So here's a great puzzle:

in a universe ruled by the second law of thermodynamics,

how is it possible to generate the sort of complexity I've described,

the sort of complexity represented by you and me and the convention center?

Well, the answer seems to be,the universe can create complexity,but with great difficulty.

In pockets,there appear what my colleague, Fred Spier, calls "Goldilocks conditions" --

not too hot, not too cold, just right for the creation of complexity.

And slightly more complex things appear.

And where you have slightly more complex things, you can get slightly more complex things.

And in this way, complexity builds stage by stage.

Each stage is magical because it creates the impression of something utterly new appearing almost out of nowhere in the universe.

We refer in big history to these moments as threshold moments.

And at each threshold, the going gets tougher.

The complex things get more fragile,more vulnerable;

the Goldilocks conditions get more stringent,

and it's more difficult to create complexity.

Now, we,as extremely complex creatures,desperately need to know this story of?how the universe creates?complexity despite the second law,

and why complexity means?vulnerability and fragility.

And that's the story that we tell in big history.

But to do it, you have do something that may, at first sight,seem completely impossible.

You have to survey the whole history of the universe.

So let's do it.

Let's begin by winding the timeline back 13.7 billion years, to the beginning of time.

Around us, there's nothing.

There's not even time or space.

Imagine the darkest,emptiest thing you can

and cube it a gazillion times and that's where we are.

And then suddenly,bang!A universe appears, an entire universe.And we've crossed our first threshold.

The universe is tiny;it's smaller than an atom. It's incredibly hot.

It contains everything that's in today's universe,so you can imagine, it's busting.

And it's expanding at incredible speed.

And at first, it's just a blur,but very quickly distinct things begin to appear in that blur.

Within the first second,energy itself shatters into distinct forces including electromagnetism and gravity.

And energy does something else quite magical:it congeals to form matter --

quarks that will create protons and leptons that include electrons.

And all of that happens in the first second.

Now we move forward 380,000 years.

That's twice as long as humans have been on this planet.

And now simple atoms appear of hydrogen and helium.

Now I want to pause for a moment,

380,000 years after the origins of the universe,because we actually know quite a lot about the universe at this stage.

We know above all that it was extremely simple.

It consisted of huge clouds of hydrogen and helium atoms,and they have no structure.

They're really a sort of cosmic mush.

But that's not completely true.

Recent studies by satellites such as the WMAP satellite have shown that, in fact,there are just tiny differences in that background.

What you see here,the blue areas are about a thousandth of a degree cooler than the red areas.

These are tiny differences,but?it was?enough?for the universe to?move?on?to the next?stage?of?building?complexity.

And this is how it works.

Gravity is more powerful where there's more stuff.

So where you get slightly denser areas,gravity starts compacting clouds of hydrogen and helium atoms.

So we can imagine the early universe breaking up into a billion clouds.

And each cloud is compacted,gravity gets more powerful as density increases,

the temperature begins to risemat the center of each cloud,

and then, at the center,the temperature crosses the threshold temperature of 10 million degrees,

protons start to fuse,there's a huge release of energy,and --bam! We have our first stars.

From about 200 million years after the Big Bang, stars begin to appear all through the universe, billions of them.

And the universe is now significantly more interesting and more complex.

Stars will create the Goldilocks conditions for crossing two new thresholds.

When very large stars die, they?create?temperatures so?high

that protons begin to fuse in all?sorts?of exotic combinations,to?form?all the?elements?of the periodic table.

If, like me, you're wearing a gold ring,it was forged in a supernova explosion.

So now the universe is chemically more complex.

And in a chemically more complex universe,it's possible to make more things.

And what starts happening is that, around young suns,young stars,

all these elements combine,they swirl around,the energy of the star stirs them around,

they form particles, they form snowflakes,they form little dust motes,they form rocks,

they form asteroids, and eventually,they form planets and moons.

And that is how our solar system was formed,four and a half billion years ago.

Rocky planets like our Earth are significantly more complex than stars because they contain a much greater diversity of materials.

So we've crossed a fourth threshold of complexity.