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Outcrop 12:
Geologic Time
GEOLOGIC HISTORY is far more vast than human history; it began when the earth was formed around 4,600,000,000 (4.6 billion!) years ago. A lot has happened since then!
The earliest known fossils are from 3.5 billion years ago, but life on earth may have begun before then. We can’t be sure—rocks from that long ago are very rare because the rock cycle encourages rocks recycling into different rocks, destroying any geologic history. Around 541 million years ago, the Cambrian Explosion occurred, and life on earth evolved drastically from microbes to more complex beings. Scientists don’t know exactly why the Cambrian Explosion occurred, but many believe an increase to oxygen levels in the ocean may have played a part.
Geologic Time Scale
Image source: Frederik Lerouge, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Geologic_Time_Scale.png
Since the Cambrian Explosion, life has continued to evolve drastically. But there have actually been six mass extinction events since then. A mass extinction event occurs when the rate of extinction rapidly increases. The first mass extinction occurred 445 million years ago, when life mainly lived in the sea. The second extinction occurred 370 million years ago, and scientists think it lasted anywhere from 500 thousand years to 25 million years. The third mass extinction is also called the Great Dying, since it killed about 70% of land-dwelling species, and more than 95% of marine species. It took more than 10 million years for Earth’s biodiversity to bounce back. The fourth extinction occurred 201 million years ago, before the supercontinent Pangea broke apart. The fifth extinction occurred 66 million years ago, killing the dinosaurs alongside many other species, such as ammonites.
Geologic time can be hard to wrap your head around. Humans have only existed for around 300 thousand years, and much of that was existing as hunters and gatherers. The Egyptian pyramids were built 4500 years ago, but that is nothing in terms of geologic time. The sixth extinction event has been occurring for the past 10,000 years, and is human driven. Because of rapid climate change and invasive species, we live in unprecedented territory regarding the earth’s biodiversity. We can’t recover species that have been lost, but it is important to slow this mass extinction as soon as possible.
Sources:
Fox, Douglas. “What Sparked the Cambrian Explosion?” Nature News, Nature Publishing Group, 16 Feb. 2016, www.nature.com/articles/530268a.
“Episode 12: Six Extinctions in Six Minutes.” AMNH Shelf Life, American Museum of Natural History, www.amnh.org/shelf-life/six-extinctions.
“NEO Basics: Life on Earth.” NASA Jet Propulsion Laboratory, NASA, cneos.jpl.nasa.gov/about/life_on_earth.html.
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