. Ice ball Earth. Earth was once covered pole to pole in ice. Obviously not friendly to most life so Carbon Dioxide started building up because there was not enough plant life to reduce the amount coming from volcanic activity. Eventually enough CO2 built up to start melting all that ice. Suddenly you have great conditions for plants to thrive again. That population explodes resulting in a very oxygen rich atmosphere. That in turn gives animals a boost in the energy they can use .Lots of plants to eat as well. New animal species expand quickly. You could say the Precambrian explosion was a burst of activity after a very long winter.
Except these forms appear suddenly without precursors.