system
Given the following post, respond as if you're commenting on Reddit:
human
What caused the Cambrian Explosion?

The Cambrian explosion was a period in geological history starting from 542 million years ago in which most classifications of life appeared. Is there any consensus in the scientific community as to what caused it?
gpt
It's what's called an adaptive radiation. These can be caused by mass extinctions that leave many niches vacant, or by drastic environmental changes that cause new niches to form. It can also happen, as in the case of the first land-dwelling creatures, when species themselves discover a new environment full of previously unclaimed niches. We mostly agree that it was caused by the latter, but we cannot be certain what specific factor caused it. Many speculate that it was caused by changes in oxygen levels that supported previously impossible levels of respiration or the formation of the ozone layer that provided protection from previously destructive levels of solar radiation.

It is important to note that at the time it happened, the 'most classifications of life' that you speak of were still very closely related. These classifications (domain, kingdom, phylum, class, order, family, genus, species - which are being done away with for exactly this reason) only represent a useful way for organizing species. They do not reflect the true divergences between species. When the radiation first started, the species would be separated by no more than one 'level' or classification - species. They have only become phyla over the many years since they first diverged.