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Here All Along

Thought Question: How old are you?

The concept of relativity can apply to more than just spacetime. Take your age, for instance. That’s the age of you being you, since you were born. But what about the things that make you, you?

Genetic material. Biologically speaking, half of your DNA came from your father and half came from your mother. The age of an average sperm cell is about 74 days, since it takes a

Sperm, Egg, DNA

sperm cell that long to mature. The other half of your DNA came from your mom’s egg cell, which was present inside your mom before she was born. In fact, all of the egg cells she would ever have in her body were present by about twenty weeks after her conception. So, if you want to say you are as old as your DNA, well, it quickly becomes complicated to answer. Half of your DNA is about four and a half months older than your mother, and the other half is about 74 days older than your conception.

Keeping with biology for a bit here, we know that DNA is passed from generation to

Human Evolution

generation, so we could ask, “How old is human DNA?” If we are focusing on our species, sapiens, then scientists currently believe the oldest fossils are around 300,000 years old. If you focus on the genus, Homo, then the oldest of these that we know of is 2.8 million years old. At this point, how old you thought you were before reading this article becomes negligible.

“Follow the biology,” you say. Ok, let’s keep going. Let’s now go back to the earliest lifeform, because your DNA had to come from somewhere, then change over geologic time scales (via evolution) until we get to modern humans. The idea that all life on Earth is related

Fossils, ammonites
Fossils of Ancient Animals

is scientific fact; we share about 60% of our DNA with bananas, for example. To get to the first living creatures, that takes us way, way back, like billions of years back. Current fossil evidence shows that somewhere around 3.8 billion years ago, the first living things emerged on Earth. All DNA we think came from those first living organisms, yours included. So biologically, you are about 3.8 billion years old.

I want to pause here to say something about deep time. Three point eight billion years is a long, long, long, long, long, long time ago. Here’s a simple mind exercise to help us deal with it. There are 25 years in a human generation. This means that in 1,000 years, there are 40 human generations. That means that back then, your great, great, great, great-repeat 35 more times-great grandparents were alive. Ok, whew, we survived the mind experiment for a thousand years, so what about a million?

Well, as you know, a million is a thousand thousand. That would mean…what exactly?

Thousands of people at a gathering

Logically, you could just say, well, repeat what you just did, but do it a thousand more times. Already, we see that our brain’s understanding of a million years is a stretch. We feel stretched thin, trying to comprehend our hominid ancestors, to hold it in our mental grasp.

Now some hotshot comes into town, and says, “You think that’s hard, try a billion, try doing all those things you just did a thousand more times.” You think about it. You answer the hotshot, “That would mean doing the 40 generations a thousand times, and then doing the 40 generations a thousand times, and then doing the 40 generations a thousand times, and repeat this 997 more times, your logical brain says. I’ve got this…,” you say, but do you? Can you? Can you wrap that up in your mind? Whatever your answer, I think you’ll have some respect for how unfathomably long a billion years is. Your hotshot retorts, interrupting your counting, “Good job, now do that 2.8 more times and you’ve got the age of the oldest lifeform on Earth!” Ouch.

So much for biology, what about chemistry and physics? What about the atoms that make up your molecules that make you, you? How old are they? Well that’s a tricky one to answer. Now we need to delve into astrophysics and cosmology. In astrophysics, we talk about the lives of stars, how they are born and die. When stars live, they are atom-creation machines, fusing elements together in the fantastically high temperatures and pressures in their core. The larger stars do this all the way up to iron (element number 26 on the periodic table). When they die, these large stars explode in supernovas, scattering the elements made in their core. These explosions are so incredibly ginormous, they fuse heavier elements than iron, and scatter those as well. Recently, we have discovered that neutron star collisions also generate and scatter enormous amounts of heavier elements to the surrounding galaxy. The human body is mostly made of elements lighter than iron, like hydrogen, carbon, nitrogen, and oxygen, but we also need iron and many heavier elements, including cobalt, copper, zinc, molybdenum and Iodine. What this means is, we wouldn’t be here if stars didn’t live and die, scattering the elements, which are later gathered by gravity forming planets like the Earth.

Supernova, exploding star, releasing matter to space
Exploding star, releasing matter to space

According to the evidence we have, our solar system formed about 4.6 billion years ago. In order for this to happen and Earth to be among the planets that formed, all the rocks, ice, and dust that formed the Sun and the Earth (and all the other planets) must have existed prior to that, from stars that exploded long before our solar system formed. Cosmologists estimate that our Sun, because it contains a certain percentage of heavy elements, as does the Earth, is at least a third-generation star. This means multiple stars have lived and died before our Sun was even born. Since our Sun is about 4.6 billion years old, the matter that makes up our star and all of the planets is much older than that, probably by billions of years.

Ok, at this point we really can not be sure of how old our particular atoms are, but we have established that most are older than 4.6 billion years. I say most here because (and now is a good time to skip this sentence if you don’t want to listen to technical jargon) there are many radioisotopes on Earth, which, upon decaying, produce lighter elements. This is why we must ask the final question: where did all of the elements come from originally?

Big Bang, expansion of space

In the end, we must turn to cosmology. In the beginning, after the Big Bang, when temperatures and densities decreased enough for atoms to form, there were just two elements, hydrogen and helium. From these two elements and the fusion processes in stars we discussed earlier, all the rest of the visible matter in the universe was formed. Therefore, since the current scientific consensus about the age of the universe is about 13.8 billion years, that makes everything in it also that old. In your body, whether your carbon, oxygen, and nitrogen atoms formed five or seven billion years ago in a solar system far, far away originally, even they were formed from hydrogen or helium atoms that came into existence soon after the beginning of the universe, 13.8 billion years ago. So, how old are you? It seems you were here all along.


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