What sparked the Cambrian explosion?

…over the past several years, discoveries have begun to yield some tantalizing clues about the end of the Ediacaran. Evidence gathered from the Namibian reefs and other sites suggests that earlier theories were overly simplistic — that the Cambrian explosion actually emerged out of a complex interplay between small environmental changes that triggered major evolutionary developments.

Some scientists now think that a small, perhaps temporary, increase in oxygen suddenly crossed an ecological threshold, enabling the emergence of predators. The rise of carnivory would have set off an evolutionary arms race that led to the burst of complex body types and behaviours that fill the oceans today.…

But last year, a major study1 of ancient sea-floor sediments challenged that view.… oxygen could already have been abundant enough “for a long, long time before”.… The gradual emergence of predators, driven by a small rise in oxygen, would have meant trouble for Ediacaran animals that lacked obvious defences.…