For Ted Petry, working under the stands at the University of Chicago’s Stagg Field was his first job out of high school and a good one, considering there was a war on.

“They came to the high school, and they were recruiting,” he said. “The fact that there were no jobs ­available at the time, you took it.”

Petry was drawn by the promise of some $90 a month and the proximity to his South Side home. What the recruiter didn’t mention, probably didn’t know, was that the Tilden Technical High School graduate would become part of scientific history in that improvised laboratory, shaping the building blocks and even fetching the uranium for the inaugural human-made nuclear reactor.

Seventy-five years ago Saturday, on Dec. 2, 1942, the graphite bricks he helped plane into shape and assemble into a pile under the direction of physicist Enrico Fermi became a key component of the world’s first self-sustaining nuclear chain reaction, an event that opened the curtains on the Atomic Age.

And now Petry — as the university prepares to culminate its commemoration of the anniversary with a slate of events Friday and Saturday — is the only known survivor of those 49 eyewitnesses.

“I call myself ‘the last man standing’ because there’s nobody else left — probably because of my age. I was 17, 18 years old,” Petry said from his Orland Park home earlier this week. “I was just lucky enough to outlive everybody.”

Now 93, he’s been retired since 1982 and drawing a pension from his 17 years as a Chicago high school shop teacher. He is matter-of-fact about his role in the Manhattan Project. While the dream team of scientists Fermi assembled on behalf of the U.S. government believed they were racing Hitler to be able to develop the atomic bomb, Petry said his understanding of things was much more prosaic.

“You did what they wanted you to do,” he recalled. “You readjusted the pile or took care of the uranium, pressing it into the cylinders and such. And when your eight hours was over, you went home.”

A university record lists him as “Laboratory Assistant,” but Petry described the job as “a laborer, a gofer.” One of the things he’d go for was the radioactive metals ­necessary to, ultimately, split the atom.

“I used to go downtown and pick up the radioactive materials in little canisters, stick it in my pocket and bring it back,” Petry recalled. “Finally my red blood count went down” — a danger sign revealed in regular blood tests at the university hospital — “and they decided that, ‘Well, we’d better pick it up with a station wagon with a big lead container where you could put the radioactive material in it.’ But it didn’t affect me. I’ve got four good, healthy children.”

He said he doesn’t remember many details from that particular day in the room with Fermi, the other scientists and the wood-and-graphite structure known as Chicago ­Pile-1. Another witness, though, recalled the neutron counter going from clicks to a telltale “roar,” signifying the atom’s energy had been unleashed inside the pile.

“Everyone realized the significance,” the witness, ­physicist Herbert Anderson, said in 1975.

“The controlled chain reaction proved that nuclear energy could be harnessed,” adds the current Museum of Science and Industry exhibition that quotes Anderson: “It also proved the possibility of an atomic bomb.”

That exhibition, “Turn Back the Clock,” tells the story of the Bulletin of the Atomic Scientists, formed at Chicago by Manhattan Project scientists immediately after the war ended. The publication’s role as an instrument of conscience, and the history of its famed Doomsday Clock symbolizing our proximity to nuclear midnight, is traced, along with the broader story of the nuclear era.

“The 75th anniversary is this opportunity to look back on one of the most incredible scientific breakthroughs we’ve experienced,” said Rachel Bronson, the bulletin’s publisher. “My colleague Eric Isaacs has said it’s like nothing short of discovering fire.”

With that breakthrough has come not only the devastation when the U.S. dropped atomic bombs on Hiroshima and Nagasaki, Japan, in August 1945, but the fear ever since of atomic weapons being used again.

There was also a “tremendous amount of positive impact on society,” said Isaacs, executive vice president for research at the University of Chicago and former director of Argonne National Laboratory.

“Enrico Fermi was all about nuclear power after that, which led to Argonne,” he said. Isaacs also cited nuclear energy’s key role in medical advances, in space exploration and in archaeology (carbon dating), as well as the belief he retains that nuclear power can find a future role despite decades of decline in the U.S.

“The other piece is it really changed the way we do ­science in this country,” he said, ringing in an era of grand-scale cooperative science exemplified by national laboratories such as Argonne and Fermilab.

He said he finds it remarkable that amid that early big science project in the converted squash court under the old football field stands, there was room for a kid like Ted Petry.

“Can you imagine being him and being asked to work on this?” Isaacs said.