A Rule of Thumb

"Fingerprints have been found on ancient Babylonian clay tablets, seals, and pottery. They have also been found on the walls of Egyptian tombs and on Minoan, Greek, and Chinese pottery — as well as on bricks and tiles in Babylon and Rome. ...on some pottery, fingerprints were impressed so deeply that they were likely intended to serve as the equivalent of a brand label." [wiki]

There is something eternally fascinating about the ridges and whorls on our hands and feet, those unrepeatable patterns which cover most of what is termed our "volar skin", that is, skin of the palms of the hands or the soles of the feet. When I was a kid I spent hours staring at the swirls and lines, looking at where they ended - and wondering why they were there. "Designs" I called them, when I was young.

I even went through a period, when I learned about fingerprinting and the idea that everyone has completely different fingerprints, where I made everyone around me (mostly adults) squash their fingers onto my ink-pad and leave their mark on the paper which I carried around for the purpose. Of course, it wasn't washable ink, so there seemed to be an inordinate number of long-suffering, black-fingered folks around my household.

The other night I got to talking with friends about fingerprints. How do they work? Why do we have them? The conversation didn't go very far, but it did make me decide to go look it up. Forty websites later, I am still no expert, but I continue to be fascinated.


(Koala fingerprint, above, versus human, below)

For example, did you know that koalas are one of the few mammals besides primates who have fingerprints, and in fact even with an electron microscope, it is difficult to tell koala prints apart from human prints? There's a mystery story in there somewhere, like The Murders in the Rue Morgue only (hopefully) more believable (anyone met a murderous orangutan lately?). Fishers are also said to have fingerprints, which seems to me very strange: if fishers do, why not stoats? Weasels? And so on?


Spider monkeys, whose prehensile tail-tips are so sensitive and flexible that they can pick a dime up off a floor, also have prints on the bare spot at the end of their tails. Since the tails are used not only as a sort of third arm when swinging in the trees (as a safeguard from falling), but often supports the entire weight of their bodies while they feed, this would make sense: fingerprints, and other places with "friction ridges" - the volar regions - generally tend to occur where one needs to grip something. This can mean gripping an object to keep from dropping it, or (as in the case of trees) to keep it from dropping you, or simply to keep your feet steady on the rocks so you don't fall off a cliff.

But how does it work? One source I was perusing posited that there could be a Van der Waals force element, like gecko's feet. The person cited the fact that our fingertips can feel the grittiness of a powder down to about 150 microns, and then it just didn't feel gritty anymore; since Van der Waals' forces tend to show up more when something is 150 microns or smaller, he conjectured a connection.


Other sources, however, didn't support this idea, even if it appealed to me. The general belief among my local pundits was that friction ridges weren't deep enough, enclosing enough or wet enough for either suction or for cohesion; and their structure wasn't complex enough for Van der Waals. The consensus was almost entirely on friction. Given that the flesh in these dermal ridges (to use another term) are notoriously squashy (thus making crime scene fingerprints - known as "latent prints" seriously difficult to decipher), the friction thing holds up as an answer. Just as tires made of squishy gel are more likely to stick to the road than ones made of hard plastic, so do the flexible, moist areas on our hands and feet provide an excellent surface to grip with. Thus does the fingerprint contribute to our development as tool-users.


Dermal ridges develop in the womb, and are pretty much developed by seventeen weeks. The patterns on our fingers are influenced by our time in the womb: subtle stresses and tensions affect how they grow, creating uniqueness through a combination of genetics and in utero experience (as can be seen by genetically identical twins, who don't have identical fingerprints). Once the fingerprints are set, they cannot be altered easily:

"Should the top layer of skin suffer any injury, the ridges grow back after healing in the exact pattern they had before. Therefore, superficial cuts or abrasions alter fingerprint characteristics only temporarily. If the injury reaches deep into the dermis and destroys the dermal papillae, then growth of new epidermal cells is impaired and a permanent scar is created."
[New South Wales Police Department]


The way the ridges develop, oddly, depends on the arrangement of the sweat glands, rising to pores which, in the volar regions, protrude in papillae (nipple-like structures) above the baseline of the skin surface. As these grow, they also grow connections to each other in rows - and this is how the lines and whorls of the fingerprint are created.

It also explains why fingerprints - the kind the police use for identification - are often made up of what appear to be rows of dots, rather than nice smooth lines:

"Such pore holes are critical to the production of latent prints since sweat reaches the surface of the hand and efficiently coats the tops of the fingerprint ridges with sweat. Sweat glands serve as small chemical reservoirs and contain a variety of water-soluble chemical compounds, produced or stored by the body."


In other words, we leave a chemical trace when we touch things, as rows of little oily mineral sweat-dots.

For those of you who have ever worried about the old hair-on-the-palm story, you can relax: both sebaceous glands and hair follicles appear in the dermal layer of other skin surfaces but don't in friction skin. Probably for good reason. How useful would it be to have painful pimples on the palms of your hands if your best escape from predators was to swing up into a tree?

Johann Christoph Andreas Mayer recognized in 1788 that although friction ridge patterns could appear similar, they never seemed to repeat themselves. Using fingerprints' unique patterns as an identification system, however came in much later, starting with the movement to the cities in the Industrial Revolution, when people began leaving their ancestral homes, where every face was familiar, and moving into more populous environments, where they were more difficult to identify and it was harder to find out their history.

"...felons quickly learned to lie about their names, and the soaring rate of urban crime forced police to search for a more exacting way to determine and keep track of identities. The first such system was devised in 1883 by a Parisian police clerk named Alphonse Bertillon. His method, called anthropometry, relied on an elaborate set of anatomical measurements -- such as head size, length of the left middle finger, face height -- and features like scars and hair and eye color to distinguish one person from another. Anthropometry proved useful, but fingerprinting, which was then coming into use in Britain, held more promise...


Francis Galton

"In 1880, Dr. Henry Faulds published the first comments, in the scientific journal Nature, on the use of fingerprints to solve crimes. Soon afterward, Charles Darwin's misanthropic cousin, Sir Francis Galton, an anthropologist and the founder of eugenics, designed a system of numbering the ridges on the tips of fingers -- now known as Galton points -- which is still in use throughout the world. (Ultimately, though, he saw fingerprints as a way to classify people by race.)"
-- [Michael Specter, from a fascinating article on the fallibility of fingerprints in the New Yorker]


Bertillon's method was actually quite popular in France long after fingerprints had become popular everywhere else (a member of the Bonnot Gang actually sent his fingerprints to the French police because he knew they only had his physical measurements on record). This popularity after his long struggle for the legitimization of his system meant that Bertillon was able to go on to implement such innovations as mug shots, systematized crime scene photography, ways to preserve footprints and ballistics, and the dynamometer, used to determine the degree of force used in breaking and entering.

Now, after a nearly hundred and fifty years of fingerprint analysis being considered unquestionably right, despite any evidence against it in trials across the world, a few cases have brought the practice into the limelight. Much of fingerprint analysis hasn't changed since it was first created, and its status as a "science" is coming into question, since scientific method, not to mention actual studies of the practice to see how accurate it is, seem to be missing from the process.


Some people are, actually, born without fingerprints. A genetic disorder due to defects in the protein Keratin 14 lead to two different diseases causing embryos not to form friction ridges. It makes it difficult to do certain things, like turn pages or deal cards. Most of all, it makes it difficult to get certain kinds of jobs - such as school teacher, nurse, and so on. Not to mention working for the government in either law enforcement or classified work.


In the old days, safecrackers used to sand the ends of their fingers to make them more sensitive and to make their fingerprints less identifiable; but that seems to be going out of fashion in contemporary times. Nowadays, you are more likely to affect your whorls by picking up a tiny virus-based skin tumor called a plantar wart (veruca), which deforms the skin striae as it grows, making the ridges go around it. When the wart finally goes away, your striae never look quite the same...

So, the next time you are lying on the couch with a loved one's feet in your lap, have a look, and marvel at the fanciful shapes and swirling minutae of their toes. Think about how long they have been on our feet, probably millions of years, and how even though we wear shoes, our bodies still create these wonderful artworks. They really are amazing.




Links:

A simple timeline on the history of fingerprints

Michele Triplett's Fingerprint Dictionary: Every term you could possibly want to know about fingerprint analysis and police procedure.

A little YouTube of the beginning of my favorite story about safecrackers, Butch Minds The Baby

Website about Sir Francis Galton, above

Photoshop brushes which give you fingerprint effects over at DeviantArt