LOOK at the people around you and you cannot fail to notice how different they all are. Their faces, bodies, behaviours and personalities all appear to be unique.
Now consider the whole of humanity. There are about 7 billion of us alive now and by some estimates about in the past 50,000 years. As far as we know each of them is, or was, a total one-off. The same applies to all those yet to be born.
That is a staggering amount of variation within the archetype we recognise as “human”. As we delve deeper into our biology and search for ever more sophisticated ways to verify people’s identity, the ways in which we are all unique are being uncovered. Some, like DNA and fingerprints, are obvious. Others, less so.
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So your mother was right: you are very special indeed. But don’t just take her word for it. Here are 11 ways in which you are a one-off.
DNA
It’s the obvious place to start. And it is also true: DNA does make you unique – up to a point. To get a measure of just how different you are genetically from everybody else try these numbers for size.
In 2001, the human genome project reported that all humans have 99.9 per cent of their DNA in common, leaving just 0.1 per cent to account for all our myriad differences. Over the past decade, this estimate has been revised upwards to about 0.5 per cent, but even that is a very small sliver of the genome. Is it enough to account for the variation we see?
In theory, yes, in spades. The human genome contains approximately 3.2 billion letters of the DNA code; 0.5 per cent of that is about 16 million letters. The code has four letters, so the number of possible combinations is four raised to the power of 16 million – an absolutely vast number of possible human genomes, more than enough to go around everybody who has ever lived, many times over. The chances of anyone having exactly the same genome as you is zero.
That is even true of identical twins. Although they are 100 per cent genetically identical at the time of conception, from that moment on their genomes diverge, and the older they get the more individual they become ().
In identical twins (and the rest of us too), these differences come from slight changes and chance mutations every time DNA is copied. These can result in single nucleotide polymorphisms (SNPs), where a single letter of the code is changed, and also copy number variations (CNVs), where long sections of DNA are duplicated or deleted altogether.
Something similar happens with what are called epigenetic markers, which help regulate how genes are expressed. Identical twins drift apart on this measure from very early in life (), and the rest of us undoubtedly do too, adding another vast layer of potential genetic variation.
What isn’t yet known is the proportion of these genetic variations that actually make you different from other people. Many occur in non-coding regions that don’t make proteins or regulate gene expression. And even if they are in coding regions, many are likely to be neutral, altering neither a gene nor how it is expressed.
We do know, however, that tiny genetic differences can have large effects on physical traits such as eye colour or susceptibility to disease. So it is safe to say that your uniqueness as a person starts with the genome.
But it is far from being the whole story. Many other factors come into play: the environment, the physical forces that acted on you in the womb and a healthy dose of randomness. A case in point is fingerprints.
Fingerprints
Another no-brainer: everyone knows that fingerprints are unique, so it might come as no surprise that their size and shape is largely determined by genes. But the developing fetus’s fingerprints are also tweaked by subtle factors like the pressure of the walls of the womb and even the sloshing of amniotic fluid.
That means that while the fingerprints of identical twins can be very similar there are enough differences to tell them apart. Known in the fingerprinting business as “minutiae”, these differences include variations such as a ridge splitting into two in a slightly different place in each twin or a loop being wound slightly more tightly. The same goes for toe prints.
Forensics notwithstanding, no one really knows what fingerprints are for. A recent study showed that, contrary to popular belief, they don’t help with grip because they reduce, rather than increase, friction (). Other possibilities are that they protect the skin by making it more flexible, or that they improve our sense of touch by amplifying vibrations.
Whatever their purpose they clearly aren’t crucial to survival. Earlier this year, researchers identified a mutation that has caused a handful of people in just five families to be born without fingerprints (). All seem to get along fine, at least until they get to border control – the condition is also known as immigration delay disease.
Face
Faces are our most obvious badge of identity and we find it easy to recognise people by face alone. But they are perhaps not as distinct as we like to think. Even leaving aside identical twins, there are plenty of doppelgängers around. One recent analysis of several thousand Norwegian faces found that 92 per cent of them had at least one lookalike that both humans and facial recognition software struggled to tell apart ().
In another study, when asked whether two very similar photographs of faces came from the same person or not, neither human nor machine did any better than would be expected by chance. Humans got it right just 56 per cent of the time with unfamiliar faces and 66 per cent with familiar ones – surprisingly low considering how much of our identity is tied to our faces.
Gait
Since our ancestors first stood upright 1.5 million years ago, humans have all walked in more or less the same way: one foot in front of the other, swinging from the hip and rolling from heel to toe. Remarkably, every one of us to walk the Earth since may have done so in a slightly different style.
While we can’t be sure that everyone really does have a unique way of walking, studies as far back as the 1970s showed that gait differs enough for us to recognise people we know just from the way they walk at least 90 per cent of the time.
Gait changes during childhood but settles down when we stop growing. Then, differences in the length of our legs and width of our hips, plus environmental factors such as the amount of muscle we build up through exercise combine to give us a characteristic walk.
It’s something that is easy to spot, but difficult to describe, says Mark Nixon, who researches gait at the University of Southampton, UK. “We don’t have the words to describe the motions,” he says.
Computers do it better, either by tracing the lines of the limbs and turning their movements into numbers, or by tracking the movement of various points such as hips, knees and feet and measuring their changing relationship as they move.
Another way to measure gait is to have someone . This kind of system, developed by Todd Pataky’s group at Shinshu University in Japan, could potentially be used to fast-track passengers through an airport.
Yet another idea, so far in its infancy, is to use the kind of motion sensors found in smartphones. Strapped to the leg, these could measure speed, acceleration and rotation. The technique could be used as a security feature for cellphones, so that they would only work when carried by their rightful owner.
Ears
You probably haven’t paid that much attention to the exact shape of your ears, but if you look in the mirror and pull them out you will see that one is very slightly different from the other. Not only that, but each of your ears is different from everyone else’s.
This is because the human ear develops from six tiny bumps that appear on the side of the head around five weeks after conception, then gradually fuse. While genes map out the general shape, the environment in the uterus, such as how the fetus lies, affects how ears turn out. Once formed, they hardly change shape as we (and they) grow and age.
Several researchers are working on ways to recognise people by the shape of their ears. A recent analysis found that ear identification is just as accurate as face recognition when identifying people from photographs ().
In the US and the Netherlands people have even been convicted on the basis of an “ear print” left at the scene of a crime. The science of recognition by ear print is more controversial, however, because the shape of the print changes depending on the amount and direction of pressure put on the ear. At least one suspect in the US has been released on appeal after ear-print analysis was ruled unreliable.
Eyes
The iris of each eye is unique enough for several countries, including the UK, US and Canada, to accept an iris scan as proof of identity. But as anyone who has their daddy’s eyes knows, the appearance of the iris runs in families. So how can eyes that are the spitting image of the rest of the family’s be totally unique?
The answer lies in the complexity of the iris’s structure, a tangled mesh of muscles, ligaments, blood vessels and pigment cells that give it colour, depth, furrows, ridges and spots.
The colour and general texture of the iris is genetically determined, which accounts for family likenesses and for the fact that nearly everyone’s left and right eyes look much
the same. But iris-recognition systems used in airports ignore colour and texture and concentrate on the details of the ridges, furrows and freckles. These depend on the exact placement of the ligaments, muscles and pigment cells as the iris develops before birth, which is not controlled by the genes, but happens randomly. By this measure, each of your eyes is as different from the other as it is from anyone else’s.
Voice
When you speak, the sound that comes out is the sum of many parts: the noise that air makes as it vibrates through the larynx, the way it bounces around through the mouth and nose, and how it is shaped into words by the palate, tongue, lips and cheeks.
Since it is highly unlikely that two people will have a larynx, mouth, nose, teeth and muscles of exactly the same size and shape, voices end up being unique and easily recognisable.
But unlike some features, such as fingerprints and iris, we can deliberately change our voice by altering how we use the muscles of our face and larynx to create volume, pitch and tone. According to Sophie Scott, a neuroscientist at University College London, that means most people are good at changing their voice when they want to, and even when they don’t: our voices often change in response to social situations in ways we are unaware of.
Some people are clearly better at deliberately changing their voice. Scott is studying how impressionists can mimic others so convincingly. There are no clear answers yet, but she says that good impressionists seem to be highly musical, and adeptly imitate mannerisms as well as voice.
All of this means that there is no way of reliably identifying an individual voice by comparing waveforms or pitch and tone. There are a few voice signature systems on the market, but they tend to be backed up by an identity card or password, just in case a good impressionist turns bad.
Scent
Dogs have always known it, and now science can prove it: no two people smell alike. But is there really enough variation for all 7 billion of us to have a unique odour? Definitely, says George Preti of the Monell Chemical Senses Center in Philadelphia, Pennsylvania. “Just think about what nature does with only four bases in DNA. In the armpit alone there are at least a couple of dozen odorants, perhaps more, and you can have lots and lots of variation in relative amounts and concentration.”
We don’t have just one scent, of course, but several. Our various nooks and crannies have different types and quantities of secretions, and harbour different kinds of bacteria, which turn our mostly odourless secretions into scent.
A recent analysis of the volatile organic compounds in the sweat of 200 volunteers showed that out of the mix of nearly 5000 acids, alcohols, ketones and aldehydes, 44 of them varied enough to give an individual chemical profile that can be read like a fingerprint (). Many of these compounds seem to have no other function than to make us smell. They may have a role in how we identify each other, Preti suggests.
No one has invented a way of capturing the total scent of a person and using it to identify them, although the US government is said to be interested in such a technology and Preti says he is working on it too.
Heartbeat
Crooners would like to have you believe that two hearts can beat as one. In reality, no two heartbeats are the same. You wouldn’t notice by putting your ear to somebody’s chest, but it is possible to tell hearts apart by recording their electrical impulses.
An electrocardiogram (ECG) records three peaks: the P wave, which is the impulse that contracts the upper chambers, the QRS complex, which is the stronger contraction of the lower chambers, and the much smaller T wave as the heart relaxes.
Each heart varies in size and shape, so the height, length and spacing of the peaks varies from person to person. And while the spacing of the peaks changes as the heart rate speeds up with exercise or stress, the individual signature can still be discerned.
Because heartbeat is subconsciously controlled it is almost impossible to fake, and a handful of biometrics companies are working on scanners that could be used to check identity. Apple, too, is working on using heartbeat as a password to protect private information. Where they lead, others will undoubtedly follow.
Brain waves
What could be more individual than the way you think? It’s something that seems obvious, but only recently has evidence started to emerge that measurable differences exist.
Humans are born with a huge number of neurons and our brains gradually prune out an astonishing 50 per cent of them during infancy and childhood. What is left after this lengthy process – which is driven largely by experience – leaves each person with a unique brain that goes about the same tasks in a slightly different way. By listening in to the electrical activity of the brain with EEG, it is possible to see these subtle individual differences.
A 2001 study by Raman Paranjape at the University of Regina in Saskatchewan, Canada, found that a type of brain activity called alpha waves was different enough in 40 people to tell them apart. Another study found that the strength of a different kind of brain wave – gamma oscillations – also varied in 100 people while they were doing a standard test of object recognition.
Could brain differences explain why we all have different personalities? Possibly, though what isn’t known yet is whether a person’s brain waves would be recognisable if they were measured on different days or even years later. Without that, there’s no way of knowing if your brainwaves are like a fingerprint or just unique to one moment in time.
Microbiome
One aspect of your uniqueness isn’t, strictly speaking, part of you at all. It comes from the 100 trillion bacteria that live both on and in you. They outnumber the body’s cells 10 to 1 and in genetic terms they are even more dominant: microbes account for 3.3 million genes, compared with your measly 23,000. “You’re 0.7 per cent human,” says Jeremy Nicholson, a biochemist at Imperial College London.
Of the more than 1000 species that commonly live in and on the human body, each of us harbours only 150 or so, mostly in the gut (). And everyone’s bacterial population is made up of a different cast of characters.
Skin bacteria, too, vary from person to person though they are remarkably stable over time. A recent study found that a unique bacterial fingerprint is transferred from our fingers to the things we touch, such as a computer keyboard or mouse, and will hang around for up to two weeks (). Even identical twins, who are difficult to distinguish on the basis of DNA, are easy to tell apart when you check out their bacterial companions.
Bacteria also contribute to uniqueness by modifying our metabolism. All humans share a basic biochemistry, but layered on top of this is a microbial biochemistry that is much more diverse. The metabolites that microbes produce affect a range of things, including cholesterol and steroid metabolism.
“There are a few thousand basic enzymatic reactions in the human body but there are tens of thousands of metabolites – because our metabolism interacts with microbial metabolism,” says Nicholson. What this ultimately means is that without our non-human component, we wouldn’t be ourselves at all.
