How are Different Animals Related?
As far as we know, animals first evolved in the Ediacaran period, about 610 million years ago. By this point, unicellular organisms had already split into plants, animals, fungi, and other divisions. The first known animals resembled cnidarians (jellyfish), and included a number of mysterious carpet, oval, and bag-like forms that ranged up to a meter in diameter (the Ediacaran biota). It is difficult to place the way Ediacaran animals related to later forms, and this is currently under debate. By the dawn of the Cambrian, 542 million years ago, most of these animals had died out and were replaced by numerous phyla generated during what is known as the Cambrian explosion. By the end of the Cambrian explosion, about 500 million years ago, representatives of practically all 38 modern animal phyla had emerged. Even the vertebrates were represented by primitive jawless fish. At this point, there were still no land animals, however. The Cambrian period was an era dominated by invertebrates,
