Robert Krulwich, the longtime journalist and one of the hosts of “Radiolab,” the immensely popular audio program known for its offbeat investigations of big ideas, will retire from the show in January, New York Public Radio announced on Thursday.

Krulwich, 72, has hosted the show, which is produced by WNYC, with Jad Abumrad since 2005.

“There comes a time — and I think it has come — when you get out of the way and let the future come flooding in,” Krulwich wrote in a letter posted online.

Abumrad created an earlier rendition of “Radiolab” in 2002, the same year Krulwich and Abumrad met. When Krulwich came aboard a revamped version, the show was a small, seasonal venture; now it airs weekly across 589 public radio stations.

Each episode poses a scientific or philosophical quandary at the top that Krulwich and Abumrad probe over the course of the show, often leading to surprising conclusions. One recent broadcast began by asking where eels come from; another explored the secret history of square dancing.