The Phanerozoiceon is the current eon on the geologic timescale. It is the one in which abundant animal life has existed. It covers about 541 million years, and goes back to the time when hard-shelled animals first appeared. It was once thought that life began in the Cambrian, the first period of this eon.
During the Phanerozoic the biodiversity shows a general increase from near zero to several thousands of genera.
The exact time of the boundary between the Phanerozoic and the Proterozoic is slightly uncertain. In the 19th century, the boundary was set at the first abundant metazoanfossils. But several hundred kinds of Proterozoicmetazoa have been discovered in the Ediacaran. Study of the Ediacaran biota started in the 1950s. There are three dividing points at the Proterozoic-Phanerozoic boundary. It might be where the first trilobites and archaeocyatha appear; or at the first appearance of signs of burrowing; or at the first appearance of the 'small shelly fauna'. The three different dividing points are within a few million years of each other.