It’s often said that humans are 99.9% identical. and what makes us unique is a measly 0.1% of our genome. This may seem insignificant. But what these declarations fail to point out is that the human genome is made up of three billion base pairs—which means 0.1% is still equal to three million base pairs.
In those three million differences lie the changes that give you red hair instead of blonde, or green eyes instead of blue. You can find changes that increase your risk of obesity, or others that decrease your risk of heart disease; differences that make you taller or lactose intolerant, or allow you to run faster.
When I first started learning about genetic variation, I assumed these changes—the 0.1% that make us unique—only appeared in certain places, such as genes for height or inherited diseases like diabetes. I thought the rest of the genome—the other 99.9%—was fixed; that the 0.1% that was different in me was more or less the same 0.1% that was different in you. But, as it turns out, the 0.1% of DNA that is different between people is not always the same 0.1%: Variation can happen anywhere in our genomes.
In fact, one group of scientists looking at 10,000 people found variants at 146 million unique positions, or about 4.8% of the genome. Another group collected the DNA from 15,000 people and found 254 million variants, roughly 8% of the genome. And as we continue to sequence 100,000, 100 million, or all seven billion people on the planet, we will find a lot more variation. This means that humans have many more differences than we first thought.
Imagine that your DNA is a car. There are certain obvious variants you can have: blue or white, two-door or four-door, convertible or sedan. These changes represent the 0.1%. Because the other 99.9%—the engine, the seats, the steering wheel, the tires—has to be there for the car to work, we assume they are fixed.
But electric cars have shown us that we don’t need the gas cap, the gas tank, or even a gas engine any more; we can replace those things with a variant like batteries and charging ports. And maybe one day we’ll develop cars that have boosters instead of tires so we can hover over the ground.
In other words, what we believe is static may actually be variable. More than 0.1% of the car can change and it still be a car, just like the human genome.
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