More from Metamorphosis, the companion book to the film:
The sudden appearance in the fossil record of [Cambrian] animals with such complex, distinct body plans has spawned many theories but no satisfactory answers. Darwin himself acknowledged that this mystery was the single most difficult challenge to his theory.
Even more mysteriously, it appears that the most ancient phyla were metamorphic from the beginning, based on the few larval forms that have been preserved. This suggests that these Cambrian animals had not one but two or more developmental stages at the outset, a small and free-swimming larva, and a bottom-dwelling adult with little or no resemblance to its earlier form. But how such transitions could have evolved and from what, is completely unknown. (p.29)
Insects showed up in the record much later, and apparently developed metamorphosis independently; the most ancient versions developed directly into adults.
Also: Here’s where Monarch butterflies – whose remarkable migration flight is the subject of the film – end up somewhere in Mexico, looking and sounding like an orange Niagara Falls:
Probably because half the Monarchs that would have been found in all of Canada are there in a small space …
Here’s the earlier post on the migration flight that leads many to dismiss Darwin.
See also: “Problematic “land organisms from Cambrian found in soil layer under the soil