IN THE END, after hours of carefully orchestrated testimony from antiabortion geneticists and biologists, last week's Senate abortion hearings evolved into something more than a biological show trial solely because of the testimony of a single man.

Despite repeated protestations about trying to be fair and impartial in the complex issue of abortion, Sen. John East (R-N.C.) called hearings that began and nearly ended with seven witnesses who all testified that human life begins the moment the egg is fertilized by the sperm. As biology lessons go, the hearings were first rate. Never has an audience had more distinguished instruction in ninth grade biology and never has so much attention been paid in the Senate to zygotes, eggs and sperms and so little to the interest of those who are already here.

As a civics lesson, the hearings debased the deliberating body that sponsored them, all the way until the very end when Dr. Leon Rosenberg, chairman of the human genetics department at the Yale University School of Medicine, took the microphone. Rosenberg was invited to testify only a week before the hearings amidst gathering criticism that the hearings were nothing more than an attempt to build a public record in support of a Human Life Bill banning abortions. Several members of the Senate subcommittee on separation of powers publicly expressed doubts about the hearings and disassociated themselves from them, leaving East to chair them in splendid isolation.

Before this freshman protege of Sen. Jesse A. Helms, the antiabortion senator from the tobacco country of North Carolina, Rosenberg cut an almost quixotic figure, an elequent but single voice of dissent pleading the cause of ethics and religion over biology. Rosenberg gave the hearing something none of the other witnesses, with all of their medical and scientific credentials, could provide. In dissent, he gave the hearings respectability and when he was done, East -- the erstwhile political science professor -- had gotten himself one memorable civics lesson.

Section 1 of the Human Life Bill states that "Congress finds that present-day scientific evidence indicates a significant likelihood that actual human life exists from conception" and extends to the fertilized egg all of the protections of the 14th Amendment available to people.

Rosenberg took the biologists on squarely, saying he knows of no scientific evidence that "bears on the question of when actual human life exists." The "notion embodied in the phrase . . . is not a scientific one, but rather a metaphysical one," he said. When conception occurs, there is a living cell "with the potential for human life. . . . But in my view, there is an enormous difference between the potential for human life and . . . actual human life.

"I believe we all know this bill is about abortion," said Rosenberg. If society feels compelled to ban abortion, he argued, then let it be through a constitutional amendment. "But don't ask science or medicine to justify that course, because they cannot. Ask your minister, your priest, your rabbi or your God, because it is in their domain that this matter resides."

Of all the witnesses, it was Rosenberg who most clearly engaged East's mind, and it was to him that East put the difficult question of when to protect human life.

"I have asked myself that question ever since I was invited to testify," replied Rosenberg, and for all his superior scientific knowledge, he stressed that his answer would be strictly personal. "I would protect it at the point of viability, at the point that the human being can exist on its own, outside the uterus."

"Is viability any more easy define scientifically than conception?" asked East.

There would not be complete consensus on when a fetus is viable, Rosenberg said, but there would be greater consensus about that than about when human life actually begins.

Rosenberg concluded with a plea that East not take the 7-to-1 witness ratio as accurately representing the views of the American scientific community. The great majority, he said, support his view. "I would hope you would continue to search for other scientific opinion."

East, to his credit, went out of his way to thank Rosenberg. "What we missed" in the original Supreme Court decision legalizing abortion, he aid, was "public dialogue on the issue." At the end of his hearings, East began to provide that.

East has scheduled further hearings on the legal and religious implications of the Human Life Bill. He can stack the rest of the hearings with people who agree with him, or he can learn from the testimony of Dr. Leon Rosenberg and turn his hearings into the kind of thoughtful public dialogue that can help America resolve the abortion controversy.

In the search for good laws, there is a lot to be said for listening to all sides. Sen. East, who in the words of one of his witnesses has opened a can of worms, certainly ought to try.