Answer:
Animals evolved during the Cambrian Explosion because oxygen on Earth had become abundant.
Step-by-step explanation:
Photosynthesis could be responsible for the rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian. This rise of oxygen concentration allowing the evolution of larger animals having complex respiratory and circulatory systems. So we can conclude that evolution in the physical and genetical characteristics of animals occur during the Cambrian Explosion.