How old is the earth?

I've recently been trying to get a better grasp on the geologic time scale, and thought others might like to share in pondering what some of those gigantic numbers really mean. To do this, we'll do a comparison with the one thing we have some experience with that spans this huge range of numbers: distance.

So what kind of numbers are we talking about here? Lets start way back to the beginning when this little rock called home (ok, or "Earth") came into existence. This happened roughly 4,500,000,000 years ago - or typing a few less zeros, 4.5 billion years ago.

Not quite sure how big 4.5 billion is? Consider the following: suppose a millimeter represents 1 year. That makes me "3cm old" since 1cm = 10yrs, and I'm almost 30 years old.

Continuing this line of reasoning (using the metric system, just to keep the math easy), we can get an idea of some larger numbers of years: 1 meter = 1000 yrs, 1 kilometer = 1 million yrs, and so on.

Next, pick your favorite reference points and see how big the distances get! How long ago was U.S. Declaration of Independence signed? About 20cm (or 8 inches). The Black Death that swept through 14th century Europe? 0.7 meters. The fall of the Roman Empire? 1.5 meters. Jesus? 2 meters. The age of the earth according to young earth creationists? About 10 meters. The last "ice age" in North America? About 13 meters. When did dinosaurs go extinct? About 65 kilometers (5 miles is about 8 km). And so on...

So with this time-distance comparison in hand, lets start back at the beginning.

That 4.5 billion years works out to 4,500 km (about 2,700 miles). The circumference of the earth is about 40,000 km (24,900 miles) you can think of this distance as about 1/10th the way around the world - say, from southern California to northern Maine, or roughly from New York to southern Florida and back.



U.S. map showing what 4.5 billion, 3.5 billion & 2 billion years
look if you equate 1 year = 1 millimeter. Click to enlarge.

Think about it - all adult humans are only a few centimeters old while the history of the earth spans comparatively enormous continental distances (see the figure above)!

With that, I'll leave you to ponder some other major milestones in the history of our little blue planet:

• Life appeared about 1 billion years after earth formed, with the oldest known evidence of life dating back to around 3,500,000,000 years ago or earlier (that's 3,500 km - roughly New York to the Grand Canyon in southern Utah).
• About 2,000,000,000 (yup, billion) years passed before multi-celled life appeared (This brings us to about the distance from Denver to Chicago - 1,500 km),
  
• and things didn't really get "interesting" until the famed Cambrian explosion around 530,000,000 years ago, when life diversified into the major groups we recognize today (for example, this is when we see the first plants, animals, etc.)
  
• During the next 50,000,000+ years (50 km or about 30 miles) organisms continue to diversify, though it takes around 300,000,000 more years for early mammals to arrive on the scene (yes, I skipped over a whole lot of invertebrates, fish, amphibians, and reptiles there), which brings us furry critters into existence a little over 200,000,000 years ago. The history of mammals' on earth (only 200 km /120 miles) is relatively quite recent!
  
• So what about humans? The first hominids appeared around 7,000,000 years ago (that's right, it took approximately 4.493 billion years since earth was formed 4.5 billion years ago before anything vaguely human came into existence - about 3.493 billion years after the first known evidence of life!) - as far as mammals go, 7 km out of 200 km is also quite recent, at least in my opinion.
  
• Finally, modern humans are thought to have appeared around 200,000+ years ago - a little over 200 meters ago!!!