The Cambrian Explosion refers to a time in Earth’s history, about 541 million years ago, when lots of new and different animals appeared. As geologists, we call it an explosion, which you may think is a very fast event, but it actually took place over 20 million years! Scientists are still trying to figure out the reason that all these new animals appeared, but they have some ideas. One idea is that oxygen levels increased on Earth and this let more animals evolve. Another idea is that the formation of our ozone layer, which protects us on Earth from harmful rays from the sun, let more animals live, since conditions on Earth were not as harsh.