What happened during the Cambrian explosion?

Question

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The Cambrian explosion refers to a sudden and rapid diversification of animal forms that occurred around 540 million years ago, during the early Cambrian period. This marked the first appearance of complex, multicellular organisms in the fossil record, including many of the major groups of animals that still exist today. There is no evidence of large-scale volcanic activity or asteroid impacts during this time that could explain the sudden burst of biodiversity. Similarly, while the Cambrian explosion may have involved the evolution of some multicellular organisms from single-celled predecessors, this process would not have accounted for the wide array of new forms that appeared so quickly.

What Happened During the Cambrian Explosion