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  • 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.

If you really want to understand history deeply,

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地質年表入門|ビッグヒストリープロジェクト (Introduction to the Geologic Time Chart | Big History Project)

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    leo に公開 2021 年 01 月 14 日
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