“Happy Birthday to you, happy birthday to you. Happy birthday dear…”
Jessica. She just turned 23 – well, so she thinks. Her birth certificate gives her age
on paper, but physically her body parts are all different ages. Yours too. For example,
the outer layer of your skin regenerates itself every two weeks. Even urban legend says the
human body replaces itself every 7 years.
So… how old are you, really?
To work this out we need to travel back in time. It’s 1945 and the first atomic bomb
is detonated at a test site in New Mexico.
In the blast, something called Carbon-14 is emitted. Remember that the regular Carbon
atom has 6 protons and 6 neutrons. It's sometimes called Carbon 12. Carbon-14 has 6 protons
and 8 neutrons–it kind of takes two neutrons from nitrogen when they slam together in the
explosion.
It’s a small difference, but enough that scientists can measure.
Carbon-14 occurs naturally in trace amounts, but nuclear weapons testing until 1963 meant
there was a lot more in the atmosphere. Scientists measure the amount of 14C in the atmosphere
every year and there’s still some hanging around from the 50s and 60s.
Okay, you may be asking yourself what 14C has to do with you. Well, it tends to hang
around in the air, it dissolves into oceans and ends up in plants and our food. Somehow
it will make its way inside of you.
But carbon-14 isn’t harmful to us, so our body starts to use it just like it uses regular
carbon atoms. Our bodies sub in 14C to build our cells, proteins and even our DNA.
Because the amount of 14C in the atmosphere changes every year, scientists discovered
how to date your organs by looking at the amount of 14C in them. It’s carbon dating,
the same method scientists used to figure out the age of the Dead Sea Scrolls and when
the woolly mammoth became extinct. To date humans, it was labelled bomb-pulse dating.
And it turns out not many of your body parts are actually as old as you. Your colon cells
are four days young and remember that outer layer of your skin? It’s about two weeks
old.
By using bomb-pulse dating, scientists in Sweden worked out your fat cells have a lifespan
of 10 years. And many of your brain cells – called neurons – regenerate too.
One study found that in a 90 year old brain, the neurons in the hippocampus, the memory
centre, were only 20-30 years old.
And this is the real discovery of bomb-pulse dating. We used to think the neurons you were
born with were the ones you died with. In certain areas of your brain they have the
ability to regenerate.
There is one part of your body that’s been with you since birth – your cortical neurons.
They make up your cerebral cortex, this outer layer of your brain. It’s responsible for
your higher functions, like thought, consciousness, language and your perception of the world
around you.
The reason we know your cortical neurons have been with you your whole live is because of
18 years of atomic bomb testing. Wherever you are in the world and however old you are,
you have little pieces of that world history living inside of you.
Carbon-14 from bomb pulses is actually part of your DNA.
So when you celebrate your next trip around the sun, wish your cortical neurons a big
happy birthday. They’re one part of you that’s really as old as you claim to be.