摘要
Much mystery surrounds the sudden and mushrooming appearance of animals in the fossil record about 540 million years ago. Known as the ＂Cambrian explosion,＂ the change took place from zero evidence to all the basic types of creatures in today＇s world within a few million years. Although paleontologists believe that can＇t be the full story, they have met tremendous difficulties in finding Pre-Cambrian animals.
