This story is about 8 minutes long.

The atoms in your body were not always yours. Right now it’s more like you’re borrowing them.

Most of them are 13.7 billion years old, made shortly after the big bang, and being you is just a single page in their long and incredible story.

If we understood how this happens, we’d be able to see how interlinked we are with the rest of the universe.

To do this, we have to look into a field called ‘astrochemistry’, which is the study of molecules in the universe. It tells that that your atoms have not all come from the same place. This isn’t surprising, as the average 70kg person is made of about seven octillion atoms (7 x 1027).

We know that about two-thirds of them are hydrogen atoms, and we can trace their history. They were made just after the Big Bang and floated in vast, ghostly clouds in interstellar space for billions of years.

In the complete silence of space, they would have been witness to one of the most sublime visions in the universe: the formation of the Milky Way through a veil of a nebula.

The rest of your atoms were here too, but they ended up having a somewhat different experience. They found themselves being pulled into a gravity well of a giant star during its formation, like being caught in a colossal rip tide.

While the star formed around them, they would have felt the temperature and pressure creep upwards increase over a tens of millions of years, until it surpassed 200 million degrees Fahrenheit.

Initially they would have been in darkness, then a dull glow as the star began to ignite, then a light brighter than anything we could imagine coming from every direction.

Your atoms spent billions of years in this pressure cooker, being pulled in the ebb and flow of the internal storms of the star. During this time, some of them fell deep into the star’s core. Here they were subject to pressure like nowhere else in the universe, and atoms of hydrogen fused together to become atoms of helium, releasing photons in a blast of light.

Every photon of light from the Sun and every star in the night sky originated from this process.

Eventually, the star would run out of fuel and their light began to dim. Pressure built as the star, starved of fuel, looked for other atoms to fuse until it had none left. After billions of years, the star went out in a supernova explosion, a blast with so much force as to be visible across a galaxy.

The searing explosion fused other atoms, creating atoms like oxygen and carbon. The shockwave pushed them back into the hydrogen gas cloud.

But after billions of years of rest, the explosion had disrupted the cloud and seeded it with the new atoms. As its shockwave impacted surrounding gas, it compressed a bunch of hydrogen and oxygen together to form icy water.

The gas, ice, and new rocky elements began to clump together and grew larger and larger.

From the dust cloud, thousands of new, smaller stars were forming. One was our Sun.

||||Some of the gas cloud was pulled into the gravity well of the Sun, destined to be pulled in and repeat the earlier experience. But some of the gas and asteroids found themselves not being pulled in, but to orbit the star instead.

‘Accretion disc’

Over time they collided, forming larger and larger asteroids in a series of impacts until they became the size of planets, and the asteroid bombardment continued for hundreds of millions of years.

As it formed, the Sun released a series of shockwaves that impacted the new planets and determined the shape of our solar system. They pushed most of the gas outwards where it formed the gas giants Jupiter, Saturn, Neptune, and Uranus.

The heavy, rocky material closest to the sun was left behind by the shockwave, and it formed the small, rocky planets Mercury, Venus, Earth, and Mars with thin atmospheres.

Eventually the shockwaves and asteroid bombardments slowed down, and all of the atoms that would eventually form you found themselves in one place; Earth.

Here on Earth, they began to cycle through the air, land, and water.

While they may temporarily come together to form molecules, the atoms themselves practically never degrade.

have been the same since the early stars fused them.

||Move down?||Some of your atoms might have been used by the very first life, perhaps as part of their DNA or their cell walls, now long dead and their atoms returned to the great cycles of the Earth. Perhaps some of the oxygen in your blood was last coursing through the veins of a Tyrannosaurus, or an ancient warlord. In the chaos of Earth’s ever changing chemistry, it’s hard to tell.

One thing we know for sure though is that your carbon atoms were once, not too long ago, part of a plant.

Originally, the atoms of carbon from which we’re made were floating in the air, part of a carbon dioxide molecule. The only way to recruit these carbon atoms for the molecules necessary to support lifeβ€”the carbohydrates, amino acids, proteins, and lipidsβ€”is by means of photosynthesis. Using sunlight as a catalyst the green cells of plants combine carbon atoms taken from the air with water and elements drawn from the soil to form the simple organic compounds that stand at the base of every food chain. It is more than a figure of speech to say that plants create life out of thin air.

Michael PollanThe Omnivore's Dilemma

Water goes through a similar process. It’s in a constant cycle of rain and evaporation driven by the heat of the sun, while occasionally getting drunk by a plant or animal. You might have seen a diagram of the water cycle like the one below in school, but what it doesn’t make clear is the scale of this process. Water is exchanged across the entire surface of the Earth, for billions of years at a time.

The water cycle. Image credit: NASA

Bill Bryson writes one of my favourite passages in any book in A Short History of Nearly Everything:

Every atom you possess has almost certainly passed through several stars and been part of millions of organisms on its way to becoming you. We are each so atomically numerous and so vigorously recycled at death that a significant number of our atoms β€” up to a billion for each of us, it has been suggested β€” probably once belonged to Shakespeare. A billion more each came from Buddha and Genghis Khan and Beethoven, and any other historical figure you care to name. (The personages have to be historical, apparently, as it takes the atoms some decades to become thoroughly redistributed; however much you may wish it, you are not yet one with Elvis Presley.) So we are all reincarnations β€” though short-lived ones. When we die, our atoms will disassemble and move off to find new uses else-where β€” as part of a leaf or other human being or drop of dew. Atoms themselves, however, go on practically for ever.

Bill BrysonA Short History of Nearly Everything

As far as our bodies go, our atoms are completely replaced about once every ten years.

Steve Grand in his book Creation: Life and How to Make It points out that because our atoms are in constant flux, we are more like a wave than a permanent thing. He invites us to do a quick thought experiment:

Think of an experience from your childhood β€” something you remember clearly, something you can see, feel, maybe even smell, as if you were really there.

After all, you really were there at the time, weren’t you? How else would you remember it?

But here’s the bombshell: You weren’t there.

Not a single atom that is in your body today was there when that event took place.

Matter flows from place to place and momentarily comes together to be you. Whatever you are, therefore, you are not the stuff of which you are made.

If that doesn’t make the hair stand up on the back of your neck, read it again until it does.

To put it another way, the atoms that comprise your mind and body don’t belong to you; you are just renting them for a few years.

After another 5 billion years, the Sun will enter the final stage of its life cycle when it will expand into a red giant.Β The Sun’s outer layers will grow until they engulf Mercury, Venus, and finally the Earth. Any life that has not found a way to leave the Earth by this point will be, in a word, cooked.

The Sun will swell into a red giant, scorching then engulfing the Earth. Image credit: James Gitlin

Eventually, like the star that preceded it, the Sun will explode into a nebula, returning atoms to a cold, dark cloud in space. At this point, the star cycle begins anew. Thousands of new stars will form, each with a new set of planets, moons, and maybe new forms of life.

The Sun will explode into a nebula, like countless stars before it

Cosmologists believe that this cycle of death and rebirth of the stars will repeat about one hundred times, before the final star in the universe exhausts all fuel and the galaxies go dark.

What will follow is an era, longer than anything that we have seen so far, of black holes. All matter including your old atoms will either be consumed by them or flung into deep space from their gravity. After countless ages even the black holes disappear, evaporating into Hawking radiation.

The view from a large asteroid in orbit around a black hole

This is the end of the line of what scientists know what will happen for sure. There are a couple of potential scenarios for what happens next.

One possibility is that, after the remaining matter and radiation has spent an eternity (1034 years, a number so large that it scarcely makes any sense) travelling through the remnants of the universe, it will decay into more basic particles. These particles will fill the universe in a ‘thermal equilibrium’, where every place in the universe is almost as cold as absolute zero, and no further exertion of energy becomes possible. No stars, or life will be possible. This is called the heat death of the universe.

A more hopeful possibility is that the expansion of the universe slows, and starts to reverse. After billions of years brings all of its matter back together into a single point. This is a reversal of the Big Bang, called the ‘Big Crunch’.

Many scientists believe that while the Big Crunch will be the end of our universe, it may be followed by something spectacular; a new Big Bang that creates a whole new universe. If this is so, then our own universe may be just one in an endless series of oscillating universes that go on forever.

“The very dust that blows along the street

Once whispered to its love that life is sweet.”

Hallam Hawksworth
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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.