If you really want
to understand history deeply, 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.