His name’s William; we call him Billy. That video is him on his birthday.

It’s a strange thing, because he’s barely begun to exist and yet he’s already about 1% of the way through the time he’ll exist for. He hasn’t even learned to speak yet, but he’s going to have to learn so much so quickly. It seems unfair. So I thought I’d write down a few things that he ought to know about the world.

Imagine that you’re holding hands with your mum, my wife. She’s standing to your left. To her left, she’s holding hands with her mother. To her left, her mother, and so on. You’d only have to walk about 350km, about the distance from London to Rochdale, before you’d bump into our common ancestor with chimpanzees. She would probably be hairy, squat, mainly quadripedal: very much like a chimp, in fact, although actually no more closely related to chimps than she is to you. You’d have been walking past things that were more like apes than humans for 100km or more. And at no point would there have been a break: Every daughter would have been the same species as her mother. The only reason that we think of “chimps” and “humans”, and for that matter “fish” and “fungi” and “beech trees”, as different groups is because all the ones in between are dead.

Humans are not separate from the rest of life. Every living thing is, literally, your (admittedly distant) cousin.

2. Every living thing, including you, evolved, and no one designed it.

Imagine you have a population of some creature. They’re little single-celled organisms, and they just eat the materials in the environment, and they reproduce. When they reproduce, their children are pretty much the same as their parents, but sometimes they have little, random changes. Maybe that change makes the child slower, or maybe it makes them unable to reproduce. But one organism gets lucky: When it reproduces, the little random change makes its children a bit better at gathering food, and so they can make more children than their rivals.

As the generations passed, the offspring of that lucky creature would take over the population, simply because they would have more children than anybody else’s children. That’s evolution; that’s, really, the whole thing. Mutation — those random changes — and natural selection, the term we give to the inevitable tendency of those creatures better suited to their environment having more children.

Of course, it gets more complicated. Over millions of generations, and millions of years, mutations and the simple working of natural selection might lead some of these creatures to specialise differently. Perhaps some stop eating the available materials and start eating other creatures. Maybe one develops a chemical which allows it to gather energy from the sun. Slowly you end up with millions of different species. But it is all the result of random chance, combined with non-random natural selection. Nothing is guiding any of it.

3. That means we’re not special.

Obviously, we are special. No other species that we know of is as clever as we are; certainly no other earthly species has built the things we have, learned the things about the universe that we have. But that’s just what happened. It’s not that life was destined to lead upwards from lowly bacteria to mighty humans. Intelligence isn’t what the universe was put here for. It just happened to be a good way for one particular species to survive and have children.

You might not know this, if you watch films which talk about the “next stage of human evolution” in which we all become psychic, or if you see those ascent-of-man diagrams going from hunched ape to proud-walking, spear-carrying human. But that wasn’t predestined. It just sort of happened. If we started it all again four billion years ago, something very different, and maybe not intelligent, would be the dominant species on the planet.

It also means that we’re not super-special rational beings. We’re animals with instincts and limited brains, like all the others. Nothing about human life can be understood properly without first understanding that we’re evolved. When all the arguments are happening about how much of modern life is about genetics and how much is about culture, it’s always worth remembering that the reason we have culture at all is because we evolved in such a way that we could.

4. You, and everything else, are made of atoms. And atoms are tiny.

The physicist Richard Feynman was once asked: What one sentence conveys the most scientific information the most efficiently? He said: “Everything is made of atoms.”

While it’s not — quite — true, exactly, it’s true enough. Technically, there are some important things in the universe that aren’t made of “atoms”. But the point is: Everything you see is constructed from tiny particles. To get an idea of how tiny, you could watch this video, but the takeaway point is, atoms are so small that if you took a grapefruit and blew all its atoms up to the size of a blueberry, the grapefruit would be the size of the Earth.

There’s no way you can imagine anything that tiny. But the point is that the universe is built on scales that we can’t possibly grasp with our imaginations.

My old physics teacher, Mr Moss, got out an ordinary globe in class once, a bit bigger than a football. This is the Earth, he said, and this — he held up a tennis ball — is the moon. How far away do you think they are? We all held the “moon” a metre or so away from the “Earth”. He shook his head and moved it to the corner of the room, maybe five metres away. And where do you think the sun would be, he said. We shrugged; the corner of the school field, he said, perhaps a quarter of a mile away. And the nearest star, Proxima Centauri? It wouldn’t fit on the planet, he said.

There is a man-made object, a space probe called Voyager 1, which has been travelling at around 60,000km/h, faster than almost anything else ever made by humans, for more than 37 years. It has just, finally, possibly, made it outside of the little bubble of space that we call our solar system. It won’t get near any other stars for 40,000 years, when it will pass quite close to a star called Gliese 445 — and that’s only because Gliese 445 is hurtling towards us four times as fast as Voyager is travelling. Humans will, probably, never reach the stars, for the simple reason that the stars are an incredible distance away. And these are all nearby stars, in our little bit of our little galaxy, a handful of light years away. The galaxy itself is 100 thousand light years across and contains 100 billion stars; the nearest other galaxy is Andromeda, about 2.5 million light-years away; there are at least 100 billion galaxies in the bit of the universe we can see, some of them 13 billion light years away.

Again: Your imagination is just not built to deal with these scales. You can run thought experiments like that one, or the inflated-atoms-of-a-grapefruit one, but you will never be able to truly imagine the scales the universe operates on, because you were designed by evolution to deal with medium-sized things moving at a medium pace. You deal in metres and kilometres, hours and seconds and years — the universe deals in femtoseconds and aeons, Planck lengths and parsecs. You will never grasp, with your human brain, what these things mean.

6. It’s also really old, and you — and all humans — are really new.

I’ll stop banging on about scale in a moment. But you, my beautiful boy, will live for perhaps a century. That’s hugely longer than the majority of your ancestors could hope to live, but it’s also a tiny fraction of how long humans have been around — depending on which kind of humans you mean, it’s perhaps one 2,000th.

But humans themselves have only been around for an eyeblink, as far as the Earth is concerned. The dinosaurs have been dead for 65 million years, or about 300 times as long as modern humans have existed. Multicellular life — the first animals and plants — arose about 550 million years ago. There is fossil evidence suggesting that the very first living things, tiny basic bacteria-like cells, appeared about 4 billion years ago, just a few hundred million years after the Earth came into being. The traditional metaphor is: If the history of life were compressed into a year, humans would come into existence about 25 minutes to midnight on 31 December. The pyramids would have been built a few seconds after 11:59pm. Billy: You were born about 8 milliseconds before midnight. Welcome to the world.

7. There’s no reason to think life goes on after death.

The universe is vast and ancient, and we are tiny and ephemeral. You can see why people would like to believe that after our firefly-flicker existences, we go on to new, eternal lives elsewhere.

And they may be right: You have to make up your own mind about that. But there’s no reason you have to believe it. There’s no suggestion that there is anything non-physical in your body: As mentioned, you’re made of atoms. As far as we know, it is how those atoms are arranged that makes you you, including the atoms in your brain. As far as we can tell, the mind is the brain; there’s nothing to suggest that, once the arrangement of atoms that makes up your brain is broken up, anything remains of your mind.

We can’t, of course, prove that’s true. But nothing we’ve found in the world so far suggests that it is. The only time you can reasonably expect is the four or five thousand weeks you get, if you’re lucky, on Earth. Make the most of them, and whether or not you end up believing in something afterwards, don’t rely on it.

As you go through life, you’ll hear variations on the phrase “science doesn’t know everything”. And it’s true: There are lots of things science doesn’t know. But you’ll hear people also say that because science isn’t complete, or perfect, it means that everything science thinks it knows may one day be overturned.

There’s a marvellous essay by the science fiction writer Isaac Asimov, called “The relativity of wrong“, which was written in response to someone who told him that, just as the wisest minds once thought the world was flat, so everything we think we know now could be overturned. Azimov’s reply was:

John, when people thought the earth was flat, they were wrong. When people thought the earth was spherical, they were wrong. But if you think that thinking the earth is spherical is just as wrong as thinking the earth is flat, then your view is wronger than both of them put together.

The view that the Earth is spherical was indeed wrong, and was overthrown. But it was overthrown by the discovery that it is an oblate spheroid, very, very gently bulging around the waist, about 44km thicker across than it is top to bottom, which isn’t much when you consider the Earth is about 12,000km through. “The Earth is a sphere” is wrong, but “the Earth is flat” is much, much wronger. And what revealed the “wrongness” of the sphere hypothesis was the scientific method, which is, broadly speaking, always honing in on less and less wrong answers, all the time.

If evolution, or relativity, or any of the great theories of science, are wrong, this is how they will be shown to be wrong: Not by being overthrown, but by being refined, made less wrong. But the basic discoveries, that living beings have evolved by natural selection or that time and space are two aspects of the same thing, will remain true. There are too many strands of evidence showing that to be the case, and it is only wishful thinking or ignorance that makes some people unable to accept that.

And 9…

Normally, these sort of pieces end with “I want you to know I’ll always love you”, or something. But I’m going to say that every day, in person, so you’ll know that by the time you’re old enough to talk. Instead, I want you to know that Fight Club is the best film; Mario Kart Wii is the best game; Faith No More are the best band; and I haven’t worked out what the best book is yet, but in the meantime, if you read lots of Iain M. Banks and Terry Pratchett you won’t go far wrong. Happy birthday, son. I’m glad you liked the big cardboard box.