Sometimes I have pondered whether my DNA might be found on continents east and west, hither and yon, from here to the back of beyond.
My scenario is I scratch my head in an airport in Melbourne, or in Indonesia or England and a flake of skin falls off. Then another traveller picks up my skin-flake on the sole of their shoe and carries it to another destination. Rinse (pun intended) and repeat.
I just added up all the airports I have departed from internationally, and it adds up to 25 (most of them multiple times, so LOTS of skin flakes could have been flaked).
Thus, if a forensic team were to do an intensive investigation into the DNA found in a random spot in any part of the world, what would be the chances of my DNA being found there?
It’s totally possible that a skin flake could hitch a ride onto an international flight and back home to someone’s street. But finding it would be like looking for a needle in a haystack. DNA is also a pretty fragile molecule. The chances of it getting picked up in a random sample even in your hometown are incredibly small, and the chance of it hitching a ride over several hops decreases significantly with every potential hop, and over weeks/months and unpreserved DNA sample will deteriorate significantly.
I wasn’t concerned that someone would be searching for MY DNA, just whether it might turn up inadvertently if (for example) a soil sample from Outer Mongolia was tested for whatever reason. And I understand that DNA degrades, but given that DNA can be extracted from bodies/etc many years old, why couldn’t mine?
These are both the same scenario, though, whether intentional or unintentional. If you’re inadvertently testing a random soil sample from Outer Mongolia, the chances of your DNA showing up there are infinitesimally tiny, essentially zero for all practical purposes.
When retrieving DNA from a cadaver, you have a sample that’s well-preserved, and tons of material from one single sample. It’s well-preserved because a grave is generally fairly well-protected from the elements and isn’t subject to a lot of movement or interference, and may even be somewhat oxygen-free depending on the preparation and burial. Plus there’s a ton of material to work with, you can just keep sampling different spots until you find one where there’s a non-degraded sample.
That’s pretty much the opposite scenario from what you described, which is finding DNA from a flake of skin in the soil, fully exposed to the elements, on the opposite side of the planet. Given the time it would take for a random skin flake to make its way from the armrest at an airport gate all the way to Outer Mongolia, which would be months to years depending on the airport, the sample would’ve certainly deteriorated to an unusuable state. Again that’s before we even get into the unlikelihood that any given testing sample happens to contain your particular DNA.
If the real question is whether your DNA would show up if you analyzed every single particle of soil everywhere in Outer Mongolia, I’d still say the chances are so small as to be effectively zero.
There wouldn’t be much point to it’s being picked up if it’s not being checked against some DNA database, even if only (as it would have to be) on a “what have we here” fishing expedition - and then what could be learnt from it?
It’s an interesting thought experiment. Call it “homeopathic DNA”; how dilute is the “solution” of all the OP’s lifetime flaked skin versus the whole surface of the Earth?
The land surface of Earth is ~149E6 km2. Which is 149E12, or 1.49E14 m2. The surface area of an adult human is ~2m2. A human sheds & renews all their skin ~1x/month. A human lives ~70 years or 840 months. The rest is details.
Which suggests a single human creates about 1680m2 = 1.68E3 m2 of skin samples per lifetime. Dividing by the surface area of the Earth we get ~1E-11.
Ignoring sample degradation over those 70 years, if the OP’s lifetime shed was scattered evenly over the Earth’s land it would form about 1 part in 100 billion of the local soil everywhere.
In the semi-real world, the scattering would not be evenly distributed, even with the help of the rest of humanity and modern travel patterns. Lots would be concentrated in or around places the person lived or worked, and rather less would be sitting in Central Antarctica. As well, some (large) fraction would eventually be washed into the oceans, not just be left sitting on land.
And of course, shed skin is not eternal. Good thing or we’d be wading through a deep morass of accumulated dead human and animal skin every day. Eeeew!
Once we put some of the factors I skipped into the math, we’re talking single parts per multiple trillions. That’s for skin. DNA in turn is a very minor constituent of skin. DNA constitutes 1 part in ~300 of a human. So the OP’s DNA fraction of Earth’s soil turns into 1 part per ~quadrillion.
Not zero, but darn close to zero. All in all, the Earth is big and the OP is small. Very small.
As I was trimming my fingernails one time, I ran a plot where someone picked up the nail clippings and placed then at a crime scene. Maybe I should quit clipping them in the back yard??
And what about hair? Surely that can travel as well as skin flakes.
Have you ever heard about the phantom of Heilbronn? It was a woman who had apparently been participating in all kinds of crimes, often murder. And it was impossible to find a pattern. She was everywhere and nowhere, and driving police nuts.
Want me to spoiler it? No, better read it yourselves. Depending on who you are and where you work, your DNA could be perhaps not everywhere, but in places it should not.
Are we really breathing in some of Caesar’s last breath?
The story goes that in 44 BC in Rome, Julius Caesar was assassinated by a group of his own senators, crumpling to the floor with a final gasp. This last breath contained around 25 sextillion (that’s 25 followed by 21 zeroes) air molecules, which would have spread around the globe within a couple of years. A breath seems like such a small thing compared to the Earth’s atmosphere, but remarkably, if you do the math, you’ll find that roughly one molecule of Caesar’s air will appear in your next breath. - SOURCE
And we know DNA can certainly be in a breath you exhale.
Exhaled breath condensate is an airway-derived specimen type that has shown significant promise in the diagnosis of asthma, cancer, and other disorders. The presence of human genomic DNA in this sample type has been proven, but there have been no reports on its utility for the detection of respiratory pathogens. - SOURCE
Now, the chances of Caesar DNA surviving till now in a breath you take I think would be very small to non-existent.
Decades ago, I decided that if I ever plan a serious crime, I’ll stop by my barber first, collect a bag full of clippings from his dumpster, then spread them around the crime scene. I’m sure I’m not the only one to think of this.
OP reminds me of a question I once saw that was apparently from an MIT admission test. The question was simply, in every breath you take, how many air molecules are the same ones exhaled by Julius Caesar in his last breath?
The question required general knowledge about lung volume and the volume of the atmosphere, and I think calculating the dynamics of oxygen and CO2 regeneration was optional and one might have gotten away with assuming a static atmosphere. I worked it out with the latter simplifying assumption and I think the answer was, reasonably assuming a well-mixed atmosphere, you’re definitely getting some Caesar in you, and I don’t mean the drink!
