In the history of the Earth, the Phanerozoic eon is the current geologic eon; it is the successor to the Proterozoic, Archean and Hadean eons and accounts for 12% of the Earth’s existence, covering a space of time from 541 million years ago until today.
The Phanerozoic eon is divided up into three eras: the Paleozoic, Mesozoic and the Cenozoic. The Paleozoic era, which spans 541 to 252.17 million years ago, contains the following periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Let’s consider the first three divisions of the Paleozoic era:
Cambrian period (541–485.4 million years ago)
The Cambrian period is marked by the appearance of mineralised organisms and the most rapid increase of the number of phyla on Earth: between 580 and 530 million years ago, Life seems to have experienced an acceleration – this evolutionary development is known as the Cambrian explosion. The Chordata, the phylum of vertebrates that at present includes mammals, fish, amphibians, reptiles and birds, originated during this period.
“The earth has music for those who listen.” – George Santayana
Ordovician period (485.4–443.4 million years ago)
The Ordovician period was characterised by high global sea levels.The wide shallow continental seas were becoming evermore diversified with molluscs, arthropods and fish; the genera of marine fauna increased fourfold. Although the first vertebrates – fish – appeared in the Cambrian, jawed fish evolved in the late Ordovician. Also, photosynthesis-based life made its first tentative steps out of the sea.
“Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty.” – Albert Einstein
Silurian period (443.4–419.2 million years ago)
The most significant event of the Silurian period was the emergence of terrestrial arthropods; these tiny ancestors of centipedes and arachnids pioneered the Earth’s surface. Another impressive new sight was the evolution of bony fish and the development of mosses and vascular plants – the phylum of the flowering plants, ferns, et cetera.
“The clearest way into the Universe is through a forest wilderness.” – John Muir
See other: History of the Earth