A school board being sued in a US court for questioning the theory of evolution has begun presenting its case.
Darwin's idea of evolution is widely accepted by scientists
Pennsylvania's Dover Area school board requires science teachers to say that evolution is unproven, and to raise "intelligent design" as an alternative.
A biology professor and leading advocate of intelligent design told the court that evolution alone could not explain complex biological processes.
Michael Behe said he believed God was behind them.
Some parents are suing the school board, saying that intelligent design is a religious belief and should not be taught, because it violates the United States constitutional separation of church and state.
The Dover school board instructs its teachers to read a statement to 14-to-15-year-old students before classes on evolution, saying that Charles Darwin's theory is "not a fact", and that there are "gaps in the theory".
Students are then referred to an intelligent design textbook for more information.
Intelligent design is being promoted in schools across more than 20 states in the US.
It holds that the development of life cannot be explained solely by evolution, and that the guiding hand of an intelligent force must have been at work.
Although it does not name God specifically, the theory has been adopted by some Christian groups, who promote it ahead of creationism, the literal interpretation of the biblical book of Genesis.
'Not a theory'
Prof Behe was the first witness called by the school board, after the dissenting parents presented their case.
He said evolution should still be taught, as "any well-educated student should understand it", but said it could not fully explain the biological complexities of life.
He said intelligent design questions whether life at the molecular level could have evolved through natural selection.
"That's the most poorly supported aspect of Darwin's theory," he told the federal court.
Prof Behe's stance is rejected by the faculty at the university where he works, and the scientific mainstream.
The head of the American Association for the Advancement of Science says that intelligent design "is not even a theory".