The Phanerozoic eon, in which we currently find ourselves, 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 last three divisions of the Paleozoic era:
Devonian period (419.2–358.9 million years ago)
Even though the Devonian period is called the Age of Fish, it is perhaps more famously marked by our vertebrate ancestors taking to the land. The first vertebrates lived as amphibians; their skeleton – as opposed to exoskeleton animals – is on the inside of their body allowing their skin to be more porous.
Pectoral and pelvic fins of these amphibious tetrapod vertebrates have been observed to gradually change into harder extremities – legs. Some of the oldest fossilised footsteps have been discovered in Poland, dated 397 million years ago. All present-day tetrapods (everything from humans and frogs to crows and geckos) to can trace their ancestry to these Devonian pioneers.
Also, some plants developed a woody covering for back support which allowed them to grow taller than other families of plants in their search for sunlight. The first forests appeared in the Devonian period.
Carboniferous Period (358.9–298.9 million years ago)
The Carboniferous period intensified the forestation of Earth even more. Meanwhile, amphibious lifeforms started to diversify; in order to escape the competition, some gradually developed more tough skin to venture out of the water for longer periods of time. Over time, these animals also managed to lay eggs with a more hardened shell which allowed them to nest on land – these land-lovers would become the ancestors of reptiles.
Permian Period (298.9–252.17 million years ago)
During the Permian, all the continents of the world finally coalesced into one supercontinent, named Pangaea (meaning ‘the entire Earth’). As the globe warmed up and the ice retreated, many areas of Pangaea became very arid; many of the Earth’s forests therefore dried out. Reptiles, however, thrived in this transformed environment.
The Permian ended with the most extensive extinction event recorded in paleontology: the Permian-Triassic extinction event, also know as the Great Dying. 90% to 95% of marine species became extinct, as well as 70% of all land organisms. It is also the only known mass extinction of insects. Its cause is still debated but the most dominant theory is an environmental disaster caused by volcanoes in Siberia.
Ross: I’ll be at the bottom of the dating barrel. The only guys below me will be four-divorce-guy, murderer-guy, and… and geologists.
– Friends (1999) Season 6, Episode 2; “The One Where Ross Hugs Rachel” [No. 123]
See other: History of the Earth