Whorls apart: the genetics of fingerprints
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When’s the last time you looked at your hands? Not just a glance, but really studied them? The fine parallel lines that form unique patterns not only on the ends of your fingers, but also cover the palm of your hand and the soles of your feet. The deep creases that interrupt this intricate design when you form a fist. The swirling lines on your fingertips perforated by microscopic sweat glands, leaving a unique identifying stamp on everything you touch, saying “I was here”.
The fancy, scientific term, for fingerprints is ‘dermatoglyphs’, literally your ‘skin carvings’ and they’re unique to primates…and koalas. Yes, chimps and koalas also have fingerprints and they’re pretty much indistinguishable from human fingerprints, leading to the really rather delightful story where half a dozen chimps were stolen from the ape house at London Zoo in 1975, and the police had to take the apes’ fingerprints for analysis. You may be pleased to know that the chimps, in that instance, had not committed any crimes.
We humans and chimpanzees are very closely related so it makes sense to have this shared similarity, but koalas are much more closely related to kangaroos and wombats, neither of which have fingerprints. This is an example of convergent evolution; two distantly related groups both coming up with the same solution to a shared problem independently of each other, the same way that Disney and Dreamworks both came up with animated films about anthropomorphised insects in 1998 - A Bug’s Life and Antz - even though they were working separately.
But why do we have fingerprints, when so many other animals do not? Because both koalas and primates both came up with the same solution, we can work backwards to discover the problem the solution is solving. If that logic sounds like we’re grasping at straws here, it’s because the answer is actually grasping - at straws, or leaves or fruits or whatever else takes your fancy. The combination of ridges, furrows and sweat glands maximises the friction between the skin and whatever surface you want to touch, regardless of whether it’s wet or dry, which is jolly useful if you don’t want to drop a slippery wet eucalyptus leaf you’ve just gone out of your way to grab.
And it’s not just grip. Your fingertips are among the most sensitive parts of the body, packed with nerve endings that detect the vibrations you make when you run the corrugated skin over a surface. The parallel ridges in your fingerprints have perfectly coevolved with your vibration sensors - Pacinian corpuscles if you want to give them their proper name - as the spacing between the ridges and furrows amplifies the narrow range of vibrations the sensors can detect. All this combined means your fingertips can detect objects as small as 40 micrometres or half the width of a human hair. Every single time you run your hands through your hair, your fingerprints are performing a feat of microscopic engineering.
With all this beautifully evolved biology at the tips of our fingers, you would be forgiven for thinking that we must have a pretty firm grasp on the genetics that create these miniature masterpieces. Well until very recently, how our fingerprints develop has been somewhat of a mystery.
There are three main patterns in fingerprints: arches where the lines flow left to right in an arching pattern; loops, where the lines start to form an arch but then turn around and double back on themselves; and whorls where the lines go round and round in circles.
In January of this year, a team of Chinese geneticists wanted to pin down the genes that control the development of fingerprint patterns, so they turned to a technique called genome wide association study, often called GWAS for short, which looks for consistent patterns of genetic differences between people.
Now of course, any two people could have a different genetic variant at any point along the genome, that’s why we’re all unique individuals. My melanin genes might be different from yours which is why we have different hair or skin colours, plus my height genes might be different, the genes for my blood type, you get the picture. So if you were to map out all of the places in the genome where two people have slight differences in the genetic code, so many places would light up that you wouldn’t be able to make sense of any of it. Who knows which one of the thousands of variations is the one that controls the thing you’re interested in.
The power of genome wide association studies is their size. To look for which parts of the genome controlled fingerprint development, the team collected the DNA of more than 23,000 people of Asian and European ancestry. With that many people, you can narrow down which bits of the genome change when you compare people with arches to people with loops to people with whorls. In this case, the analysis found at least 43 regions on the genome where changing the genetic code in these regions leads to different fingerprints.
Once you’ve got these regions, a grid reference or GPS point if you like, you can look on the map and see what’s actually happening at that location. It turns out that one of the most influential of the 43 regions happened to coincide with regulating embryonic limb development. Not the development of the skin, but the development of the whole limb.
Our best guess now, is that fingerprints are shaped by the different stretching and elongating forces as a foetus’s hand grows in the womb. This means our fingerprints are like an archival record, tracing the stages our hands went through from a smooth round bump poking out from an embryo to a complex machine of muscle and sinew with five unique protuberances that can curl and flex.
So pay more attention to your hands, because not only can they perform microscopic miracles, they can also tell you who you were before you were even born.
