This story starts with Where Our Atoms Came From (Part 1).

Once they found their way to the Earth, your atoms began to cycle through the air, land, and water.

At this point, it’s hard to track exactly where your atoms went but we do know that they have been very well recycled.

The average person is made of seven octillion atoms, which is a number so great that it is almost impossible to conceptualise. Seven octillion is a seven followed by twenty-five zero’s. If you had seven octillion standard sized bricks, they’d fill the volume of the planet Jupiter… four times over.

With so many atoms scattered wide across the planet, the story of your atoms is also the story of the Earth. They have formed part of the magma ocean underneath the continents to the peaks of mountains, and helped make the bodies of millions of creatures before they made you.

A handful of your atoms may have been used in one of the very first molecules of RNA at the birth of life on Earth.

Others may have been used in one of the mandibles of a five-eyed Opabinia regalis, a tiny three-inch predator during the Cambrian Explosion.

Others may have been part of the muscle tissue of both a Tyrannosaurus and its prey.

In the chaos of Earth’s ever-changing chemistry, your seven octillion atoms have truly been everywhere.

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 — up to a billion for each of us, it has been suggested — 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... When we die, our atoms will disassemble and move off to find new uses else-where — as part of a leaf or other human being or drop of dew.

Bill Bryson, A Short History of Nearly Everything

After 4 billion years, your atoms came together through the food you ate and the air you breathed to create your hands, heart, and the eyeballs you’re using to read these words.

Your carbon atoms were very recently part of a plant, and considering our modern diet, it was probably corn or wheat.

The plant absorbed carbon dioxide molecules floating in the air and using the Sun’s light as a catalyst, the green cells of the plant combined them into a long carbohydrate molecule. You recently ate it as part of bread, corn starch, or sugar.

The last glass of water you drank has also gone through a monumental journey. It’s part of a constant cycle of rain and evaporation driven by the heat of the Sun, while occasionally getting diverted through the belly of an animal or a plant.

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 atoms in your saliva might have once been a wave that pushed Christopher Columbus’ Santa Maria across the Atlantic, or an avalanche that toppled Hannibal’s elephants down the Alps. Your distant ancestors may be partially made of the iceberg that sank the Titanic.

But the atoms that we have now are not the atoms that we’ll keep. In fact, they are 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, our bodies 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 — 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.

Steve Grand, Creation: Life and How to Make It

The atoms that comprise your body don’t belong to you – they are nomads, and they’re just staying with you for a little while.

After Earth

After another five billion years of cycling around the Earth, all atoms on our planet will be scorched by the Sun as it expands into the final stage of its life, a red giant.

The Sun’s outer layers will expand 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, quite simply, completely cooked.

Eventually like the ancestor stars that preceded it, the Sun will explode, returning new atoms to cold, dark clouds in space. Then the star cycle begins anew.

Hundreds of new stars will form, and your atoms will be split amongst a new set of planets, moons, and maybe new forms of life.

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 the remaining hydrogen and helium, and the galaxies will go dark.

The end of the road?

What will follow is an era of black holes. All matter, including your old atoms, will either be consumed by them or flung into deep space by their gravity.

After countless ages, where the time scales are so long that the eras of stars and planets that preceded it begin to look like a momentary blip, even the black holes disappear by evaporating into nothing but radiation.

This is the end of what scientists know will happen for sure.

Or will it all begin again?

But there is much speculation about what happens next.

One possibility is that, after any surviving atoms and radiation have spent an eternity travelling through the cold, dark remnants of the universe, they will decay into the quarks that they consist of.

These particles will fill the universe in a ‘thermal equilibrium’, where every place in the universe is barely above a temperature of absolute zero, and no further exchange of energy becomes possible.

This means no stars, life, or intelligence will be possible. It may remain in this state for all of eternity. This is called the heat death of the universe.

A second, more hopeful possibility is that the expansion of the universe itself slows, and is gradually reversed by the pull of its own gravity.

After hundreds of billions of years, every atom and flash of radiation are brought back together until they rush to collapse into a single point. This is a reversal of the Big Bang, called the ‘Big Crunch’.

Many scientists believe that the Big Crunch will be the end of our universe, but others think it may be followed by something spectacular.

The Big Crunch may trigger a new Big Bang, one that creates a whole new universe, which could one day have new stars, new planets, and new life – or perhaps something entirely different.

In this distant universe, the energy that once made your atoms may be part of some wonderful form that’s beyond our imagination.

Or perhaps something that’s a little familiar. It may form the light of a distant star that touches the retina of a creature who wonders about the universe, and where their own atoms came from.