Remember when you were a kid, and all of the sudden, for no apparent reason, you shot up several inches one summer? Like growth wasn’t a slow, glacial process but an abrupt one? Sort of like some of the theories on evolution that suggest mutations aren’t as gradual as we think. (A Cambrian Explosion, per Jim C.)
Although, when you look closely at the theories, the idea of a real ‘explosion’ is probably a misnomer:
The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 542 million years ago, of most major animal phyla, as demonstrated in the fossil record. This was accompanied by major diversification of other organisms.[note 1] Before about 580 million years ago,[note 2] most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude[note 3] and the diversity of life began to resemble that of today. All present phyla appeared within the first 20 million years of the period, with the exception of Bryozoa who made its earliest known appearance in the upper Cambrian.