We have a strong sense of continuous, coherent existence – yet from the cells that make our bodies to our defining character traits, we are in a constant state of change


9 December 2020

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Each of us shows many differing faces over time

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Are you always the same person?

MY MOM sometimes jokes that it is fortunate she didn’t meet my dad when he was in college, because she wouldn’t have liked him. She was (and is) a self-described goody two shoes. Dad not so much, but presumably even less so when keg parties were involved.

We know that we change over time. Our bodies grow, then age; we mature and our views shift; our memories sharpen and fade. Yet for most of us, our sense of self is seamless and continuous. You are the same old you, right?

Let’s start with the physical. Some of our cells, notably neurons in the brain, are with us from before birth, and can live more than 100 years. “Most of the nerve cells in the brain are actually as old as we are,” says molecular biologist Jonas Frisén at the Karolinska Institute in Sweden.

But most of our cells aren’t. Some, including certain kinds of white blood cell, live for only days. How quickly our skin cells are replenished changes as we age, but in general it takes about a month. The notion that the liver regenerates every 40 days or so is a myth: our liver cells live 200 to 300 days.

On the level of atoms and molecules, meanwhile, we are exchanging material with our environment with abandon. Think of your body like a grassy field, says Frisén. “It’s the same lawn from year to year, but each strand of grass is completely different.”

But what about less tangible …