Our cells, each composed of 100 trillion atoms made of particles from the Big Bang, are filled with all kinds of structures. These include organelles—little factories like energy-producing mitochondria—and tiny molecular machines like ATP synthase, whose rotor and shaft spin at up to 300 rpm to produce ATP, the molecules that transmit energy in our cells. The interior of our cells are also filled with all kinds of molecules randomly colliding at tremendous speeds. Water molecules, for example, zigzag at the astonishing speed of over 1 thousand miles an hour (although they only go about 4 billionths of an inch before they smack into another molecule). In addition to collisions, cells face a myriad of other threats from within and without. You might expect them to suffer the same fate as our cars and dishwashers and constantly break down. But they don’t. Your body has an ingenious three-part strategy to keep you out of the junkyard.
The biophysicist Dan Kirschner told me that just thinking about everything that could go wrong in cells used to keep him awake at night. He was learning about cell development in a graduate school course just as his wife was about to have a baby. He was so overwhelmed by the many opportunities for mistakes that he feared his daughter would be born with a neck like a giraffe.
She wasn’t. Our cells have come up with a number of clever strategies to avoid living short lives. The first is that their machinery is astonishingly reliable. Ribosomes, for instance, insert the wrong amino acid into a protein on the order of once every 10 thousand times. The machines that copy our DNA make a mistake only about one in a million to 10 million or so.
Nonetheless, nothing is perfect. Sometimes, mistakes happen. Battering collisions, UV light, and dangerous molecules like free radicals also cause damage. Ingeniously, our cells have several ways to meet these threats. For one, they are full of clever repair mechanisms—machines whose jobs are to go on patrol to look for mistakes and fix them. Our cells have error-checking molecular machines and autocorrecting feedback loops that ensure remarkable fidelity.
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A 1954 newspaper story in the Atlanta Constitution suggests a second strategy our cells have adopted to stay alive. “Bored with yourself? Tired of the same old frame and face? Take another look then. In a manner of speaking, you’re constantly being reborn. Mankind, like the automobile industry, goes in for a radical chassis change each year.” The science behind this odd claim was the work of an inventive nuclear physicist named Paul Aebersold.
Aebersold began his career at the cyclotron in Berkeley’s Radiation Lab, which pioneered the production of radioactive isotopes. Later, at the Atomic Energy Commission, Aebersold oversaw the development of isotopes for medical uses. At some point, he realized he could use his isotopes to find out how often we replace the atoms in our bodies. All he had to do was irradiate a substance like table salt, ask an extremely accommodating subject to swallow it, and trace the salt’s path with a radiation-tracking device like a Geiger counter. You can follow radioactive atoms in quantities as small as “a billion billionth of an ounce,” Aebersold proudly told a television interviewer. He found that we swap out half of our carbon atoms every one to two months, and we replace a full 98% of all our atoms every year.
Wait, what? Is that even possible? Apparently it is. Over half of you is water, and we know that we constantly replace that. Another large percentage of you is protein, and as you may recall, most proteins degrade within hours or days. We even disassemble and replace our ribosomes and large organelles such as mitochondria, which are made primarily of protein.
Aebersold had discovered another strategy that enables our cells to live so long: our cells are constantly replacing their seemingly permanent structures and old battered molecular machines with new ones. The only ones they don’t replace are our massive chromosomes. Instead, we have machines that swarm along them looking for problems and fixing them.
What if the damage to a cell is too great to repair? We have a fallback plan for that too. We simply destroy the entire cell, chop it up into recyclable units, and make a fresh one. On average, you replace most of your cells every 10 years, which amounts to about 330 billion cells a day. Those that work in the harshest conditions are retired most frequently. The damage to many cells in your intestines, which are exposed to harsh acids, is so predictable that they commit planned suicide and are replaced every two to four days. You replace your skin cells, which endure scrapes and UV light, every month or so. Your red blood cells, which take a beating as they careen through your bloodstream, are replaced every 120 days. That means you have to make almost 3.5 million new red blood cells every second. Other cells, like those in our bones, are taken out of commission less often, only about once every 10 years.
So, in addition to using reliable machines, our cells have a three-pronged motto to stay alive: ceaselessly check for errors, constantly repair, and continually replace. In a way, your body is like a major New York highway—always open and always under repair.
Adapted Excerpt from What’s Gotten Into You: The Story of Your Body’s Atoms, from the Big Bang Through Last Night’s Dinner by Dan Levitt. To be published by HarperCollins on Jan. 24, 2023. Copyright © 2023 by Daniel Levitt. All rights reserved.
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