Nature and Scientific American think we are beginning to find out:
Biologists have argued for decades over what ignited this evolutionary burst. Some think that a steep rise in oxygen sparked the change, whereas others say that it sprang from the development of some key evolutionary innovation, such as vision. The precise cause has remained elusive, in part because so little is known about the physical and chemical environment at that time.
But 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. “This is the most significant event in Earth evolution,” says Guy Narbonne, a palaeobiologist at Queen’s University in Kingston, Canada. “The advent of pervasive carnivory, made possible by oxygenation, is likely to have been a major trigger.”
More.
Possibly, but the oxygen debate has been going back and forth for quite a while. Some say a small oxygen jump, possibly triggered by a massive tectonic shift. But others say advanced life does not necessarily flow from lots of oxygen.
The difficulty is, of course, that the more we know about the Cambrian explosion, the more it is going to sound like World War II, in the sense that informed, intelligent, and honest people will offer different theories. What we are most likely closing in on is not The Answer, but reasoned ongoing historical debate.
See also: What Elizabeth Liddle doesn’t understand about the Cambrian explosion
Conclusions: What the fossils told us in their own words
and
Why they call it the Cambrian explosion
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By the way, Steve Meyer’s 2013 Darwin’s Doubt (3:30 pm today EST):
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