Question

What caused the Cambrian explosion?

Answer 1

We should consider multiple causes acting simultaneously, rather than any single stand-alone process, and that we should consider both abiotic and biotic factors.
Sea-level rise would have generated a very large habitable area lying between the base of wave turbulence. Additionally, Cambrian flooding would release nutrients such as phosphate and calcium, which would aid the formation of hard shells.

Some of the processes are biotic: an arms race sparked by near-simultaneous appearance of both predatory and defensive hard tissues across a wide range of animal groups. Some of these processes are abiotic: “Calcium concentrations in seawater increased almost three-fold in the early Cambrian, and this input may have facilitated the origin of biomineralization.

This multifaceted complexity is how scientists think about the causes of the Cambrian explosion