How Old Are You?

How Old Are You?

Time is Man-Made

Time is a construct, helping us describe and synchronise our lives. Yet our calendars are hardly in sync with nature. A solar year is an awkward 365.25 days, which we account for with leap years. And a solar day isn't exactly 24 hours either; on average, it's more like 23 hours and 56 minutes, varying depending on the time of year.

During the Carboniferous period, some 350 million years ago, Earth's faster spin meant a solar day was less than 23 hours. Our orbit wasn't any larger, but the shorter day-night cycle extended the year to 385 days.

Even in the everyday world, our concept of time is arbitrary. The ancient Sumerians invented the 24-hour day around 2000 BCE, drawn from their systems of base-6 and base-10 numbering. They could just have easily divided the day into 10 hours of 144 minutes.

Even the year 2021 is a token number, coming from the Anno Domini dating system devised by Dionysus Exiguus in the year 525. Without actually explaining how he came to his conclusion, Dionysus declared 525 years had passed since the incarnation of Jesus. Before that, the Romans simply identified the years by who was in office.

The Human Era calendar, devised in 1993, attempts to correct this by adding precisely 10,000 years to our collective count. That makes it 12021 this year.

The calendar shifts the focus from Christianity to the Holocene, marking the (once again, approximate) transition from the nomadic hunter-gatherer lifestyle to agriculture and fixed settlements.

With time such a subjective mess, let's look at the other side of this equation: identity. To know how old you are, we must settle on a firm definition of you. And this turns out to be very difficult indeed.

The Identity Paradox

What are you? Perhaps you're the sum of your physical body. And yet your building blocks are subject to endless molecular change.

Here's one example: you are what you eat. The toast you had for breakfast wasn't part of your body before—but it is now. The carbohydrates were integrated into your cells and converted to energy, or stored as glycogen for later use. Now you're you plus a piece of toast.

Or perhaps you're the sum of your thoughts, memories, and experiences—what some describe as the self. But the self is also subject to continuous change. Any sense of consistency is also an illusion.

"No man ever steps in the same river twice, for it's not the same river and he's not the same man." - Heraclitus

Elaborating on the philosophical puzzle posed by Heraclitus, Plutarch once described how the ancient Ship of Theseus decayed over time. It prompted the Athenians to replace the vessel plank-by-plank until, after thirty years, it comprised none of the original wood.

This created a paradox around the ship's identity. The craft was both qualitatively and numerically different—arguably not the same ship on which the mythical king had sailed. Yet there was no definitive point in time when it stopped being the Ship of Theseus.

Illustration of the philosophical thought experiment on the Ship of Theseus

The Ship of Theseus is a thought experiment highlighting the paradox of identity.

The same problem occurs with personal identity. Every thirty seconds, a new plank is laid down, until we look back ten years and realise we're made of all new planks. But when did we become different?

If we're changing continually, then our sense of identity evolves along a curve, a series of incremental deviations such that any continuity of self is only an illusion.

Working Memory and The Self

Hang on, where did we get thirty seconds from? This argument comes down to the span of working memory.

Your brain has the capacity to store incoming sensory information only briefly in short term memory, allowing you to follow a conversation or type out a phone number. After around 30 seconds, sensory input fades from awareness, sometimes persisting in parallel in long term memory. It's a bit like RAM, if you prefer your analogies digital.

Sensory information is organised into different types of memories, persisting in working memory for around 30 seconds

Sensory information is organised into different types of memories, persisting in working memory for around 30 seconds.

This 30-second awareness may be the narrowest band of time we can attribute to our psychological sense of self, before it's scattered over numerous emotional and cognitive dimensions in transition.

We may feel an ongoing sense of self lasting for days, months, and years, but this feeling is a cognitive illusion, simulated by the brain to make sense of reality.

Our own experience highlights the paradox. There's no perceptible change at the stroke of midnight when we turn 18 and are catapulted into adulthood. Yet over some hazy transitional period, we progress through various physical and mental state changes such that we no longer identify as children.

Similarly, profound life experiences ripple through us over time. It takes a few seconds to learn of a breakup or loved one's demise, but months or years to work through the grief that ultimately changes us as people.

Identity is what we see as important amid this continual state of flux. It isn't a series of boxes we check off from one day to the next, providing definitive borders to the self. It's a series of overlapping micro and macro experiences, integrated consciously and unconsciously to give rise to who we are right this moment.

Instantaneous Quantum Biology

So, in a psychological sense, you're thirty seconds old now? Great. What about in a physical sense? How fast does our biology change?

I already mentioned the molecular conversion of food into energy, a process that occurs inside our cells: the miniature biological factories that make up all tissues including the brain, heart, skin, and muscles.

Interestingly, different cells are destroyed and replaced at varying rates, but if we average them out, cells are actually 7-10 years old. There are a few cell types that persist our whole lives, but for the most part it's a Ship of Theseus situation.

Different cells types have vastly different lifespans, ranging from days to decades

Different cells types have vastly different lifespans, ranging from days to decades.

While sperm cells mature in the testes over a couple of months, they have a finite lifespan once they leaves the body. So if you identify as your sperm, you have a measly lifespan of three days. Not that anyone identifies as their sperm. I'm just saying.

But let's dive deeper. Cells also change in their qualitative and quantitative nature, building and destroying the biological molecules that sustain your existence. Consider the hormone insulin, which is very much part of you biologically. Insulin is produced and destroyed in the space of just 5-6 minutes.

Or how about the energy molecule ATP? You remember ATP. You learned about it in biology and hated it instantly because it meant memorising the godforsaken electron transport chain.

Assuming you didn't repress the memory, you'll remember that ATP is critical for storing and transferring energy in cells. It carries a string of three phosphates which release energy when cut loose. So far, so good.

But now we must delve from biochemistry into quantum physics; something that gives me heart palpitations and I'm not sure if that's a good thing.

The nucleus of each phosphorus atom has a quantum property called spin. And interactions between quantum spins cause the phosphates to become entangled. This is the phenomenon where particles, devoid of any connection, can influence each other across space.

Quantum entanglement in the human body: superposition between phosphates in ATP

Quantum entanglement occurs in the human body between phosphates cleaved from ATP.

Quantum entanglement is a real devil. Einstein didn't like it; he pointed to it as an example of just how little we know quantum mechanics. Yet here we are today, getting on for a century later, using applied quantum entanglement in computers. The devil is real, even if his existence still makes no sense.

The thing that bothered Einstein is that particles can remain entangled even once they're separated by vast distances. Somehow, they remain invisibly connected, allowing them to signal information to each other. For desperate want of better terminology, Einstein called it "spooky action at a distance."

Quantum entanglement allows distant particles to communicate with each other faster than the speed of light

Quantum entanglement allows distant particles to communicate with each other faster than the speed of light.

It's happening right now inside your body. Entangled phosphates are broken apart physically by cell enzymes, yet keep on chatting to each other as they travel through your brain. They can remain in this state of quantum superposition for a day or more. And when the superposition collapses, it happens instantaneously.

Say that again. Instantaneously. That's how fast quantum changes occur in your cells—how fast your body changes on the smallest physical scale.

And what do we mean by an instant? Experiments on entangled photon pairs suggest the collapse occurs at one million times the speed of light.

I told you Einstein hated it.

Generations and Genes

I'm not sure I can handle being a speck of an instant old. I may as well not exist at all, which really conflicts with my sense of being and everything. The good news is we can turn this argument around and run away completely in the other direction. It's all about context.

Earlier we saw that ova can live for 50 years. Technically, ova are the mature, fertilised eggs that mark the beginning of pregnancy. In their immature state, egg cells are called oocytes.

Most of your mother's oocytes were made when she herself was in the womb. By the time she was born, she carried 7 million of them. They died off rapidly within her ovaries so that by the time she reached puberty, only 700,000 remained.

And of course, one of those eggs produced you.

So how old are you? In some sense, you're as old as your mother. You already existed, in part, when she herself was a foetus, gestating away inside your grandmother.

Female foetus carrying millions of oocytes (immature eggs)

Females produce oocytes during foetal development, meaning you once lived inside your grandmother.

This yolky leap of logic means that you're at least as old as your mother.

But if we're reducing our origins to egg cells and Russian Doll biology, why not go the whole hog and simply identify as our DNA? How old are you then?

Your genetic code has been passed down by a long and unbroken line of ancestors. Current estimates propose there are around 30 nucleotide mutations in every gamete (sperm or egg cell). That's uniquely yours—all original work. The remainder of your DNA is a merger of 6 billion nucleotides coming from your parents, which in turn came from their ancestors.

How far back does your DNA inheritance go? Consider that modern Homo sapiens emerged around 160,000 years ago. Or that archaic Homo sapiens evolved around 300,000 years ago.

Do we just stop there? Hell, no.

Our immediate hominin ancestor didn't give birth to a Homo sapiens baby and produce a new species in a single day. The evolution of species is yet another Ship of Theseus story.

Evolutionary trees don't reveal this subtlety because they're simplifications of massive intergenerational complexity. In reality, the forks are blurry, multi-pronged, and subject to considerable overlap.

Timeline of human evolution

Homo sapiens is the only extant (living) species of human.

Genetic analysis between humans and chimpanzees suggests there were long periods of hybridisation between our common ancestors around five million years ago. This created various intermediate species of australopithecines, which gave rise to numerous species of humans. There's really no definitive point in time we can say humans began.

In this sense, our DNA isn't bordered by a single generation or a single species. It flows on and on, all the way back to the origin of mammals, which arose from chordates (loosely, vertebrates), which arose from the broader yet animal kingdom. Do we dare go back further?

Illustration of the evolution of humans from bacteria

Your ancestry flows uninterrupted for billions of years to the origin of life itself.

The oldest human gene isn't a human gene at all. It originated in ancient bacteria, handed down through countless generations to reside in our genome today.

This remarkable gene—called glutamate-ammonia ligase (GLUL)—has remained functionally intact for more than two billion years. It's a clean biological link from bacteria to humans, and exists in every living organism today.

And the GLUL gene is just one of many, passed from bacterium to bacterium, which trace back to the beginning of life on Earth.

How old are you? If you identify as your genetical material, you're at least 3.5 billion years old, born in a sweaty hot spring we now like to call the primordial soup. Ah, the good old days.

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Becky Casale Author Bio

Becky Casale is the founder, keyboard smasher, and drinks lady at Science Me. If you like her content, please take a hot second to share it with your favourite people. If you don't like it, why not punish your enemies by sharing it with them?