No more than 540 million years ago there was a huge boom in the diversity of animals on Earth. The first larger animals evolved in what is today known as the Cambrian explosion. In the time that followed, the animals evolved and grew larger, but concurrently with the evolution of animals, the oxygen level in the atmosphere dropped and this temporarily slowed radiation. However, subsequent oxygenation and growth of algae added energy to the food chain and got the explosion of life going.
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Source: Phys.org