The Phanerozoic, the eon of visible life, got its name from a mistake
Geologists named our current eon the Phanerozoic, Greek for visible life, because they once thought life itself began with it. They were wrong. Complex soft-bodied creatures flourished before it, and microbes go back far further. Still, the last 539 million years are when shells, forests, fish and eventually humans filled the planet.
The American geologist George Halcott Chadwick coined the term in 1930. At the time, the lack of older fossils suggested that life started in the Cambrian, the eon's first period. Since then, traces of several hundred kinds of soft-bodied animals from the preceding Ediacaran period, a burst known as the Avalon explosion, have been studied from the 1950s onward. Scientists now think complex life existed at least as far back as the Tonian period, and simple microbial mats earlier still, in the Archean.
The eon began about 538.8 million years ago, when animals evolved hard shells that fossilise well. It splits into three eras, the Paleozoic, Mesozoic and Cenozoic, and 12 periods. The Cambrian, running from roughly 539 to 485 million years ago, saw more new animal body plans appear than any other period, with armoured arthropods like trilobites dominating the seas. The following Ordovician brought a wave of diversification in corals, nautiloids and early jawless fish, while the first arthropods ventured onto Gondwana, a continent with no animals, and algae washed ashore gave rise to early land plants.
Across the Paleozoic, arthropods, molluscs and chordates rose to prominence, and backboned animals eventually conquered dry land. The Mesozoic produced dinosaurs, including the ancestors of birds, along with turtles, crocodilians, lizards, snakes and mammals. The Cenozoic opened with the disappearance of all non-avian dinosaurs, pterosaurs and marine reptiles, clearing the way for birds and mammals to diversify, and humans appeared late in it.
Beneath all this, the continents kept moving. Plate tectonics gathered them into Pangaea, the most recent supercontinent, which later broke apart into the landmasses on today's maps. Everything before the eon is lumped together as the Precambrian, now divided into the Hadean, Archaean and Proterozoic.
Source: Phanerozoic
