Have you ever wondered if something can stay the same even when all its parts are replaced? There’s a famous French proverb about a knife: if you replace the handle when it wears out and the blade when it breaks, is it still the same knife? This idea can also apply to our bodies because, like the knife, parts of us are constantly being renewed. Even as you read this, some of your cells are dying and being replaced. It’s a bit strange to think about, but if parts of us didn’t die, we couldn’t be alive.
This idea isn’t new. It dates back to ancient Greece with a story about a ship owned by Theseus. After Theseus defeated the Minotaur, the people of Athens preserved his ship by replacing each part as it wore out. Eventually, none of the original parts were left. So, was it still Theseus’s ship? This question makes us think about what truly defines something: is it the parts, the shape, or something else entirely?
Our bodies are like that ship. Every day, about 330 billion of your body’s 30 to 40 trillion cells are replaced. This means that roughly every 100 days, you have a new version of yourself. But not all cells are replaced at the same rate. For example, neurons in your brain, which hold your thoughts and memories, don’t regenerate much after your brain stops growing. This is why brain injuries can be so serious.
Cells die in two main ways. The first is called apoptosis, where cells die in a controlled way as part of their natural life cycle. The second happens when cells are damaged beyond repair. Your body has a system to clean up these dead cells efficiently.
Different types of cells have different lifespans. For instance, red blood cells, which carry oxygen, last about 120 days. Skin cells are replaced even faster, with about 500 million being shed every day. In contrast, the cells in your heart and bones can last for years.
Interestingly, not all of your body is made up of living cells. About a third of your body mass is non-living. This includes things like the fluid in your blood and the calcium in your bones. Even your hair and nails are made of dead protein called keratin.
Moreover, your body is home to countless bacteria and microbes, which play a crucial role in your health. These tiny organisms are as numerous as your own cells, and they help with digestion and other essential functions.
Life is a balance between death and regeneration. Just like the Shinto shrine in Japan that is rebuilt every 20 years, our bodies are constantly renewing themselves. This cycle is essential for health and survival.
So, the next time someone asks, “Who are you?” you can tell them that you’re a constantly changing collection of cells, both living and non-living, working together to make you who you are. Stay curious and keep exploring the wonders of the human body!
For more fascinating insights into the natural world, check out the PBS series “Out of Our Elements.” It explores the molecular stories behind everyday phenomena, revealing the extraordinary in the familiar. And if you want to support more educational content, consider joining the community on Patreon.
Imagine you are the captain of Theseus’s ship. Create a comic strip or storyboard that illustrates the journey of the ship as its parts are replaced over time. Reflect on whether it remains the same ship and why. Share your creation with the class and discuss your thoughts.
Create a timeline poster that shows the lifespan of different types of cells in the human body. Include details such as how often they are replaced and their functions. Present your timeline to the class and explain the importance of cell renewal in maintaining health.
Keep a diary for a week, noting down any changes you observe in your body, such as hair growth or skin shedding. Research and write a short paragraph each day about the science behind these changes. Share your findings with a partner and discuss the concept of body renewal.
Participate in a class debate on the topic: “If all your cells are replaced over time, are you still the same person?” Prepare arguments for both sides and engage in a lively discussion. Consider philosophical, biological, and personal perspectives.
Research the role of microbes in the human body and create a role-play activity where you act as different types of microbes. Explain your function and importance to the human body. Perform your role-play for the class and discuss the symbiotic relationship between humans and microbes.
Here’s a sanitized version of the provided YouTube transcript:
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There’s a French proverb that asks if the handle of a knife is replaced whenever it’s worn out and the blade is replaced whenever it’s beyond repair. Well, does the knife itself last forever, or does the knife become a new knife each time? We can ask the same question about our bodies because, like this knife, bits of us are always breaking down and being replaced. Even right now, parts of you are dying. Some of your cells even died before you were born, and some will never come back. These are all very strange and mildly uncomfortable things to think about, but the fact is, if parts of us weren’t constantly dying, we couldn’t be alive.
What’s even stranger is that a significant fraction of your body is made up of cells that were once alive but are no longer. So, how much of you is dead? How much of you has been replaced like the knife? And has any part of you always been you?
Hey, smart people! Joe here. I’m going to put this away now because it’s getting a little weird. Versions of that thought experiment actually go back millennia. A Greek philosopher, Plutarch, is credited with one of the first versions, and it starts with a ship. This guy named Theseus, who may or may not have actually existed, sailed his ship to Athens after defeating the Minotaur. To honor Theseus, the people of Athens preserved his ship for hundreds of years. Whenever part of the ship needed to be fixed, they replaced it plank by plank until eventually no original pieces remained. So, is it still Theseus’s ship?
Take this stack of blocks. I can replace it brick by brick until it’s a new version of itself. It looks the same, so what defines the essence of the stack? Is it its shape? Is it the number or the pattern of the colors? Perhaps it’s the same stack, or maybe it’s something else.
In Japan, people have been doing something similar since the 7th century. This Shinto shrine gets rebuilt piece by piece every 20 years. There’s basically a new shrine—or is there? It aligns with Shinto beliefs around the impermanence of nature and the idea that the only way something can last is if it’s eternally rebuilt. Some might say there’s always been just one ship of Theseus or one stack of blocks, a thing bigger than its pieces. Others might argue that when a part of a thing is replaced, then that thing becomes new.
The Greek philosopher Heraclitus once said it’s impossible to step in the same river twice. The outlines may be the same, but the water before is long gone. So it is with our own bodies. Roughly 330 billion of your body’s 30 to 40 trillion total cells are replaced every day. Of course, not all of your cells get replaced, but this means every 100 days, at least in total cell numbers, you regenerate a new version of you. Are you the same you 100 days from now?
Clearly, all of this cell regeneration must come with an equivalent amount of cell death; otherwise, you’d be like a huge walking tumor. This story of how your body manages its own death piece by piece gets interesting because different cells die faster or slower than others, and some not at all. A lot of this cell death happens before you’re even born. For instance, the cells that help grow the tiny bones in your middle ear do their job in the womb and die right before you are born. If your body didn’t direct some very particular tissue to die at precisely the right time, you’d have webbed fingers today.
Broadly speaking, cells die in two ways. Cell death that’s programmed by your body is called apoptosis. The second kind occurs when cells are damaged beyond repair by toxins or environmental extremes. Cleaning that up is a bit messier, but your body’s janitors are good at what they do.
Think of your body like a car. You have to do maintenance to keep it running. If you couldn’t ever get rid of old parts, you would have a problem. Luckily, that doesn’t happen because you are dying inside right now in a carefully programmed and beautifully orchestrated way. So, what part of you is dying the fastest? You have around 200 different types of cells acting as the building blocks for everything that you are, from bones to blood, intestines to eyeballs, and everything in between.
On the most immortal end are your neurons. Networks of these cells hold your thoughts, memories, and consciousness. Killing and trying to replace neurons could really mess all that up. Neurons do make new connections and trim others as you learn or forget, but after your brain is done growing, you’re pretty much stuck with the neurons you’ve got. This is why brain or spinal injuries are often so permanent.
Interestingly, scientists have found that in one part of your brain, the hippocampus, neurons can regrow. This brain region is important in learning and memory, and about a third of it gets pruned and replaced throughout your life. They proved this in a fascinating way. Massive amounts of radioactive carbon were released into the environment by nuclear bomb tests in the 1950s and 60s, and people alive during this time had radioactive carbon inside neurons in their hippocampus. The only way it could have gotten there is if they were making new neurons during their life in that part of their brain.
The majority of your body’s bulk and mass is made up of fat and muscle cells, which can last for decades. Of all those muscle cells, the ones in your heart stick around the longest. The cells that help build your bones last about 10 years. About 86% of the 30 trillion cells that you lose every few months are blood cells, specifically red blood cells, which carry oxygen from your lungs to your tissues and bring carbon dioxide back. These cells make up about half of your blood by volume and last about 120 days. Some white blood cells, which only make up about 1% of your blood, last only hours.
In your gut, about 12% of your cellular recycling happens. The cells that line your colon last less than a week before being replaced. You also lose about 500 million skin cells a day, which means in a lifetime you lose enough skin to cover about a thousand bodies.
So, where does all this dead material go? Well, everywhere! Every time you bathe, cough, sneeze, poop, or spit, you’re sending bits of dead you out into the world. About two-thirds of your body mass is made up of stuff that will eventually die and regenerate.
This brings up a couple of interesting questions. If you fall in love with someone and spend decades of your life together, are they literally a different person than who you first met? And how much of you will never be alive in the first place?
Let me explain that second question. Almost a third of your mass is made up of stuff that’s not alive. For instance, the fluid outside of our cellular membranes—things like the plasma in your blood, tears, mucus, lymph, spinal fluid, and even milk—makes up about 25% of your mass. Another 7% is hard material like the calcium in our bones and the minerals in our teeth. Then you’ve got non-living stuff like the dead protein keratin that makes up your hair and fingernails.
Remember all those trillions of red blood cells you’re constantly losing and making more of? They have no nucleus, no mitochondria, and they can’t make more of themselves. They don’t metabolize. Are they actually even alive? Does that mean all of us have trillions of non-living cells flowing through our veins keeping us alive?
That’s the kind of thought that can mess up your whole day. Your connective tissue—things like ligaments, tendons, and cartilage—is made of stretchy or spongy webs of protein and other molecules, again made by cells but not made of cells. Three-fourths of your connective tissue and all your joints and muscles is just this non-living molecular scaffolding.
All in all, a sizable fraction of what you probably consider “you”—the thinking, living being watching this video—is actually not alive. Of course, every person’s body is unique, and all kinds of things can influence your cellular turnover and the number of living cells that you possess. A healthy liver has around 240 billion cells, but a diseased liver could have as few as 172 billion. Is the person that that belongs to less alive than a healthy person or not?
Some cancer cells are cancer because they’ve lost the genetic software that told them when to die. Is a person with cancer more alive on a cellular level? The answer is less clear than you might have thought before watching this.
We can go even further. Much of the life that you carry within that living body is not you. The human body is home to about as many bacteria and other microbes as it is human cells. As you digest and process your way through life, the balance of life within you shifts back and forth.
We think of dead and alive as two sides of a coin, but it’s more like a spectrum—a gradient. It has beginnings and ends, but any point between them is a mix of both. Maintaining the right balance of dying and regeneration turns out to be essential to health and complex life itself.
In a way, we’re a lot like that Shinto temple, aren’t we? If your body had a philosophy, perhaps that would be it: nothing can truly last as it is. Life is a balance of death and regeneration.
Anyway, the next time someone asks, “Who are you?” you’ve got a much more interesting answer to give them. Stay curious!
Have you checked out “Out of Our Elements” yet? It’s a new PBS series that explores the molecular stories underpinning our everyday natural world. From the first molecule in the universe to the water we drink, “Out of Our Elements” shows that if you look closely enough, what may seem familiar can actually be kind of extraordinary. Check it out through the link in the description and be sure to tell them I sent you!
As always, we’d like to thank everyone who supports the show on Patreon. If you’d like to join them, we would really appreciate your support.
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This version removes any informal language, personal anecdotes, or potentially inappropriate content while maintaining the core ideas and concepts presented in the original transcript.
Cells – The basic structural and functional units of all living organisms. – In biology class, we learned that human bodies are made up of trillions of cells that perform various functions.
Change – A process through which something becomes different. – Philosophers often discuss how change is a constant part of life, affecting everything from ecosystems to personal beliefs.
Body – The physical structure of a living organism. – The human body is a complex system that includes organs, tissues, and cells working together to sustain life.
Renewal – The process of making something new, fresh, or strong again. – In biology, the renewal of skin cells is a continuous process that helps maintain healthy skin.
Life – The condition that distinguishes living organisms from non-living matter, characterized by growth, reproduction, and response to stimuli. – Scientists study the various forms of life on Earth to understand how different species adapt to their environments.
Death – The end of life; the cessation of all biological functions that sustain a living organism. – In philosophy, death is often explored as a natural part of the life cycle and a subject of existential inquiry.
Neurons – Specialized cells in the nervous system that transmit information through electrical and chemical signals. – Neurons in the brain communicate with each other to process information and control bodily functions.
Bacteria – Microscopic single-celled organisms that can be found in various environments, some of which can cause disease. – While some bacteria are harmful, others play essential roles in processes like digestion and nutrient cycling.
Health – The state of being free from illness or injury and maintaining physical and mental well-being. – Maintaining good health involves a balanced diet, regular exercise, and adequate rest.
Identity – The characteristics and qualities that define an individual or a group, often explored in philosophy as a concept of self. – In biology, genetic identity is determined by the unique sequence of DNA in an organism.