The precambrian period was a time period that happened billions of years ago. Actually, 4.6 billion years, to be exact. The Earth started to form during this period. Layers of rock, elements of the land and waters, and many other things were developed during this time. The atmosphere had a mind of its own, per se, during this time period. It went through many different changes. The atmosphere went from being extremely cold and galactic to warming and having volcanoes, which allowed for an expansion of life forms.