The Phanerozoic Eon is a geologic time division that extends from roughly 542 million years ago until the present. The name derives from Greek and means “revealed life,” because the Phanerozoic Eon is defined as the period of time during which hard-shelled macroscopic multicellular organisms, beginning with trilobites, archaeocyatha, and a few other early genera, existed. The Phanerozoic is the most recent of four geologic eons that divide up time on Earth since its formation: the Hadean, Archean, Proterozoic, and Phenerozoic.
Although it only encompasses about 10% of the Earth’s total age, it is throughout the Phanerozoic Eon that the life with which we are familiar evolved and covered the planet. Prior to the Phanerozoic, the only living things were numerous unicellular organisms and some blob-like and disc-like early multicellular organisms called the Ediacaran biota.
The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. In Greek, these terms mean: early life, middle life, and recent life. The eras are separated from one another by massive extinctions, the most recent occurring only 65.5 million years ago, wiping out all the non-avian dinosaurs and paving the way for the evolution of modern mammals.
In terms of life, the Phanerozoic can be divided up into a number of informal “ages.” At the beginning of the eon, in the Cambrian period, a massive evolutionary explosion occurred, referred to as the Cambrian explosion. During this remarkable period of diversification and evolution, all of the major body plans still used by all animals to this day evolved. This is often called the “Age of Invertebrates” because of the numerous invertebrate genera which emerged and filled up the oceans. This was the early Paleozoic, when life had barely yet touched the land.
During the middle Paleozoic, fish were the most numerous organisms, and it is accordingly called the “Age of Fish.” This is around the Silurian and Devonian period. The largest of these fish, the apex predator Dunkleosteus, was the size of a school bus and had a bite with similar force to that of a Great White Shark. The land was still largely lifeless at this point, but some fungi and wormlike creatures colonized the coasts.
During the late Paleozoic, in the Carboniferous and Permian periods, reptiles evolved and gained the ability to lay eggs with hard shells, allowing them to be less dependent on water and conquer most of the land. As plants moved onto the land as well, massive tropical forests flourished. When these plants died and were compressed over hundreds of millions of years, they created the energy-rich coal beds which we mine today. The late Paleozoic is sometimes called the "Age of Tetrapods."
The Mesozoic era was dominated by the dinosaurs and is accordingly called the Age of Dinosaurs. This is the period of ancient history that gets the most attention from paleontologists and the lay public. Most recently, during the Cenozoic, we have the "Age of Mammals," which culminated in the evolution of Homo sapiens, now quite obviously the dominant organism on Earth.