Fossils Challenge Theory of Rapid Animal Evolution in Cambrian

Most major animal groups appear for the first time in the fossil record some 545 million years ago in a relatively short period of time known as the Cambrian explosion. The explanation of this sudden arrival is a scientific conundrum.

The fossil record suggests that exceptional evolutionary activity took place over 10 million years at the base of the Cambrian and generated the ancestors of nearly all the animal groups living on Earth today, as well as others that failed to see modern times.