By this time, oxygen levels in the atmosphere had risen enough to support both a protective ozone layer and the possibility of fire on land.
# Metadata
Title: Formation of the Ozone Layer Movietime: 9:40 Geographical Time: 600 Ma
Around 600 million years ago, Earth reached a major atmospheric milestone: oxygen concentrations became high enough to allow the formation of a stable ozone layer in the upper atmosphere. This thin shield of O₃ molecules absorbed harmful ultraviolet radiation from the Sun, dramatically reducing UV exposure on Earth's surface.
The presence of ozone was critical for enabling complex life to move out of the oceans and onto land. For the first time, the surface was shielded from lethal solar radiation.
Around the same time, geologic evidence — including the appearance of charcoal deposits — indicates that oxygen levels were also sufficient to support combustion. This is the earliest indication of fire on Earth, showing that terrestrial environments were becoming chemically and energetically dynamic.