There was no ozone layer in the very beginning because the process of photosynthesis had to build it up over a long time.
The ozone layer built up starting over 2 billion years ago, and was dense enough (with enough ozone to protect biological organisms from lethal UV light) by about 600 million years ago. Prior to this period, life was restricted to the ocean.
Source: albany.edu/faculty/rgk/atm101/ozone.htm
There was no oxygen in the early atmosphere of the earth because there was no vegetation. When life finally began, vegetation provided the needed oxygen.
The land received too much harmful solar radiation
Oxygen
yes
Photosynthesis by plants
reacted out with oxygen as oxygen levels rose.
Oxygen
Oxygen
Nitrogen and Oxygen.
Oxygen
yes
oxygen
oxygen
Nitrogen and Oxygen.
oxygen
No! Stromatolites converted the toxic atmosphere into a breathable one.
no
photosynthsis
Photosynthesis by plants