There was no oxygen and layer of ozone in the atmosphere of earth billions of year ago.
The geologic time scale is a “calendar” that show events that occur in Earth history. It is used by geologists and other Earth scientists in order to describe the timing and relationships of events in geologic history.
Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon as reliable clocks in order to date ancient events.
The early Earth had no ozone layer and no free oxygen. Without an oxygen atmosphere very few things could live on the early Earth which did not need oxygen for their survival.
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