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The Phanerozoic Eon is the most recent of the four geological eons and is defined as the period of time during which macroscopic hard-shelled multicellular organisms existed. It is divided into three eras and is the time when life as we know it evolved and covered the planet. The eras are separated by massive extinctions, and the Phanerozoic can be divided into a number of informal “ages”. The Mesozoic era was dominated by dinosaurs, and the Cenozoic had the “Age of Mammals,” which culminated in the evolution of Homo sapiens.
The Phanerozoic Eon is a geological time division extending from approximately 542 million years ago to the present. The name derives from the Greek and means “revealed life”, because the Phanerozoic Eon is defined as the period of time during which macroscopic hard-shelled multicellular organisms existed, starting with trilobites, archaeocytes and a few other primitive genera. The Phanerozoic is the most recent of the four geological eons that divide time on Earth since its formation: the Hadean, the Archean, the Proterozoic and the Phenerozoic.
While it comprises only about 10% of Earth’s total age, it is during the Phanerozoic Eon that the life we are familiar with evolved and covered the planet. Before the Phanerozoic, the only living things were numerous unicellular organisms and some early multicellular blob- and disk-like organisms called Ediacaran biota.
The Phanerozoic is divided into three eras: Paleozoic, Mesozoic and Cenozoic. In Greek, these terms mean: early childhood, middle age and recent life. The eras are separated from each other by massive extinctions, the most recent occurring just 65.5 million years ago, wiping out all non-avian dinosaurs and paving the way for the evolution of modern mammals.
In terms of life, the Phanerozoic can be divided 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 major body plans still used by all animals to this day evolved. This is often called the “age of invertebrates” due to the many genera of invertebrates that emerged and filled the oceans. This was the early Paleozoic, when life had just touched the earth.
During the Middle Paleozoic, fish were the most numerous organisms, and consequently it is 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 force similar to that of a great white shark. The land was still largely devoid of life at this point, but some fungi and worm-like creatures colonized the shores.
During the late Paleozoic, Carboniferous and Permian periods, reptiles evolved and gained the ability to lay hard-shelled eggs, allowing them to be less dependent on water and to conquer most of the land. As plants also moved onto land, huge tropical forests flourished. When these plants died and were compressed over hundreds of millions of years, they created the energy-rich coal seams we mine today. The Late Paleozoic is sometimes called the “Age of the Tetrapods”.
The Mesozoic era was dominated by dinosaurs and is therefore called the era of dinosaurs. This is the period of ancient history that most attracts the attention of paleontologists and the lay public. More recently, during the Cenozoic, we had the “Age of Mammals,” which culminated in the evolution of Homo sapiens, now obviously the dominant organism on Earth.
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