This is the United States Geologic Survey version of the geologic timescale. They have divided the four and a half billion year history of the planet up into eons, eras, periods, and epochs. Eons are the largest divisions. Those are divided up into eras which are divided into periods which are divided into epochs. Originally all of these divisions were only known relative to each other but with the advent of radiometric dating and paleomagnetic measurements we can assign actual numerical dates to these different divisions. Those are given over here in the right hand column along with their uncertainty measurements. Let me just scroll back to the beginning of history. Okay, the Earth begins back here four and a half billion years ago or so. There are no rocks from this time period. Most geologists believe the Earth was a magma ocean at this point which is pretty cool to think about. We scroll through the Archean and the Proterozoic. At the end of the Proterozoic here, this is where we start to get actual fossils. Shelled organisms in the ocean and things like that. The Phanerozoic eon begins here about 542 million years ago. Note all of these different divisions now because we know a lot more about the planet at this point. There are several mass extinction events that happened in the history of the planet but we are only going to cover 2 of them in this lesson. One of them happened here at the end of the Permian. And the other one, more famous, happened here at the end of the Cretaceous. The present time is the Holocene epoch and that is right here.