Well, by counting the number of isotopes

in a sample of rock

that's undergone radioactive decay,

geologists have estimated the Earth's birthday,

when it first formed from a solar nebula,

to be 4.6 billion years ago.

But just how long is that really?

Here's some analogies

that might help you understand.

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

until the present day

as a single calendar year.

On January 1st, the Earth begins to form.

By March 3rd, there's the first evidence

of single-celled bacteria.

Life remains amazingly unicellular until November 11th

when the first multicellular organisms,

known as the Ediacaran fauna, come along.

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

is the Cambrian Explosion of life,

a major milestone,

when all of the modern phyla started to appear.

On December 10th at 1:26 p.m.,

the dinosaurs first evolve

but are wiped out by an asteroid

just two weeks later.

On December 31st, the mighty Roman empire

rises and falls in just under four seconds.

And Columbus sets sail

for what he thinks is India

at three seconds to midnight.

If you try to write the history of the Earth

using just one page per year,

your book would be 145 miles thick,

more than half the distance

to the international space station.

The story of the 3.2 million year-old

Australopithecine fossil known as Lucy

would be found on the 144th mile,

just over 500 feet from the end of the book.

The United States of America's Declaration of Independence

would be signed in the last half-inch.

Or if we compared geologic time

to a woman stretching her arms

to a span of six feet,

the simple act of filing her nails

would wipe away all of recorded human history.

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

from the moment you're born

to your first day of high school.

Your first word,

first time sitting up,

and first time walking

would all take place while life on Earth

was comprised of single-celled organisms.

In fact, the first multicellular organism

wouldn't evolve until you were 12 years old

and starting 7th grade,

right around the time

your science teacher is telling the class

how fossils are formed.

The dinosaurs don't appear

until three months into 8th grade

and are soon wiped out right around spring break.

Three days before 9th grade begins,

when you realize summer is over

and you need new school supplies,

Lucy, the Australopithecine, is walking around Africa.

As you finish breakfast

and head outside to catch your bus

44 minutes before school,

the Neanderthals are going extinct throughout Europe.

The most recent glacial period

ends as your bus drops you off

16 minutes before class.

Columbus sets sail 50 seconds before class

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

The Declaration of Independence is signed

28 seconds later

as you look for an empty seat.

And you were born 1.3 seconds before the bell rings.

So, you see, the Earth is extremely,

unbelievably old

compared to us humans

with a fossil record

hiding incredible stories to tell us about the past

and possibly the future as well.

But in the short time we've been here,

we've learned so much

and will surely learn more

over the next decades and centuries,

near moments in geological time.