The Cambrian Period was the first period of the Paleozoic Era, spanning 53.4 million years, begining 538.8 million years ago at the end of the Ediacran Period, and coming to a close 485.4 million years ago at the begining of the Ordovician Period.

You may know it as "Cambrian Explosion", an event that saw the extreme diversification of life and the begining of nearly every animal group around today. Before this life was pretty simple, but the cambrian saw the rise of fast predators and sneaky pray. It isn't entirely clear what caused this drastic change, but it was likely caused by increased oxogen produced by phytoplanton, and changes in ocean chemistry caused by erosion.