John T. Scopes was a 24-year-old physics, chemistry and math teacher at the public high school in Dayton, Tennessee, when local community leaders persuaded him to answer the ACLU’s call for a defendant in a test case challenging the Butler Act.

That law, passed in March 1925, made it illegal to teach any theory contradicting the Biblical version of creation, as presented in the Book of Genesis. While filling in as a substitute teacher in a biology class, Scopes used a textbook that promoted the theory of human evolution introduced by the English naturalist Charles Darwin in his 1871 book The Descent of Man.

Arrested on May 9, 1925, Scopes asked some of his students to testify against him in front of a grand jury in Nashville (about 150 miles northwest of Dayton) to ensure that his case would go to trial. On May 25, the grand jury indicted Scopes on the charge that he “did unlawfully and willfully teach…certain theory and theories that deny the story of Divine creation of man as taught in the Bible and did teach thereof that man descended from a lower order of animals.”

Scopes’ indictment opened the way for what would become known as the “trial of the century,” or the “Scopes Monkey Trial.” Heading up the prosecution team was William Jennings Bryan, a three-time Democratic presidential candidate and a devout Christian who often spoke passionately and publicly about the Bible’s teachings. Scopes’ defense was led by the renowned Chicago attorney Clarence Darrow, a member of the ACLU known for his defense of labor unions and opposition to the death penalty.

The trial took place in the blisteringly hot month of July 1925, at the Rhea County Courthouse in Dayton. Thousands of visitors, including journalists and prominent academics, poured into the small town to see the clash of evolutionism versus creationism, rural fundamentalism versus worldly urban sophistication.

Darrow and his colleagues on the defense team made no attempt to maintain their client’s innocence of the charges against him, but argued that the Butler Act itself was unconstitutional. As a result, Scopes’ particular situation gave way to a larger debate over the validity of Darwin’s theory versus the authority of the Bible.

The drama reached its peak on the seventh day of the trial. The courtroom had grown so packed, and the heat so intense, that Judge John T. Raulston ordered that the trial proceedings be moved outside. As the judge had prohibited the defense from calling any scientific witnesses, Darrow took the highly unusual step of calling Bryan himself onto the stand as an expert on the Bible.

Under cross-examination, Darrow got his opponent to admit that creation was not completed in a week, but “might have continued for millions of years”—in effect, that not all of the Bible’s teachings should be taken literally—considered a major triumph for the defense.

The lawyers’ heated exchange might have continued, as Bryan wanted to put Darrow on the stand the following day, but Judge Raulston put a halt to that and declared Darrow’s cross-examination irrelevant to the case. Darrow responded by asking the jury to find his client guilty, so they could move on and appeal the verdict to a higher court. The jury did so, finding Scopes guilty of violating the Butler Act and fining him $100.

The verdict allowed Bryan to claim a nominal victory, but it was a short-lived celebration: he died of apoplexy five days after the trial ended. On the other hand, the case had allowed Darrow and the ACLU to present arguments on behalf of evolution to a larger audience than ever before.

On appeal, the state supreme court upheld the Butler Act’s constitutionality but acquitted Scopes, on the grounds that he had been excessively punished. The Scopes Monkey Trial would become the basis for the acclaimed 1955 play “Inherit the Wind,” as well as a 1960 film of the same name starring Spencer Tracy. Scopes himself left teaching and became a chemical engineer in the oil industry.

The “trial of the century” faded from the public’s memory, but the Tennessee law would remain on the books for four more decades, and evolution would not become part of accepted school curricula until the 1960s.