Final answer:
Free oxygen began accumulating in the Earth's atmosphere about 2 billion years ago.
Step-by-step explanation:
Free oxygen began accumulating in the atmosphere about 2 billion years ago. The interaction of sunlight with oxygen can produce ozone (which has three atoms of oxygen per molecule, as compared to the two atoms per molecule in the oxygen we breathe), which accumulated in a layer high in Earth's atmosphere. This ozone provided protection from the Sun's damaging ultraviolet radiation.