According to the figure shown below, about how long ago did oxygen first appear in Earth's atmosphere? About how long ago did oxygen reach 50 percent of its current abundance in Earth's atmosphere? 
Definitions:
U-joint Working Angles
The operational angles at which universal joints efficiently transfer rotational power between shafts.
Full Rotation
The movement of an object in a circular path, completing 360 degrees around an axis.
PTO Driveshafts
PTO driveshafts, short for Power Take-Off driveshafts, are components used to transfer mechanical power from a vehicle, like a tractor, to another piece of equipment or machinery.
Solid Steel Stock
A form of steel material that is completely solid, used for a variety of industrial and construction applications.
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