Why did the “Cambrian Explosion” happen at the time it happened when no event like it happened before it for billions of years of Earth’s history?

One of the most mysterious times in the history of Earth is that of the so-called “Cambrian Explosion,” when in a short geologic period of time (few millions of years) most of the animal ancestors of all animals alive today (including humans) suddenly appeared. Why did the “Cambrian Explosion” happen at the time it happened when no event like it happened before it for billions […]