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How Long Does DNA Last?

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Even minimal exposure to forensic science on shows like CSI and NCIS will impress upon a viewer what a whopping big deal DNA analysis is. It’s the opposite of circumstantial evidence: undeniable proof of someone’s identity that is impossible to fake, short of swapping out one sample for another. The technique may be applied to murder victims or long-dead English kings or illegitimate children and their custody-dodging fathers—any subject from which intact genetic information can be extracted—and that's what makes DNA as valuable a tool in anthropological study as it is in police investigations. For a long-dead subject, DNA has an expiration date, but when exactly is it?

The entire formula for human life is encoded in the sub-microscopic molecules of deoxyribonucleic acid, and has been throughout all stages of evolution. Like fingerprints, genetic code is particular to an individual, which makes it a unique identifier in the absence of other information, like modern dental records. DNA, however, is fragile, and breaks down over time. How long the decomposition process takes will vary with the circumstances under which it is found. Take, for example, if DNA is exposed to the elements: Like the human body itself, DNA decays with increasing rapidity in the presence of heat, water, sunlight, and oxygen. Those essential conditions of life also speed the process of death, potentially rendering DNA useless for analysis in a matter of weeks.

Scientists have estimated that under the most ideal conditions, DNA can theoretically survive for a maximum of one million years. Although a team of researchers recently claimed to have discovered 419 million-year-old genetic material belonging to prehistoric bacteria in the Michigan Basin, others in the field have loudly contested the claim, especially in light of an earlier sample thought to be 250 million years old, but later proven contaminated by the presence of modern DNA. The oldest actual DNA samples hail from Greenland (the icy one, as opposed to Iceland, the green one), extracted from beneath a mile of ice, a “perfect, natural freezer” for DNA preservation. The 450,000 to 800,000-year-old samples provide evidence of green life on the now largely barren landmass.

As far as human genetic material goes, the record for oldest Neanderthal DNA is held by a 100,000-year-old sample found in a Belgian cave. The longest-lasting sample of human DNA was discovered in northeastern Spain, and boasts a survival age of 7000 years. In both cases, techniques pioneered by Dr. Rhonda Roby allowed researchers to use mitochondrial DNA rather than the type found in the cell nucleus; although mitochondrial DNA only contains only partial genetic information, it provides sufficient evidence for identification and is present in greater abundance than nuclear DNA, increasing its odds of surviving.

How long does DNA last? The short answer is that it’s complicated, and determined by a number of unpredictable factors such as weather and the organism’s final resting place. Your DNA may be the molecular legacy you leave behind, but once you’re dead, there’s not really much you can do about it.

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Big Questions
Why Does the Queen Have Two Birthdays?
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CHRIS JACKSON, AFP/Getty Images

On April 21, Her Majesty Queen Elizabeth II will turn 92 years old. To mark the occasion, there are usually a series of gun salutes around London: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park, and a 62 gun salute at the Tower of London. For the most part, the monarch celebrates her big day privately. But on June 9, 2018, Her Majesty will parade through London as part of an opulent birthday celebration known as Trooping the Colour.

Queen Elizabeth, like many British monarchs before her, has two birthdays: the actual anniversary of the day she was born, and a separate day that is labeled her "official" birthday (usually the second Saturday in June). Why? Because April 21 is usually too cold for a proper parade.

The tradition started in 1748, with King George II, who had the misfortune of being born in chilly November. Rather than have his subjects risk catching colds, he combined his birthday celebration with the Trooping the Colour.

The parade itself had been part of British culture for almost a century by that time. At first it was strictly a military event, at which regiments displayed their flags—or "colours"—so that soldiers could familiarize themselves. But George was known as a formidable general after having led troops at the Battle of Dettingen in 1743, so the military celebration seemed a fitting occasion onto which to graft his warm-weather birthday. Edward VII, who also had a November birthday, was the first to standardize the June Trooping the Colour and launched a tradition of a monarchical review of the troops that drew crowds of onlookers.

Even now, the date of the "official" birthday varies year to year. For the first seven years of her reign, Elizabeth II held her official birthday on a Thursday but has since switched over to Saturdays. And while the date is tied to the Trooping the Colour in the UK, Commonwealth nations around the world have their own criteria, which generally involve recognizing it as a public holiday.

Australia started recognizing an official birthday back in 1788, and all the provinces (save one) observe the Queen's Birthday on the second Monday in June, with Western Australia holding its celebrations on the last Monday of September or the first Monday of October.

In Canada, the official birthday has been set to align with the actual birth date of Queen Victoria—May 24, 1819—since 1845, and as such they celebrate so-called Victoria Day on May 24 or the Monday before.

In New Zealand, it's the first Monday in June, and in the Falkland Islands the actual day of the Queen's birth is celebrated publicly.

All in all, just another reason it's great to be Queen.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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What Is the Meaning Behind "420"?
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Whether or not you’re a marijuana enthusiast, you’re probably aware that today is an unofficial holiday for those who are. April 20—4/20—is a day when pot smokers around the world come together to, well, smoke pot. Others use the day to push for legalization, holding marches and rallies.

But why the code 420? There are a lot of theories as to why that particular number was chosen, but most of them are wrong. You may have heard that 420 is police code for possession, or maybe it’s the penal code for marijuana use. Both are false. There is a California Senate Bill 420 that refers to the use of medical marijuana, but the bill was named for the code, not the other way around.

As far as anyone can tell, the phrase started with a bunch of high school students. Back in 1971, a group of kids at San Rafael High School in San Rafael, California, got in the habit of meeting at 4:20 to smoke after school. When they’d see each other in the hallways during the day, their shorthand was “420 Louis,” meaning, “Let’s meet at the Louis Pasteur statue at 4:20 to smoke.”

Somehow, the phrase caught on—and when the Grateful Dead eventually picked it up, "420" spread through the greater community like wildfire. What began as a silly code passed between classes is now a worldwide event for smokers and legalization activists everywhere—not a bad accomplishment for a bunch of high school stoners.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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