Back in the 1980s, when DNA forensic analysis was still in its infancy, crime labs needed a speck of bodily fluid—usually blood, semen, or spit—to generate a genetic profile.
That changed in 1997, when Australian forensic scientist Roland van Oorschot stunned the criminal justice world with a nine-paragraph papertitled "DNA Fingerprints from Fingerprints." It revealed that DNA could be detected not just from bodily fluids but from traces left by a touch. Investigators across the globe began scouring crime scenes for anything—a doorknob, a countertop, a knife handle—that a perpetrator may have tainted with incriminating "touch" DNA.
But van Oorschot's paper also contained a vital observation: Some people's DNA appeared on things that they had never touched.
In the years since, van Oorschot's lab has been one of the few to investigate this phenomenon, dubbed "secondary transfer." What they have learned is that, once it's out in the world, DNA doesn't always stay put.
Objects bearing DNA of a participant who never touched them, reported the Marshall Project.
In a sense, this isn't surprising: We leave a trail of ourselves everywhere we go. An average person may shed upward of 50 million skin cells a day. Attorney Erin Murphy, author of "Inside the Cell," a book about forensic DNA, has calculated that in two minutes the average person sheds enough skin cells to cover a football field. We also spew saliva, which is packed with DNA. If we stand still and talk for 30 seconds, our DNA may be found more than a yard away. With a forceful sneeze, it might land on a nearby wall.
To find out the prevalence of DNA in the world, a group of Dutch researchers tested 105 public items—escalator rails, public toilet door handles, shopping basket handles, coins. Ninety-one percent bore human DNA, sometimes from half a dozen people. Even items intimate to us—the armpits of our shirts, say—can bear other people's DNA, they found.
The itinerant nature of DNA has serious implications for forensic investigations. After all, if traces of our DNA can make their way to a crime scene we never visited, aren't we all possible suspects?
Forensic DNA has other flaws: Complex mixtures of many DNA profiles can be wrongly interpreted, certainty statistics are often wildly miscalculated, and DNA analysis robots have sometimes been stretched past the limits of their sensitivity.
But as advances in technology are solving some of these problems, they have actually made the problem of DNA transfer worse. Each new generation of forensic tools is more sensitive; labs today can identify people with DNA from just a handful of cells. A handful of cells can easily migrate.
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