The ginkgo is a living fossil. It is the oldest surviving tree species, having remained on the planet, relatively unchanged for some 200 million years. A single ginkgo may live for hundreds of years, maybe more than a thousand. They’ve survived some of our world’s greatest catastrophes, from the extinction of the dinosaurs to the atomic bombing of Hiroshima.

So what’s the secret to their longevity?

In the rings and genes of Ginkgo biloba trees in China, some of which are confirmed to be more than 1,000 years old, scientists are starting to find answers.

“In humans, as we age, our immune system begins to start to not be so good,” said Richard Dixon, a biologist at the University of North Texas. But in a way, “the immune system in these trees, even though they’re 1,000 years old, looks like that of a 20-year-old.”

He and colleagues in China and the United States compared young and old ginkgo trees, ranging in age from 15 to 1,300 years old, in a study published Monday in the Proceedings of the National Academies of Science. By examining the genetics of the vascular cambium, a layer or cylinder of living cells behind the bark, they found that the ginkgo grows wide indefinitely through old age.

That’s because the genes in the cambium contain no program for senescence, or death, they say, but continue their program for making defenses even after hundreds of years. Old trees also produce just as many seeds and their leaves are just as resourceful as those of young trees. Though it has yet to be tested, the researchers believe other old trees — think of the 4,800-year-old bristlecone known as Methuselah in eastern California — may have a similar pattern of genetic programming.

Although ginkgos live long, they do age. The trees grow up and out: Up, with a cell-generating region called the apical meristem, and out, with the vascular cambium. Over time, weather or other things damage the apical meristem, limiting a tree’s height. And each year, leaves die and fall off.

But the cambium, contained within the tree’s trunk, remains intact and active. Cell division tends to slow down after the age of 200, they found. But the cells are still viable. They generate defenses and carry water and nutrients so the tree grows and stays healthy.

Sometimes trees may be reduced to just hollow stumps, but with the cambium intact, they can still produce leaves and flowers or even live as stumps.

Eventually, even ginkgo trees die. But a big question remains: Why?

Essentially, trees like ginkgo could live forever, says Peter Brown, a biologist who runs Rocky Mountain Tree Ring Research and was not involved in the study. “Being modular organisms, every year they’re putting on new wood, new roots, new leaves, new sex organs,” he said. “They’re not like an animal, like us. Once we’re born, all of our parts are there, and at a certain point they just start to give out on us.”

The trees don’t necessarily die of old age, he says. Something — pests, drought, development — kills them first.

He and others presume that studies on other trees like redwoods or Methuselah would produce similar results. And though humans are quite different from trees, contemplating them serves some purpose.

Peter Crane, an evolutionary biologist and author of “Ginkgo: The Tree that Time Forgot,” said contemplating long-lived trees might help us to see further into the future than many of us tend to look

“It’s kind of a way of calibrating how quickly our world is changing and reminding us that we shouldn’t always be thinking of the short term.”