All 70 or so phyla of complex organisms appeared at once at the start of the Cambrian era with very few multicellular organisms evidenced before. While some recent findings point to a few complex phyla of origin before, most of them have not been found.

Changes in oxygen level is one theory put forth to explain it. Some kind of tipping point may have been reached that injected lots of oxygen into the oceans. Organisms in current very-low-oxygen environments are rather sluggish and seem more interested in mere survival above hunting or hiding, which generally matches Precambrian macro-life.

See wikipedia entry: http://en.wikipedia.org/wiki/Cambrian_explosion