On the projector, he placed a picture of the hand of a gibbon, another human cousin. “There’s the opposable thumb,” he said, wiggling his own. “But theirs is a longer hand because they live in trees, and their arms are very long.”

Mr. Campbell bent over, walking on the outer part of his foot. He had intended to mimic how arms became shorter and legs became longer. He planned to tell the class how our upright gait, built on a body plan inherited from tree-dwelling primates, made us prone to lower back pain. And how, over the last two million years, our jaws have grown shorter, which is why wisdom teeth so often need to be removed.

But too many hands had gone up.

He answered as fast as he could, his pulse quickening as it had rarely done since his days on his high school debate team.

“If that really happened,” Allie wanted to know, “wouldn’t you still see things evolving?”

“We do,” he said. “But this is happening over millions of years. With humans, if I’m lucky I might see four generations in my lifetime.”

Caitlin Johnson, 15, was next.

“If we had to have evolved from something,” she wanted to know, “then whatever we evolved from, where did IT evolve from?”

“It came from earlier primates,” Mr. Campbell replied.

“And where did those come from?”

“You can trace mammals back 250 million years,” he said. The first ones, he reminded them, were small, mouselike creatures that lived in the shadow of dinosaurs.

Other students were jumping in.

“Even if we did split off from chimps,” someone asked, “how come they stayed the same but we changed?”