you have to break it down into manageable chunks.

This is called periodization,

breaking history down into periods.

So, we're familiar with that in human history,

where historians talk about the Renaissance

or they talk about the Ming Dynasty

or they talk about the Old Kingdom of Egypt.

Well, these human history periods

tend to be pretty vaguely defined

because different people would put the start of the Renaissance

at different dates

and it probably started at a different time in Italy

than it did in England.

And so these are informal periods.

Now in geology,

where we're reading the record of Earth history

that's written in rocks,

we've found it's necessary to have very much more formal,

concrete, well-defined periods.

And so geologists break Earth history up

into very long intervals that are called eons,

and then shorter periods which are called periods,

and then still shorter intervals that are called epochs

and even stages, and all of those periods

had been defined very precisely

and agreed upon at international level

by all the geologists.

And that's the basis on which we reconstruct Earth history.

Let's start with the broadest divisions of geological time,

the eons.

There are four of these and their names are Hadean,

Archean, Proterozoic, and Phanerozoic.

What do those names mean?

Well, Hadean starts at the beginning of the Earth

four and a half billion years ago

and it lasts for the first half billion years.

And that was the time when Earth was being assembled,

accreted as big comets and asteroids

fell out of the sky and heated the Earth up

so that some parts of it were even melted at times.

So it must have been a really awful place,

kind of like hell, which is why the geologists call it

the Hadean.

Then, once the accretion of the Earth was over,

things quieted down and in fact, it became very slow

and tranquil.

The Earth was very quiet

and there are two long eons that cover that period,

first the Archean and then the Proterozoic.

And then, about a half billion years ago,

life gets going in complicated life, like animals and plants,

and that's called the Phanerozoic

because-- well, that means visible life

and it's because fossils are around.

And so here's a really good piece of understanding

about the Earth.

The Earth had rapid change at the beginning in the Hadean

because it was hot.

It had slow change

in the Archean and the Proterozoic

because it had cooled down,

but in the Phanerozoic,

because of having complicated life around,

the rate of change increased again

and of course now it's very fast

with our human technological civilization.

At a finer level, each of those eons

is divided up into intervals that we call eras.

So the Phanerozoic eon is divided up into the Paleozoic,

meaning old life; Mesozoic, meaning middle life;

and the Cenozoic, meaning young life.

And each of those eras in turn is divided up into periods

with names like Cretaceous and Ordovician and Cambrian.

Now, each of those periods

has its own personality, its own character

which is as familiar and distinctive

to a geologist as, say, Classical Mayan

or the Industrial Revolution would be to a human historian.

For example, the Cretaceous

is the last of the periods when the dinosaurs were alive.

Now, maybe you have a favorite period of human history,

maybe the High Middle Ages, for example, or the Renaissance.

Well, you might think about

whether there might be a geological period

that you find particularly interesting.

One of my favorite periods is the Ordovician.

Let me tell you about a couple of really cool things

that happened during the Ordovician.

So first of all, at that time, Gondwanaland,

which was the supercontinent that includes Africa,

was drifting across the South Pole.

So there were glaciers

in what is now the heart of the Sahara,

where it's blazing hot and as dry as you can imagine.

Boy, were the first geologists

to find those glacial deposits ever surprised.

And another thing that happened in the Ordovician

had to do with our bodies because before that time,

fish had a single big fixed mouth up at the front end

and it was during the Ordovician

that jaws first evolved.

And we today, hundreds of millions of years later,

are still using the jaws that we inherited

from those creatures of so long ago.

So every time you talk or eat or share a kiss,

you are using a souvenir of the Ordovician.

Now, if you find the Ordovician interesting,

it's easy to remember its dates because it ended

444 million years ago.

And the beginning is almost as interesting to remember.

It started 488 million years ago.

So this is how geologists have periodized Earth's history

and made it possible to understand what happened

in the past.