Human DNA can be found everywhere humans go, and that’s basically OK.
While true crime documentaries and police procedural dramas may reasonably lead us to conclude that we’re only likely to get human DNA on something by bleeding, spitting, sneezing, defecating, ejaculating, decomposing, or otherwise oozing directly on it, human DNA can be found in sweat, fingerprints, hair follicles, and the dead skin that flakes off of us everywhere we go. Our DNA follows us like clouds of miasma, and we inhale, swallow, and bathe in strangers’ DNA on a regular basis.
Of course, that doesn’t mean that there’s always nuclear DNA present to test—which is why we hear about larger, grosser samples swabbed up in all those dreadful TV shows. But there’s quite often more than enough mitochondrial DNA flaking off in our normal detritus to identify us as human, so we can find little pieces of ourselves in places we might not expect. Well-documented examples include…
1. Hot dogs and (probably) other processed meats
If you’ve been watching the news, you know it’s been a bad week for processed meat products all around; the WHO has decided they cause cancer, and a report by a group called Clear Food stated that up to 2% of hot dogs contain human DNA.
But observers predicted human DNA would be found in meat products years ago, and it probably applies to all categories of processed meat:
The possibility of finding human DNA in meat products is “quite likely” due to abattoir workers continually cutting themselves during production, experts revealed yesterday.
Conducting a hearing in response to the recent horse-meat scandal across Europe, the South African committee said it was probable they would find traces of human tissue in meat meant for public consumption, but this poses “no threat” to the consumers who eat it.
That would explain even the presence of nuclear DNA—and as noted above, you don’t really need to actually bleed in the meat to get your mitochondrial DNA there; dead skin and hair are enough. So it’s quite plausible, and maybe more likely than not, that a statistically significant percentage of hot dogs contain human DNA. It isn’t unsanitary, it isn’t dangerous, and it shouldn’t gross us out.
Snopes has identified some issues with the Clear Food study that cast some doubts on its methodology, but honestly, the amount of time humans spend around these products during processing should make this a no-brainer. Of course hot dogs contain trace amounts of human DNA, and so do any number of other foods you consume on a regular basis.
(PS: Speaking of things Snopes has written about, that meme about human flesh being found in McDonald’s hamburger meat is completely false.)
2. gonorrhea and (probably) numerous other bacteria
“When you gaze long into a bacteria’s genome sequencing,” Friedrich Nietzsche never said, “it gazes back at yours.” Scientists were said to have been “surprised” to learn in 2011 that gonorrhea carries around human DNA, but truth be told, most probably were not. While neisseria gonorrhoeae is the first bacteria species known to participate in lateral gene transfer with its human hosts, it almost certainly isn’t the first to do so, and biologists have long suspected that this is a thing that happens.
3. ordinary house dust, though not as much as we’ve been told
While the often-cited factoid that 70% or more of dust is dead skin is probably horseshit, dead skin is ubiquitous enough in our environments that at least a low single-digit percentage of dust in an occupied house is likely to be made up of human skin. That’s more than enough to put our mitochondrial DNA there, which means that we’re inhaling little bits of each others’ DNA all the time.
4. a 400,000-year-old human fossil
In 2013, scientists tested DNA recovered from a fossilized 400,000-year-old femur discovered in Spain’s aptly-named Sima de los Huesos and found that it belonged to one of our Denisovan ancestors. (While Neanderthals and Denisovans were not members of homo sapiens sapiens, they were hominids and therefore are classified by biologists as human.) This is by far the oldest human DNA ever tested, and gives us hope that comparatively new ancient DNA—such as that of the mysterious ancient Sumerians—might one day be tested.
5. 14,000-year-old poop
Scientists examining fossilized fecal samples (coprolites) collected in Oregon’s Paisley Caves discovered two curious facts: they dated back to 12,300 BCE, and they contained human DNA. This completely changed our narrative about the early history of human settlement in the Americas. It also reinforced the durability of human DNA; recovering it from a prehistoric femur is one thing, but recovering it from someone’s morning constitutional is another.
6. paper money
While the most surprising piece of data from a 2014 study of paper currency was that it contained over 3,000 species of bacteria (including hundreds that hadn’t even been classified yet), human DNA was also pretty much universally present.
7. live animal organs
Scientists have been splicing human and animal DNA for over a decade, giving us exciting new options for organ transplantation.
8. non-primate genome databases
Even scientists working in controlled settings manage to spread their DNA where it doesn’t belong; one 2011 study found that nearly 1 in 5 “non-primate” genome sequences were contaminated by human DNA.
9. post-reentry rocket debris
One 2014 study found that about 35% of DNA samples exposed to the high temperatures and volatile environment of rocket reentry survived intact. Even falling to the Earth from space isn’t enough to effectively destroy DNA.
In other words: not only can our DNA get pretty much everywhere, but it’s pretty hard to get rid of, too. As much as we like to think of our world as a pretty sterile place, and our biochemical fingerprint as something relatively private, the truth is that we’re spreading durable pieces of ourselves, and picking up durable pieces of each other, wherever we go.