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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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Words
Why Is 'Colonel' Spelled That Way?
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English spelling is bizarre. We know that. From the moment we learn about silent “e” in school, our innocent expectations that sound and spelling should neatly match up begin to fade away, and soon we accept that “eight” rhymes with “ate,” “of” rhymes with “love,” and “to” sounds like “too” sounds like “two.” If we do sometimes briefly pause to wonder at these eccentricities, we quickly resign ourselves to the fact that there must be reasons—stuff about history and etymology and sound changing over time. Whatever. English. LOL. Right? It is what it is.

But sometimes English takes it a step too far, does something so brazen and shameless we can’t just let it slide. That’s when we have to throw our shoulders back, put our hands on our hips and ask, point blank, what is the deal with the word “colonel”?

“Colonel” is pronounced just like “kernel.” How did this happen? From borrowing the same word from two different places. In the 1500s, English borrowed a bunch of military vocabulary from French, words like cavalerie, infanterie, citadelle, canon, and also, coronel. The French had borrowed them from the Italians, then the reigning experts in the art of war, but in doing so, had changed colonello to coronel.

Why did they do that? A common process called dissimilation—when two instances of the same sound occur close to each other in a word, people tend to change one of the instances to something else. Here, the first “l” was changed to “r.” The opposite process happened with the Latin word peregrinus (pilgrim), when the first “r” was changed to an “l” (now it’s peregrino in Spanish and Pellegrino in Italian. English inherited the “l” version in pilgrim.)

After the dissimilated French coronel made its way into English, late 16th century scholars started producing English translations of Italian military treatises. Under the influence of the originals, people started spelling it “colonel.” By the middle of the 17th century, the spelling had standardized to the “l” version, but the “r” pronunciation was still popular (it later lost a syllable, turning kor-o-nel to ker-nel). Both pronunciations were in play for a while, and adding to the confusion was the mistaken idea that “coronel” was etymologically related to “crown”—a colonel was sometimes translated as “crowner” in English. In fact, the root is colonna, Italian for column.

Meanwhile, French switched back to “colonel,” in both spelling and pronunciation. English throws its shoulders back, puts its hands on its hips and asks, how boring is that?

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Big Questions
Why Do Cats Love Scratching Furniture?
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Allergy suffering aside, cat ownership has proven health benefits. A feline friend can aid in the grieving process, reduce anxiety, and offer companionship.

The con in the cat column? They have no reservations about turning your furniture into shredded pleather. No matter how expensive your living room set, these furry troublemakers will treat it with the respect accorded to a college futon. Do cats do this out of some kind of spite? Are they conspiring with Raymour & Flanigan to get you to keep updating home decor?

Neither. According to cat behaviorists, cats gravitate toward scratching furniture mostly because that love seat is in a really conspicuous area [PDF]. As a result, cats want to send a message to any other animal that may happen by: namely, that this plush seating belongs to the cat who marked it. Scratching provides both visual evidence (claw marks) as well as a scent marker. Cat paws have scent glands that can leave smells that are detectable to other cats and animals.

But it’s not just territorial: Cats also scratch to remove sloughed-off nail tips, allowing fresh nail growth to occur. And they can work out their knotted back muscles—cramped from sleeping 16 hours a day, no doubt—by kneading the soft foam of a sectional.

If you want to dissuade your cat from such behavior, purchasing a scratching post is a good start. Make sure it’s non-carpeted—their nails can get caught on the fibers—and tall enough to allow for a good stretch. Most importantly, put it near furniture so cats can mark their hangout in high-traffic areas. A good post might be a little more expensive, but will likely result in fewer trips to Ethan Allen.

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