be put before the Supreme Court for final arbitration. It is exactly the sort of issue that causes conservative Republicans to scrabble to get one of their persuasion on to the Supreme Court.

Eleven parents from eight families in York, Pennsylvania, with support from the American Civil Liberties Union and Americans for Separation of Church and State, are arguing that under the First Amendment their children should not be taught "intelligent design" - the theory, ridiculed by scientists, that life did not evolve but that species emerged fully developed, the creation of a supreme intelligence. Even states such as Pennsylvania - hard-nosed, no-nonsense, industrialised and far from the preaching fields of the Southern Christians - have been teaching "intelligent design" since the Supreme Court ruled in 1987 that they could no longer legally teach "creationism", a theory that takes at face value the Bible's account of God creating the universe in six days.

Teaching science and religion has always been treacherous territory in the US. The persecution of Protestant sects in Tudor and Stuart Britain propelled waves of religious refugees to America, ensuring that the founding fathers of the new republic decreed that church and state should be divided, and that public schools should not teach religion.

In godly America, the publication of Darwin's theory of evolution in 1859 in The Origin of Species caused consternation in the schools. Until then, American teachers had taken comfort in thinkers such as the British philosopher William Paley, who believed life was so self-evidently extraordinary, it could only have been the product of a godlike presence. In many states the teaching of Darwin's irreligious theory was promptly outlawed.

In the early part of the 20th century, cases pitching evolution against divine intervention worked their way through the US courts, notably the infamous 1925 "Monkey Trial" in Dayton, Tennessee, where John T Scopes, a schoolmaster, was convicted of teaching Darwin's theory in defiance of the state's laws. Before long, to avoid repetition of Scopes's profanity, many states removed all mention of evolution from their textbooks.

Biblical purity, however, came at a price. The results of decades of teaching irrationality and promoting scientific ignorance became evident when, in 1957, the Eisenhower administration suffered international humiliation. The Soviet Union, its cold war nemesis, became the first country to launch a space satellite, demonstrating that godless communism could beat capitalism when it came to understanding science.

In response, the US government investigated science teaching and issued textbooks placing evolution firmly at the centre of biology lessons. Unable to hold off the centre's imposition of evolution, backwoods states such as Louisiana and Arkansas and more than 20 others began teaching "creationism" - which recognised the historical accuracy of Adam and Eve in Eden and Noah's Ark - alongside Darwin.

After 20 years of classroom and courtroom battles, the Supreme Court dealt the Christian fundamentalists a grievous blow in 1987 by ruling that the teaching of biblical creationism in public schools was illegal because it contradicted the First Amendment guarantee of a division between church and state. But Darwin's victory was short-lived. Hard on the heels of the banishment of creationism came "intelligent design", postulating that life was created by a higher force rather than evolution from a submarine soup - though not necessarily by God. In classrooms, Christians turned to "intelligent design" to drown out Darwinism.

Since 1984 in York, Pennsylvania, the school board has instructed pupils that evolution is merely one theory among many. "Because Darwin's Theory is a theory, it continues to be tested as new evidence is discovered," says the York schools biology syllabus. "The Theory is not a fact. Gaps in the Theory exist for which there is no evidence." Students are directed to digest the intelligent design bible Of Pandas and People, described as a "companion text" to the standard Darwinist textbook Biology.

At issue in Harrisburg is whether teaching "intelligent design" in lieu of "creationism" is contrary to the 1987 Supreme Court ruling. The parents say it is teaching religion. The schools board, supported by a Christian legal advice group, the Thomas More Law Centre, contends it is not a religious concept and is therefore a legitimate alternative to teaching evolution alone.

While its adherents insist there is no theological dimension to intelligent design, the concept appears to embrace creationism. One witness told the Harrisburg court that the concepts were so interchangeable to the publishers of Of Pandas and People that the word "creationism" in early drafts was simply replaced in later editions with the phrase "intelligent design".

Intelligent design may not seem persuasive or plausible, but it is hugely popular among God-fearing Americans. According to Gallup, nearly half of all Americans believe "God created man pretty much in his present form within the last 10,000 years", while 38 per cent said that though evolution played its part, God none the less took a hand in the process. Only 13 per cent of Americans believe God had no part in the evolution or creation of humans.

There is no clearer evidence of the divide between Americans and Europeans than the difference in their attitudes towards science and religion. According to a Northwestern University study, more than 60 per cent of those living in Austria, Britain, Germany, Italy, the Netherlands and Spain, and more than 75 per cent in Denmark and France, believe evolution is "definitely" or "probably" true.

Among those Americans who believe intelligent design deserves its place alongside Darwin in the classroom is George W Bush, and it is not the first time he has defied the spirit of the Enlightenment. Science-defying irrationality has guided his policies on issues such as global warming, the right-to-die case of Terri Schiavo and drug tests for athletes. The president's contribution to the intelligent design debate was suitably irrational.

In August Bush declared, "Part of education is to expose people to different schools of thought"; he added that he thought intelligent design should be on the syllabus "so people can understand what the debate is about".

The Harrisburg verdict will reverberate across the US, giving a heads-up to the 28 states planning to introduce the teaching of rival theories to evolution. Parents in Sacramento, Califor- nia, will also take note. They are claiming in a US district court that they were denied their civil rights when their school district refused their request to teach an alternative to evolution. Until the Supreme Court eventually comes to decide the matter, the spirit of the irrational will continue to run riot in biology classes across America.