Cervantes body discovered? How DNA testing works with ancient corpses.

What’s the Shelf Life of DNA? And Has Miguel de Cervantes’ Expired Already?

Answers to your questions about the news.
March 18 2015 4:33 PM

What’s the Shelf Life of DNA?

How to identify 500-year-old authors and resurrect extinct species.

Dr Jo Appleby, a lecturer in bioarchaeology at Leicester University, addresses a press conference in front of an image of the skeleton of Britain's King Richard III, at the university in central England, on Feb. 4, 2013.
Forensic anthropologist Francisco Etxeberria speaks about Miguel de Cervantes’ burial investigation on March 17, 2015, in Madrid. Etxeberria said his team had identified “some fragments” of the Don Quixote author, who died in 1616 a week after William Shakespeare.

Photo by GERARD JULIEN/AFP/Getty Images

Spanish researchers announced on Tuesday that they have found what they believe to be the body of Don Quixote author Miguel de Cervantes. Forensic anthropologists have indicated that they may try to use DNA testing to confirm the discovery. In 2013, Forrest Wickman explained how long DNA lasts and how it can be used to reveal the identities of ancient remains. His article is reprinted below.

Forrest Wickman Forrest Wickman

Forrest Wickman is a Slate senior editor. He writes and edits for Slate’s culture blog, Brow Beat.

The body of Richard III has been found beneath a parking lot in Leicester, England, according to experts from the University of Leicester. DNA testing was used to match the infamous king with DNA from a descendant of his sister. What’s the shelf life of DNA?

About a month to a million years, theoretically. The decay rate of DNA depends on the conditions of its storage and packaging. Above all, it depends on whether the DNA is exposed to heat, water, sunlight, and oxygen. If a body is left out in the sun and rain, its DNA will be useful for testing for only a few weeks. If it’s buried a few feet below the ground, the DNA will last about 1,000 to 10,000 years. If it’s frozen in Antarctic ice, it could last a few hundred thousand years. For best results, samples should be dried, vacuum-packed, and frozen at about -80 degrees Celsius. Even then, ambient radiation is likely to render DNA unrecognizable before it celebrates its millionth birthday.

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Some scientists contend that DNA could survive beyond our current theoretical estimates. In fact, several scientists have claimed to find DNA hundreds of millions of years old. In 2009, a team of researchers reported that they had found 419-million-year-old DNA inside ancient salt deposits in the Michigan Basin. If confirmed, it would be the oldest DNA ever discovered. However, some experts who study ancient DNA are highly skeptical of these claims, noting that they usually turn out to be the product of contamination in the lab. Other scientists studying bird bones have estimated that under ideal conditions, DNA has a half-life of approximately 521 years, meaning that it would be broken down so much as to be useless after about 1 million years.

Despite what John Hammond and Mr. DNA might tell you, amber does not actually do a good job of keeping DNA fresh. While the fossilized tree sap can preserve insect skeletons for tens of millions of years, the DNA inside the insects breaks down very rapidly. When the organism dies, enzymes are released that begin breaking down the DNA almost immediately. Similarly, Egyptian mummies may look well-preserved—many of the proteins in their hair and muscles are intact—but their DNA has typically decayed rapidly in the heat. As a general rule, outward appearance is not a good indicator of whether DNA is still intact.

Probably the oldest DNA ever found was discovered in frozen mud taken from the base of an ice sheet in Greenland. It is estimated to be 450,000 to 800,000 years old. The sample contained genetic material from butterflies, pine trees, and other organisms. The frozen sludge broke the record previously held by plants frozen in ice in Siberia, which grew there 400,000 years ago. Neanderthal DNA has been found that is about 100,000 years old. When it comes to modern humans, the oldest DNA recovered so far has been only about 5,000 to 7,000 years old. In 2008, researchers used DNA samples that were thousands of years old to sequence the genome of the extinct woolly mammoth. While many have wondered whether we might be able to clone one of the creatures, such an endeavor presents mammoth challenges. While Spanish researchers successfully resurrected an extinct species of ibex in 2009, it died of breathing difficulties seven minutes later, likely because of flaws in its DNA.

In their efforts to identify Richard III, researchers used a kind of DNA called mitochondrial DNA—so-called because it’s contained in the mitochondria of the cell rather than in the nucleus. Mitochondrial DNA does not contain the complete human genome, making it not as useful for many researchers’ purposes. However, because it’s more abundant—there are often hundreds of mitochondria in a cell, and only one nucleus—the odds are better that you could find mitochondrial DNA intact than nuclear DNA.

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Explainer thanks Christina Warinner of the University of Oklahoma.

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