Four ways to understand the Earth's age - Joshua M. Sneideman

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.