Answer:
The Cambrian Explosion!
Explanation:
Sounds exciting right?
WELL IT IS!
The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when practically all major animal phyla started appearing in the fossil record. It lasted for about 13 – 25 million years and resulted in the divergence of most modern metazoan phyla.
So in your case the answer would be 542 million years ago or the first animals with exoskeletons appear.
