These are my parents, they're clearly humans too.

My grandfather, human.

My great-grandfather and great-great-grandfather?

Humans without a doubt.

But my grandfather 185 million generations removed?

Not a human.

He was a fish.

So imagine you could take a picture of every ancestor down the line and put them in a big stack.

Every father's father's father's father's [etc.] father.

And well, that would be a very big stack, with me at the top and my fishy forefather all the way down at the bottom.

Maybe we should lay this stack down on its side, that would be a little safer.

As we journey back in this stack, let's pull out a few snapshots from history.

1,000 generations back we're just a few inches in, and we find a human.

10,000 generations ago, just two steps further and well, still human but not like we'd recognize.

75,000 generations ago, that's a million and a half years back, not a human.

That's Homo erectus.

Now here we are just a few hundred steps back in our journey, a million and a half generations ago.

This ancestor looks more like today's Old World monkeys, but still a primate.

15 million generations ago and this ancestor looks more like a tree rodent than a monkey.

Let's jump all the way back to my 120 millionth great-grandparent, this is 12 kilometers back down the line.

This is a decidedly non-human, non-primate shrew-like mammal, but it's kinda cute!

My 165 millionth great-grandparent is not even a mammal, it's a prehistoric lizard that predates even the dinosaurs.

They've got their own photo album, their own stack, that joins up with ours somewhere around here.

In fact, every species has their own stack that branches off somewhere down the line.

And here we are 185 million generations ago at our ancestral fish.

You see the resemblance?

So where along the stack was the first human?

There wasn't one.

Photo #4,632?

Human.

Photo #79,221?

Well, that's Homo erectus.

There's no single point where one became the other.

Every photo that we pull from this stack looks pretty much like the photo on either side.

Every generation is the same species as its parents, and the same species as its children.

Homo erectus had Homo erectus parents and Homo erectus kids.

Our fishy ancestor?

Had fishy fathers and fishy children.

You can never pinpoint the exact moment that a species came to be, because it never did.

It's just like how you used to be a baby, and now you're older.

But there was no single day when you went to bed young and woke up old, although sometimes it feels that way.

There was no first human.

It sounds like a paradox, it sounds like it breaks the whole theory of evolution.

But it's really a key to truly understanding how evolution works.

Evolution happens like a movie, with frames moving by both quickly and gradually, and we often can't see the change while it's occurring.

Every time we find a fossil, it's a snapshot back in time, often with thousands of frames missing in between, and we're forced to reconstruct the whole film.

Life is what happens in between the snapshots.

Instead of a nice smooth road, this is a journey on stepping stones, and we give each one their own name.

Stay curious.

This journey was inspired by Richard Dawkins' book "The Magic of Reality" and there is a link down in the description.