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Microbes that have been hibernating deep below the Pacific Ocean since the reign of the dinosaurs have been revived in the lab. Some may be 100 million years old, perhaps making them the longest-lived life forms on Earth.

We already know that microbes can survive deep below our planet’s surface, even though nutrients are generally scarce. Biologists suspect that the microbes enter a minimally active mode to stay alive. But whether they can emerge unscathed has been unclear.

Now a team led by Steven D’Hondt at the University of Rhode Island and Yuki Morono at the Japan Agency for Marine-Earth Science and Technology has studied about 7000 individuals of a bacterium found living in mud 75 metres beneath the sea floor, 5700-metres-deep in the South Pacific Ocean.

“We didn’t know whether we had fully functioning cells or zombies capable of doing very few things,” says D’Hondt.

In the lab, the researchers gave the microbes nutrients laced with distinctive isotopes of carbon and nitrogen. Within 10 weeks, these isotopes began showing up inside the microbes, indicating that they had begun to feed like typical bacteria.

That is remarkable considering what the bacteria have been through, says Jens Kallmeyer at the GFZ German Research Centre for Geosciences in Potsdam. He says the mud in which the bacteria were found is capped by layers of silicon dioxide that no microbe could penetrate.

This implies that the microbial populations have been trapped since the mud was buried under the silicon dioxide an estimated 101.5 million years ago. Given that this mud contains few nutrients, survival must have been challenging. “Nowhere else on Earth do you find sediment as close to totally dead as this,” says Kallmeyer.

The microbes may be even more astonishing than that. Although they can probably gather sufficient nutrition from the mud to repair cellular damage, it isn’t clear if the mud contains enough nutrients to fuel cell reproduction. “They may have divided since they were buried, or they may not,” says Virginia Edgcomb at the Woods Hole Oceanographic Institution in Massachusetts. “I don’t think anyone knows.”

If cell division is difficult there, some of the bacterial cells might be as old as the mud itself. “I mention this possibility in talks and it drives some researchers nuts,” says D’Hondt. Many biologists are unsettled by the idea that individual bacterial cells could survive for 100 million years.

There have been a handful of claims for even older microbes on Earth. One team claimed in 2000 to have resurrected microbes trapped inside 250-million-year-old salt crystals, but some researchers suspect that the microbes were seen as a result of sample contamination, which is unlikely to be the case in the new study.

Because the deep-sea microbes must have patched and repaired themselves countless times, it is perhaps down to philosophers to decide whether any individual cell really is 100 million years old. D’Hondt believes they qualify.

“I sometimes use the metaphor of my grandfather’s hammer,” he says. “My grandfather gave a hammer to my father and my father gave it to me. We’ve replaced the head twice and the handle three times, but it’s still the same hammer.”