According to the tale of Caesar’s last breath, each breath we draw probably contains a few of the molecules that escaped with his last. That assumes of course, that those 10²² molecules were more-or-less uniformly mixed with the 10⁴⁴ molecules in the atmosphere. When acting blind, as with the simple inert molecules in the air, this great thermal mill only homogenizes. But if purposed with more versatile actors, it becomes the universal constructor that knits order into every cell in our bodies, molecule by molecule. When instead of gases, the molecules are snippets of DNA — barcodes if you will — they can be used to discreetly, and unambiguously, mark almost anything. With the right molecular touch, it is possible to determine where something first came from, and where it has been.

Researchers at the University of Aveiro in Portugal are developing DNA barcode tags that can be harmlessly applied to a wide variety of products, even foods or liquids. Each tag is a unique combination of DNA base pairs that attach to most surfaces, and can later be collected, amplified, and sequenced. The power of this technique lies in the uncountable micro-matings that take place in the DNA solution where the primary goal is to determine like from unlike. Every known sequencing method has some margin of error, and the chance of false positives or false negatives exists whenever the signal to noise ratio is too low.

For example, in some schemes a DNA “fog” might be used to spray violent protesters when there are not enough law enforcement personnel to immediately subdue the lot of them. That tag will be unique, and mark anyone who bears it, at least for a while. Over time however, the signal will spread and degrade. Multiple tags could be used to mark multiple events or increase reliability of a single event. Clearly though, finding a way to contain your marking agent at the outset is the cleanest option.