HARRISBURG -- The millennias-old meditation on the origins of the species arrived in a Harrisburg courthouse yesterday, when a group of parents filed a federal lawsuit that aims to prevent a York County school district from pushing "intelligent design" as an alternative to evolutionary theory in a high school biology course.

The parents, backed by the American Civil Liberties Union and Americans United for the Separation of Church and State, sued the Dover Area school board. The plaintiffs say the intelligent-design theory is a "Trojan horse" that would allow Judeo-Christian biblical beliefs to be introduced in public schools.

And that would be a violation of the U.S. Constitution, as well as subsequent Supreme Court judgments, they argue.

"They want our public schools to operate like Sunday schools," said the Rev. Barry Lynn, executive director of Americans United, a Washington-based organization that funds legal challenges when it believes church and state are getting too cozy.

Dover school board members, as well as many taxpayers in the district, want to offer their students what they say is a more balanced presentation of how the Earth and its inhabitants may have gotten here.

"I think it's a downright fraud to perpetrate on the students of this district to portray one theory over another," school board member William Buckingham told The Associated Press.

In October, the Dover board voted 6-3 to require its science teachers to tell students that evolution is an incomplete scientific theory, and that intelligent design -- the idea that the universe, its earth and its creatures are so complex that they must have been built by an intelligent creator -- is a viable alternative.

Evolutionary theory, which has been around in various forms since the early 1800s, gained credence as a scientific standard following Charles Darwin's 1859 manifesto "The Origin of Species."

Since October, three of Dover's school board members have resigned amid rancorous public debate on the issue, but the intelligent design requirement, which appears to be the first public school mandate of its kind in the United States, has remained.

The board members, even if they wanted to, could not have blocked evolution-related lesson plans outright, because the teaching of evolution in public schools is required by the state Board of Education.

The biology class at the heart of the flap is taught to ninth-graders. The evolution lesson would begin in January, and the suit's plaintiffs hope the board will reverse course before then.

If it doesn't, the litigants may seek an injunction, through which a U.S. District Court judge could force the district to drop the requirement.

The theory of intelligent design has been tested in the courts before, when it was known simply as creationism.

In 1987, the U.S. Supreme Court said creationism couldn't be taught in public school science courses, because such instruction would promote certain religious beliefs over others.

Dover's is the year's second high-profile legal tussle over evolution. Last month in Atlanta, a trial opened over whether a warning sticker in a biology textbook, saying evolution is "a theory, not a fact," violates church-and-state separation provisions.

Western Pennsylvania's most recent evolution-related fracas came in 2002, when the West Greene School District in Waynesburg considered allowing a creationism seminar to be conducted on school grounds. After being threatened with an ACLU lawsuit, the district backed off.

Dover, a bedroom community for York and Harrisburg, is in the state's conservative, GOP-leaning midsection. Many there believe that groups like the ACLU are sticking their nose in somebody else's business.

Steve Farrell, a Dover High School alumnus, says the requirement will help bring "God back into the school." Anyone who would fight the intelligent-design mandate is "taking a stand against God," he said.

The parents who filed the suit said school board members are not trying to broaden students' horizons as they claim, but instead are forcing their own religious beliefs on students.

"The Dover school board created the policy for religious reasons," said Tammy Kitzmiller, whose ninth-grade son is about to take the biology course.

Kitzmiller and other opponents of the Dover board said the theory of Judeo-Christian creationism could have a place in public schools, but not in science classes. Instead, it belongs in a religious history course, comparing Genesis' version of creation with those from other worldwide religions.

Throughout history, various religions, both monotheistic and polytheistic, have conjured literally hundreds of versions of the planet's and species' origin.

Even though evolution-creationism controversies, at least in this country, tend to pit social libertarians against religious conservatives, some clergy in the past several decades have said that the two theories aren't necessarily incompatible.

"Evolution is not at war with religion," Lynn said.

Theistic evolutionary theory, for example, suggests our species developed just as Darwin said, but further contends that evolution is a tool created by, then manipulated by, God. And "deists" believe that God set the evolutionary process in motion billions of years ago, but hasn't been involved since, allowing life to develop on its own.