How old is the Earth?
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.