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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
What Does the Sergeant at Arms Do?
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
House Sergeant at Arms Paul Irving and Donald Trump arrive for a meeting with the House Republican conference.
Chip Somodevilla, Getty Images

In 1981, shortly after Howard Liebengood was elected the 27th Sergeant at Arms of the United States Senate, he realized he had no idea how to address incoming president-elect Ronald Reagan on a visit. “The thought struck me that I didn't know what to call the President-elect,'' Liebengood told The New York Times in November of that year. ''Do you call him 'President-elect,' 'Governor,' or what?” (He went with “Sir.”)

It would not be the first—or last—time someone wondered what, exactly, a Sergeant at Arms (SAA) should be doing. Both the House and the Senate have their own Sergeant at Arms, and their visibility is highest during the State of the Union address. For Donald Trump’s State of the Union on January 30, the 40th Senate SAA, Frank Larkin, will escort the senators to the House Chamber, while the 36th House of Representatives SAA, Paul Irving, will introduce the president (“Mister [or Madam] Speaker, the President of the United States!”). But the job's responsibilities extend far beyond being an emcee.

The Sergeants at Arms are also their respective houses’ chief law enforcement officers. Obliging law enforcement duties means supervising their respective wings of the Capitol and making sure security is tight. The SAA has the authority to find and retrieve errant senators and representatives, to arrest or detain anyone causing disruptions (even for crimes such as bribing representatives), and to control who accesses chambers.

In a sense, they act as the government’s bouncers.

Sergeant at Arms Frank Larkin escorts China's president Xi Jinping
Senat Sergeant at Arms Frank Larkin (L) escorts China's president Xi Jinping during a visit to Capitol Hill.
Astrid Riecken, Getty Images

This is not a ceremonial task. In 1988, Senate SAA Henry Giugni led a posse of Capitol police to find, arrest, and corral Republicans missing for a Senate vote. One of them, Republican Senator Bob Packwood of Oregon, had to be carried to the Senate floor to break the filibustering over a vote on senatorial campaign finance reform.

While manhandling wayward politicians sounds fun, it’s more likely the SAAs will be spending their time on administrative tasks. As protocol officer, visits to Congress by the president or other dignitaries have to be coordinated and escorts provided; as executive officer, they provide assistance to their houses of Congress, with the Senate SAA assisting Senate offices with computers, furniture, mail processing, and other logistical support. The two SAAs also alternate serving as chairman of the Capitol Police board.

Perhaps a better question than asking what they do is pondering how they have time to do it all.

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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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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