A. Oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.
Explanation:
The Cambrian explosion was an event that happened approximately 541 million years ago and it lasted for about 13 – 25 million years.
The current hypothesis for the cause of the Cambrian explosion is that oxygen levels reached a critical threshold at the start of the Cambrian explosion, making the evolution of big, mobile animals possible.
