The Lechuguilla Cave in New Mexico is a network of chambers stretching 1,600 feet underground. The bacteria that grow on the walls of its most remote recesses have been living in complete isolation for more than four million years.

In 2010, Gerry Wright, a microbiologist at McMaster University in Ontario, ran an experiment on those long-lost bacteria. He and his colleagues doused them with antibiotics, the drugs that doctors have used for the past 70 years to wipe out bacterial infections.

But many of the Lechuguilla bacteria would not die.

“Most of them were resistant to something,” said Dr. Wright. Some strains, he and his colleagues found, could resist 14 commercially available antibiotics.

Dr. Wright’s discovery didn’t fit the conventional story of antibiotics. Antibiotics were introduced in the mid-1900s. Each time a new drug was introduced, it would take years before bacteria that could resist it became common.