“We tend to think of our bodies changing only slowly once we reach adulthood. In fact, we're changing all the time and constantly rebuilding ourselves …
“Very little of our physical bodies lasts for more than a few years. Of course, that's at odds with how we perceive ourselves when we look into the mirror. But we're not fixed at all. We're more like a pattern or a process. And it was the transience of the body and the flow of energy and matter needed to counter that impermanence that led us to explore our interconnectedness with the universe.”
– Iris Schrijver, professor of pathology at Stanford University and author of Living With the Stars: How the Human Body Is Connected to the Life Cycles of the Earth, the Planets, and the Stars
Did you know a common belief is that every 7-10 years, every cell in your body has been replaced by a new cell since cells are always dying and being replaced all the time? It may be an inspiring story to know that you essentially become a “new you” and not the same person you were years ago; however, it's not quite that simple.
It's true. Individual cells – about 37 trillion of them and around 200 different types – have a finite life span, and when they die off, they are replaced with new cells.
According to carbon dating, the average age of a cell is seven years… but that doesn’t mean that every cell is replaced in seven years. Some cells, in fact, never get replaced at all, remaining with us from birth until death. These include many of the neurons in our cerebellum, and the cells that make up the lenses in our eyes.
(Benjamin Radford. “Does the Human Body Really Replace Itself Every 7 Years?” Live Science. April 04, 2011.)
There's nothing special or significant about a seven-year cycle, since cells are dying and being replaced all the time. It's not clear where this myth began; perhaps some well-meaning but innumerate person simply added up the all the lifespans of the body's various types of cells and (mistakenly) assumed that all the cells are renewed after seven years.
The conclusion? Although almost all the cells in your body die and are consistently replaced, their life cycles vary from different organs, types, and functions. Parts of your body are as old as you are and will exist to as old as you’ll ever be.
A more nuanced claim would be that most of the body changes every 10-15 years, while some parts of the body remain the same, dying only with the individual.
Can you imagine if some cells didn't? Take those brain cells, for example. If they left you, would you lose your memory in the cell replacement process? How about your human identity? Would you even be the same person as you were seven years ago? What about the person in the future… would that also be you?
No need to worry. The brain where your memories, ideas, beliefs, personality and every other detail of you exist remains the same throughout your life. Those neurons in your cerebral cortex – the brain's outside layer – stay with you from birth to death.
But, sadly, because they aren't replaced, the loss of these cells over time can cause maladies like dementia. The good news is that other areas of the brain, like the olfactory bulb that helps us smell and the hippocampus that helps us learn, can and do rejuvenate.
(Chris Opfer. “Does your body really replace itself every seven years?” How Stuff Works. 2021.)
Still, some of this is confusing. For example, an average skin cell only lasts about 14 days before dying, so why don't scars magically disappear?
It turns out scars are not skin. What we think of as scars are usually made of collagen, a stringy protein that makes up much of our structure. About 25% of the total protein in the human body is made of collagen.
Our skin is naturally full of collagen; it’s woven in a linked, somewhat random fashion. It helps keep our skin plump and taut, acting as a glue to hold our skin together. And, when we get injured, our body rushes to put a patch over the wound. It uses collagen to create this patch – but instead of making a nice, woven blanket of collagen, with fibers going in all different directions, it cross-links the collagen fibers so they mostly all point in the same direction.
Because it’s dense, it remains inert, and skin cells grow around it. Even as these skin cells die and are replaced, the scar itself remains. Over time, however, some of the collagen will be re-absorbed, and the cells grow closer to the remaining collagen. This is why scars fade and diminish over time, although they’ll never fully vanish.
How about tattoos? Tattoos are not scars, but they are another item that doesn’t fade from shedding skin – in their case, they’re injected beneath the dermis, the layer of the skin that sheds. That creates a wound, which your body tries to heal by sending macrophages (a type of white blood cell) to the area.
The macrophages swallow the ink in an effort to take the foreign particle away from the wound. But, the pigment in the ink is too large for the macrophages to remove, so the macrophages – with the ink inside – end up stuck in the dermis.
(Sam Westreich, PhD. “Science Monday: If Our Cells Replace Themselves, Why Do We Have Lifelong Scars?” Medium. January 05, 2020.)
Universal Connection
“And yes, every one of our body’s atoms is traceable to the big bang and to the thermonuclear furnaces within high-mass stars that exploded more than five billion years ago.
“We are stardust brought to life, then empowered by the universe to figure itself out – and we have only just begun.”
– Neil deGrasse Tyson, Astrophysics for People in a Hurry
Skin, brain … even scars and tattoos … “all of you is the universe, expressing itself as human for a little while” as spiritual teacher and best-selling author Eckhart Tolle says. He seems to have it right.
Scientists tell us that everything we are and everything in the universe – and on Earth – originated from stardust, and it continually floats through us even today. It directly connects us to the universe, rebuilding our bodies over and again over our lifetimes.
Did you realize how impermanent you are, and that your body is made of remnants of stars and massive explosions in the galaxies. Astrophysicist Karel Schrijver, a senior fellow at the Lockheed Martin Solar and Astrophysics Laboratory, says, “All the material in our bodies originates with that residual stardust, and it finds its way into plants, and from there into the nutrients that we need for everything we do – think, move, grow. And every few years the bulk of our bodies are newly created.”
(Simon Worrall. “How 40,000 Tons of Cosmic Dust Falling to Earth Affects You and Me.” National Geographic. January 28, 2015.)
So, most of the material of which you're made of comes out of dying stars, or stars that died in explosions. And those stellar explosions continue. In us, we have stuff as old as the universe, and then some stuff that landed here maybe only a hundred years ago. And all of that mixes in our bodies.
Our bodies are never static. We're dynamic beings, and we have to be dynamic to remain alive. This is not just true for us humans. It's true for all living things.
Karel says …
“A figure that jumped out at me is that 40,000 tons of cosmic dust fall on Earth every year. From what you wouldn't even see unless you had a microscope, to things that weigh many tons, like comets – it's all being pulled around by the gravity of the planets and the sun. The Earth can't avoid running into this debris, so that dust falls onto the Earth all the time and has from the very beginning. Eventually all that stuff, which contains oxygen and carbon, iron, nickel, and all the other elements, finds its way into our bodies.”
(Simon Worrall. “How 40,000 Tons of Cosmic Dust Falling to Earth Affects You and Me.” National Geographic. January 28, 2015.)
We're literally not what were a few years ago, and not just because of the way we think. Everything around us does this. Nature is not outside us. We are nature. And, the “new you” is universal.
“You and I are made of stardust. We are the stuff of exploded stars. We are therefore, at least one way that the Universe knows itself. That, to me, is astonishing.”
– Bill Nye, American mechanical engineer, science communicator, and television presenter
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