# What was a prerequisite for the survival of life on land?

Early Earth was anaerobic ; there was no free atmospheric oxygen and thus no ozone layer (${O}_{3}$) to shield the Earth from harmful UV rays. Earth was thus subjected to intense UV radiation , which was somewhat negated in an aquatic environment. On land however, this was not the case and UV was extremely harmful, making it impossible to live on. When the ozone layer eventually formed due to a rise in photosynthetic activity, life could colonise land without intense UV rays.