Geologically, this Earth since its formation (about 4.6 billion years ago) until now, is divided into several epochs. The first period is called "Precambrian" (meaning "Before the Cambrian") in which in the middle of this period (according to the Theory of Evolution) life began to appear. This is evidenced by the appearance of the first fossils in the form of bacteria that are 3 billion years old. The second period, called the "Paleozoic", is divided into several eras, of which the most important is the "Cambrian", where simple life suddenly "exploded" into a multitude of very complex types of living things