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How old is the Earth?

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Well, by counting the number of isotopes

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in a sample of rock

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that's undergone radioactive decay,

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geologists have estimated the Earth's birthday,

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when it first formed from a solar nebula,

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to be 4.6 billion years ago.

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But just how long is that really?

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Here's some analogies

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that might help you understand.

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For example, let's imagine the entire history of Earth

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until the present day

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as a single calendar year.

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On January 1st, the Earth begins to form.

0:00:37.124,0:00:39.459
By March 3rd, there's the first evidence

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of single-celled bacteria.

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Life remains amazingly unicellular until November 11th

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when the first multicellular organisms,

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known as the Ediacaran fauna, come along.

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Shortly thereafter, on November 16th at 6:08 p.m.

0:00:54.121,0:00:56.409
is the Cambrian Explosion of life,

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a major milestone,

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when all of the modern phyla started to appear.

0:01:01.412,0:01:03.873
On December 10th at 1:26 p.m.,

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the dinosaurs first evolve

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but are wiped out by an asteroid

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just two weeks later.

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On December 31st, the mighty Roman empire

0:01:11.794,0:01:14.952
rises and falls in just under four seconds.

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And Columbus sets sail

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for what he thinks is India

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at three seconds to midnight.

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If you try to write the history of the Earth

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using just one page per year,

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your book would be 145 miles thick,

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more than half the distance

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to the international space station.

0:01:30.817,0:01:33.005
The story of the 3.2 million year-old

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Australopithecine fossil known as Lucy

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would be found on the 144th mile,

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just over 500 feet from the end of the book.

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The United States of America's Declaration of Independence

0:01:44.872,0:01:47.295
would be signed in the last half-inch.

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Or if we compared geologic time

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to a woman stretching her arms

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to a span of six feet,

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the simple act of filing her nails

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would wipe away all of recorded human history.

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Finally, let's imagine the history of the Earth as your life:

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from the moment you're born

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to your first day of high school.

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Your first word,

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first time sitting up,

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and first time walking

0:02:07.652,0:02:09.694
would all take place while life on Earth

0:02:09.694,0:02:12.760
was comprised of single-celled organisms.

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In fact, the first multicellular organism

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wouldn't evolve until you were 12 years old

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and starting 7th grade,

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right around the time

0:02:19.584,0:02:21.276
your science teacher is telling the class

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how fossils are formed.

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The dinosaurs don't appear

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until three months into 8th grade

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and are soon wiped out right around spring break.

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Three days before 9th grade begins,

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when you realize summer is over

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and you need new school supplies,

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Lucy, the Australopithecine, is walking around Africa.

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As you finish breakfast

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and head outside to catch your bus

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44 minutes before school,

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the Neanderthals are going extinct throughout Europe.

0:02:47.819,0:02:49.190
The most recent glacial period

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ends as your bus drops you off

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16 minutes before class.

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Columbus sets sail 50 seconds before class

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as you're still trying to find the right classroom.

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The Declaration of Independence is signed

0:03:00.552,0:03:02.353
28 seconds later

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as you look for an empty seat.

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And you were born 1.3 seconds before the bell rings.

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So, you see, the Earth is extremely,

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unbelievably old

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compared to us humans

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with a fossil record

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hiding incredible stories to tell us about the past

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and possibly the future as well.

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But in the short time we've been here,

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we've learned so much

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and will surely learn more

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over the next decades and centuries,

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near moments in geological time.