Second Half of the Paleozoic Era

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

First Half of the Paleozoic Era

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

Precambrian Supereon

In the geochronology of the Earth according to the Geologic Time Scale, the Precambrian supereon accounts for 88% of all geologic time. The Precambrian lasted from the Earth’s beginnings, over 4.5 billion years ago, until the start of the Phanerozoic eon, 541 million years ago. During these 4 billion years it covered the Hadean, Archean and Proterozoic eons.

Hadean eon (~4500–4000 million years ago)

The Hadean eon is the first subdivision of the Precambrian supereon and therefore the first geological eon. It began with the formation of the Earth around 4.5 billion years ago, and ended 500 million years later, 4 billion years ago.

The name “Hadean” comes from the Ancient Greek Ἅδης, the ancient Greek god of the underworld, in reference to the hellish conditions on Earth at the time: the planet had just formed and was still very hot due to high volcanism.

During this eon, the Earth was bombarded for millions of years on end with meteors from outer space. The valleys of craters left by this onslaught of rocks filled up and became the first oceans. Also, because of the heavy bombardment, pieces of rock broke away from the Earth and formed the basis of the Moon. The Hadean is also the eon in which it is suggested that life on Earth began.

“Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy

Archean eon (4000–2500 million years ago)

The Archean eon lasted from from 4 billion years ago to 2.5 billion years ago. Although, in older literature, the Hadean is included as part of the Archean.

The name comes from the Ancient Greek Αρχή, meaning “beginning, origin” and is a reference to the fact that photosynthesis started to take place in this eon.

Even though it is suggested that life on Earth originated earlier during the Hadean eon, we known for certain that certain species of bacteria thrived during the Archean eon. It is also the eon during which the first continents were formed.

“Evolution did not end with us growing opposable thumbs. You do know that, right?” – Bill Hicks

Proterozoic eon (2500–541 million years ago)

The Proterozoic eon is the longest geological eon to date. It is the eon which precedes the Phanerozoic eon in which we currently find ourselves.

The name comes from the Ancient Greek and means “earlier life”. It represents the time just before the proliferation of complex life on Earth.

One of the most important events of the Proterozoic was the accumulation of oxygen in the Earth’s atmosphere. Also, the first appearance of advanced single-celled, eukaryotes and multi-cellular life, roughly coincides with the start of the accumulation of free oxygen. Finally, towards the end of the Proterozoic, the earliest arthropods, fungi and small shelly fauna appear.

“Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.” – Charles Darwin, The Origin of Species

See other: History of the Earth

Geologic Time Scale

When Aristotle (384-322 BC) observed that fossil seashells from rocks were similar to those he found on the beach, he concluded that those fossils were once living animals. He further deduced that the positions of land and sea had changed over time and thought these changes occurred over very long periods.

Of course, Aristotle was in no position to accurately determine the length of those long time periods. Nowadays however, thanks to radiometric dating (the measurement of decay in radioactive isotopes), we know that the Earth is approximately 4.54 billion years old.

“The geologist takes up the history of the earth at the point where the archaeologist leaves it, and carries it further back into remote antiquity.” – Bal Gangadhar Tilak, The Arctic Home in the Vedas

In order to accurately document the enormous history of the Earth, a system known as the Geologic Time Scale has been developed to divide the history of our planet into units. The Geologic Time Scale is made up of the following units of time:

• Supereon (approximately 4 billion years)
• Eon (half a billion years or more)
• Era (several hundred million years)
• Period (between tens and one hundred million years)
• Epoch (tens of millions of years)
• Age (millions of years)

Because each unit of time summarizes the major events and characteristics of a certain geological time span, there are no fixed time frames for the different units of time.

“Geology gave us the immensity of time and taught us how little of it our own species has occupied.” – Stephen Jay Gould, Ever Since Darwin: Reflections on Natural History

See other: History of the Earth