Cambrian period (570 to 500 million years ago) Cambrian period has a significant role in life history on the Earth. The primitive animals, which are available in form of fossils, occurred in this period. Sometimes, this event is called Cambrian Explosion because a little time needed for this purpose. At first, it was assumed that the first and oldest animal fossils were found in early Vendian. Animals developed the hard parts of their body to protect soft ones. For example, external shell of a brachiopod converted to a feeding chamber. Whereas, external tissue of trilobites produced feet and muscles. Animals having skeleton had a better growth rate and formed colonies at the bottom of different seas. Arthropods could swim and moved fast and developed their skeleton. The first animals were small in this period and used phosphate for their feeding. Sponges used silica and bigger animals consumed calcium carbonate as their food. Ancestors of main groups of animals developed in the sea within Cambrian.