Thursday, March 3, 2011

Geologic Time Analogies

Radiometric dating indicates that the Earth is about 4.570 billion years old. Geologists break down this 4.570 billion years time frame into a calendar as shown below. 

 Some people have a hard time getting their arms around this number (4,570,000,000); so today, I thought I would help give you a perspective of how much time 4.570 billion years really is. 

At a recent symposium which I attended, Dr. Kirk Johnson of the Denver Museum of Nature and Science attempted to help us get a grasp of time by telling us a million years was about the time it would take for your fingernails to grow to the length of 18 miles. Further calculations show that if you were alive when our solar system was created, your fingernails would now be 82,260 miles long. That would long enough to circle the Earth more than 3 times.  I bet you didn’t know that!

The analogy for geologic time that I used in my book, Ancestors of Gods, uses the length of one’s arm span as a scale of time. Holding your arms out to your sides; if the tip of your left hand represents when the earth was first formed and the tip of the middle finger on your right hand represents the present, then life first appears on earth about at the bend of your left elbow. From there to about your right bicep, life consisted of single celled organisms, mostly bacteria. At about your right bicep, the first eukaryotes, single celled organisms with nuclei, appear. The first multicellular organisms don’t appear until about two inches above your right wrist. The first fish appear at about your right wrist bone, the first reptiles at the center of your right palm, and the first mammals at the base of your middle finger. The extinction of the dinosaurs occurs near the last joint in your middle finger. If you were to take a nail file and make a light pass over the fingernail on the middle finger of your right hand, you would eliminate modern man’s time on earth; two passes would eliminate the entire history of hominids, man’s entire post-ape-ancestor lineage.

Included below are a few other interesting analogies:
1.    365 days/1 year analogy
For this analogy we assume the Earth was formed on January 1st and the present is midnight on December 31st. The oldest known rocks, 3.9 billion year old metamorphic rocks from Greenland, would date from late February. Oxygen would first appear on Earth on July 25th. Multicellular life with skeletons would appear in late November and the first mammals would appear on December 15th. The Atlantic Ocean would begin to open up, separating North America from Europe, about December 19th and the dinosaurs would die out on 27th.  Homo sapiens would first appear at 11:49PM on December 31st and the birth of Christianity would occur at 11:59PM.

2.    Trips around the Milky Way Galaxy
The Earth has circled the Milky Way Galaxy 18 times since its conception 4.6 billion years ago. About 2/5’s of an orbit ago, the dinosaurs died out. Almost 2 orbits ago, the first fish appeared on Earth. Multicellular life appeared about 4 orbits ago.  (from geologic_time_galactic/)

3.    The toilet roll analogy
For this analogy, you need a class in an auditorium. A student starts at the front of the auditorium at the present time. As the student slowly walks toward the back of the auditorium and back in time, she unrolls the toilet paper. By the time the student gets to the back of the auditorium, she stops to recognize the time the very first dinosaurs begin to appear on Earth. The student has covered about the last 5% of the Earth’s existence. The student continues across the back of the room and heads down another aisle to the front of the auditorium. By the time she gets back to the front of the auditorium, she has only gone back about 10-15% of the way back to the origin of the solar system. There are still quite a few sheets on the roll of toilet paper.

4.    The glass of beer analogy

I think the students probably came up with this analogy after class on a Friday night.

Now it is your turn. Please leave your favorite analogy or make up one of your own in the comments. To help you make one up, I have included below a time line for the various highpoints in the evolution of the Earth.

Years Ago
Origin of the Earth
Oldest Dated Crustal Rocks
Oldest Evidence for Life
First Oxygen Atmosphere/Ozone Layer Forms
Oldest Metazoan Fossils
Oldest Fossil Fish
First Land Plants
That important first step: Amphibians Evolve
Huge Mass Extinction at End of Permian Period / Close of the Paleozoic Era
First Mammals
First Birds
Atlantic Ocean first opens
Angiosperms (Flowering Plants) on the Scene
Adaptive Radiation of Mammals
Dinosaurs Go Extinct
Close of the Mesozoic Era/ Beginning of the Cenozoic Era
New discoveries of Australopithecus afarensis (Lucy) fossils from Ethiopia
Pleistocene Ice Age begins
Light from the Andromeda galaxy seen today left Andromeda
Age of Homo erectus fossils from Ethiopia
Oldest rocks in the Bahamas
Homo sapiens appears in the fossil record
Last ice sheet retreats from northern United States
Grahams Harbor, San Salvador, Bahamas floods due to rising sea level after ice sheets are reduced to modern day volume
Columbus lands in New World


Source: Ritger, S.D. and R.H. Cummins. 1991. Using student-created metaphors to comprehend geologic time. Journal of Geological Education. 9:9-11. - Calculate your own analogy to deep time. – analogy of time scaled to the human life time.

1 comment:

  1. Can you do a 4.6 billion years to 6,000 years comparison? :)