According to the figure, about how long ago did oxygen reach its current abundance in Earth's atmosphere? 
Definitions:
Natural Selection
The process by which organisms better adapted to their environment tend to survive and produce more offspring.
British Naturalist
A scientist from Britain who studies the natural world, especially plants and animals.
Mutations
Changes in the DNA sequence of a cell's genome, which can lead to variations in physical traits or the development of diseases.
DNA Copies
The process of duplicating the DNA molecule, essential for cell division and reproduction.
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