One Twin Spent a Year in Space

The story

Here is a puzzle people have wondered about for ages. What makes you YOU? Is it the recipe you were born with, or everything that happens to you afterwards? It is a hard question to test, unless you can find two people who started from the very same recipe. That is why scientists love twins.

There are two main kinds of twins. Sometimes two babies grow side by side, each starting from its own separate tiny beginning, and they are no more alike than ordinary brothers and sisters. Those are fraternal twins. But sometimes, at the very beginning, one tiny beginning, smaller than a dot, splits into two, and each half grows into a whole baby. Those are identical twins. They start with almost exactly the same recipe, the same set of instructions for building a person, which is why even their teachers can mix them up.

Now, among the astronauts of the American space agency, NASA, there were two brothers, Scott and Mark Kelly, and they were identical twins. When Scott was chosen to live on the space station for nearly a whole year, the brothers themselves offered NASA a wonderful idea: study the two of us. So Scott flew up and spent his year floating, working and sleeping far above the Earth, while Mark, who had flown to space before, stayed on the ground. The same starting recipe, and two very different years. When Scott came home, scientists compared the brothers from top to toe: their blood, their eyes, their bones, even the tiny workings inside their cells.

What did they find? Space really does change a body. While Scott floated, scientists saw some of the small workings inside his cells shift and change, in ways they did not see in his brother down on Earth. And after Scott landed, most of those changes slowly drifted back to normal. Thanks to the twins, scientists gathered precious clues about how a human body copes with living in space, clues we will need if people ever voyage to Mars.

But the deepest twin discovery is not about space at all. It is this: even identical twins, with their almost-matching recipes, are never truly identical. Their fingerprints are different, swirled into their own patterns before they were even born. Their favourite foods can drift apart. One might love drawing while the other loves climbing. Because a person is not just a recipe. A person is a recipe being cooked, day after day, by everything they live: the games they play, the stories they hear, the friends they make, the things they practise.

Scientists call the recipe part nature, and the living part nurture. And the answer to the old puzzle turns out to be both, braided together so tightly that you can never fully pull them apart.

So if you ever meet identical twins, look closely. You are watching one of nature's own experiments: the same beginning, growing into two different people. And you are an experiment too, one that has never been run before. Nobody in all of history has ever been cooked from your recipe, by your days.