Phanerozoic vs. Precambrian

Phanerozoicnoun

the period from about 5,400 million years ago until the present

Precambriannoun

the eon following the Hadean time and preceding the Phanerozoic eon; from about 3,800 million years ago until 544 million years ago

Phanerozoic

The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. It covers 538.8 million years to the present, and it began with the Cambrian Period when animals first developed hard shells preserved in the fossil record.

Precambrian

The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied.