The earliest known primitive chordate is Pikaia gracilens, a wormlike creature that swam in middle Cambrian seas. Fossils found in the Burgess Shale of British Columbia show traces of a notochord ...

All animal evolution for the last half billion years has come from tinkering with these Cambrian body plans. Then, between about 570 and 530 million years ago, another burst of diversification ...

Something dramatic happened about 500 million years ago called the Cambrian "explosion," during which an incredible diversity ...