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:
Goodwill Impairment
A decrease in the value of goodwill, indicating that the acquired company is not performing as expected.
Depreciation Expense
The systematic allocation of the cost of a tangible asset over its useful life.
Bond Coupon Rate
The interest rate stated on a bond when issued, which represents the annual interest payment divided by the bond's face value.
Unrealized Profit
A profit that has been generated on paper through an investment but has not yet been realized through a sale.
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