How old is the earth?

Friday, April 3, 2009 at 10:09 PM Bookmark and Share
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!!!


Posted by: Anonymous | 5/12/2009 5:15 PM

i cant believe anyone would believe that creationists theories are the only ones that make sence i mean how can we come from monkeys is it possible really think if we did come from monkeys wouldnt the cicle continue and think is it possible a random chemicle reaction would make our beautiful mountains NO! that didnt happen the only logic thing as scientists would say is that God in heaven is the only thing that could have happened to the earth he is our real creator and if you dont believe me youre crazy and seriously who would come up with the theory that the underground caves radomly had water flow through them and create beautiful creations NO! again the Bible tells us that there was a flood for a year and the bible was made way before we found out about the underground caves i am very young like younger than twelve and i know the truth that we come from God so you guys have to be dumb to believe we come from monkeys or aliens or chemicals and think every theory the scientiists have come up with Christians have proved wrong so you have to make up that you werehiding aliens in area 51 no one is going to believe that unless they are alien freaks no there is only one answer God made us

Posted by: Anonymous | 5/22/2009 9:31 AM

I'm not hardcore scientist nor a religious person. Something just gets to me is that, Cars did not evolve, plains, trains, plastic. None of that evolved. Who created it?

Posted by: Paul | 5/13/2010 9:43 PM

(Re: Comment #1, Original Reply: 5/12/2009 8:24 PM )
Quite a lot in that comment, Anonymous! I would encourage you to read about some of the things you mentioned. For example, we ARE a bunch of chemicals - proteins, fats, water, calcium and phosphate in our bones, and so on. Also, humans ARE classified as primates - its true. But that does not detract from our humanity!

Before we even new DNA existed, biologists had found a lot of evidence that humans and other primates all originated from some common ancester, millions of years ago. What's the evidence? Lots! For example, taking a close look at what makes primates primates, we find all monkeys and apes share common traits. Humans have all those same physical characteristics, suggesting the relationship. Its the same way you can establish that American Bison ("Buffalo") are related to cows based upon not just general appearance, but also by very detailed comparisons of their bones, muscles, organs, etc. Then came along the discovery of DNA and the study of genetics, and just as the theory would predict, our DNA also confirms we are distant cousins of the great apes - our closest relatives of all living organisms being the Chimpanzees.

Here's a nice book you can download for free from the National Academies of Sciences on Science, Evolution and Creationism:

I've included a bunch of other links below. Keep your head engaged and try to give them a listen.

Remember, "understanding" and "believing" aren't the same - whatever your religious beliefs, you should still be able to understand the scientific evidence and theories that summarize that evidence.

Many extremist creationist claims have no evidence supporting them, and recklessly make people thing religion and science are necessarily opposing forces. This doesn't mean all of religion is false, just that those claims are false. It's an unfortunate fact that some people make false assertions in the name of religion (on accident, and on purpose), and in the end this is bad for everyone.

No matter what your faith, learn to think critically about it. All religions are full of cultural and human bias - knowing this leads to a deeper understanding of one's religion. Do you know the history of the Bible according to non-christian AND christian historians?? Can you name and explain common factual and logical mistakes made by some extremist creationists? Do you know what a "false dichotomy" is? Learn how to check the facts, how to recognize biased and unbiased sources of information, and use good logic. Test yourself on all of these skills so you aren't easily fooled... Can you give examples of why the Bible can't be taken literally without logical contradictions? Would you be able to recognize Peter Popoff or Kent Hovind as a fraudster, or would you have fallen for their scams (see links below)?

Keep reading, keep your mind sharp, and good luck! :)


Posted by: Paul | 5/13/2010 9:44 PM

(Re: Comment #2, Original Reply: 5/22/2009 11:32 AM )
First, its important to be clear on what we mean by evolution (in the biological sense). This requires some science background you might not have seen yet, so I'll include links (right click and open in new window) for key concepts below.

Evolution by natural selection has to do with the change in genetic and/or phenotypic makeup of a population. This occurs because not all individuals are the same, some of those differences are genetic, and those differences enable some to survive and reproduce better than other individuals - passing along their good genes to their offspring. For an example of how this works, consider artificial selection of domestic animals, or for examples see this list at, or this post related to mouse evolution.

As for planes, trains, plastic and the like - these are all man-made. These are the only kinds of examples where asking the question "who created it?" makes sense, because in the back of our mind we already know these are all things made by people. Other things though, aren't made by anyone.

For example, the glassy volcanic rock on the shores of Hawaii wasn't made by anyone, it was made by a process - lava from the nearby volcano cooling and forming rock. Recognizing that a process created the rock then allows us to ask more specific questions, some of which we can test experimentally (e.g. does cooling lava in water versus air lead to different kinds of rock?)

Science is about understanding those processes, including the process of evolution and how it has shaped the forms of life we see in the world today.

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