The Cambrian Explosion 545 Million years ago
The Cambrian period, which began the Paleozoic era, produced the most intense burst of evolution ever known. The Cambrian Explosion saw an incredible diversity of life emerge, including the major animal groups alive today. Although it is called an “explosion”, it really took 10 to 30 million years for these groups to develop. The amazing thing is that these organisms suddenly appear in the fossil record when there were no other organisms like them before.
Among them were the chordates, to which vertebrates (animals with backbones) such as humans belong. So the Cambrian explosion produced the body plans of the organisms that exist today. By body plan is meant organisms that have two matching left and right sides of their bodies, like two eyes, two ears, two wings, etc. which still exists today.
What sparked this biological explosion isn't clear. It may be that oxygen in the atmosphere, thanks to emissions from photosynthesizing cyanobacteria and algae, were at levels needed to fuel the growth of more complex body structures and ways of living. The environment also became more hospitable, with a warming climate and rising sea levels flooding low-lying landmasses to create shallow, marine habitats ideal for spawning new life-forms.
The Cambrian Explosion evidence is found in animals with jointed, external skeletons known as arthropods—the ancestors of insects, spiders, and crustaceans. These toughened-up creatures represented a crucial innovation: hard bodies offering animals both a defense against enemies and a framework for supporting bigger body sizes. But the best evidence for this explosion of life is found in the Burgess Shale located in British Columbia, Canada. (The Canadian Rockies) This fossil find included hard shelled organisms, soft bodied organisms and plants.