The Great Dying

Permian extinction
252 million years ago

Just as the reptiles were flourishing, life on Earth faced perhaps its greatest challenge. The Permian extinction was the worst mass extinction in the planet’s history, obliterating up to 96% of marine species and similar numbers of land animals. We don’t know for sure what caused it, but massive volcanic eruptions – creating what is now the Siberian Traps – may have been to blame. In the aftermath, the first dinosaurs evolved.

See other: History of Life


300 million years ago

For the last time, all Earth’s continents came together to form one giant supercontinent. Known as Pangaea, it was surrounded by a world-spanning ocean called Panthalassa. It lasted until 175 million years ago, when it began to tear itself apart over tens of millions of years. Its shattered remnants became the familiar modern continents.

See other: History of Life

Fish that Walk on Land

From fins to legs
375 million years ago

With plants well-established on land, the next step was for animals to move out of the water. Insects were among the first, around 400 million years ago. But they were followed soon after by big, backboned animals such as Tiktaalik, a fish that looked a bit like a salamander. Fish like Tiktaalik would eventually evolve four limbs, and give rise to amphibians, reptiles and mammals. It may be a good thing it left the water when it did, as soon afterwards the Late Devonian Extinction wiped out many marine animals, including some terrifying-looking armoured fish.

See other: History of Life

Meat On The Menu

A varied diet
3.5-1.8 million years ago

Our oldest ape-like ancestors mostly ate fruit, but later species like Australopithecus branched out. As well as eating a wider range of plants, such as grasses, they seem to have eaten a lot more meat, and even butchered it with stone tools. More meat meant more calories, and less time spent chewing.

See other: What Makes Humans Human?

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

Evolution 101

‘Consider Darwin’s original phrase natural selection: everything from Cuckoo Birds that lay their eggs in the nest of other birds, to giraffes whose long necks are good for reaching food in high trees, to humans whose brains make up for their fragile bodies, are selected for, naturally.

An even better phrase would be non-random selection or maybe even non-random elimination. While all genetic mutations are generated by a random copying error or random variation completely beyond the animal’s control, the selection of those traits is not random.

Successful variations that allow you to survive and reproduce are determined by the very specific circumstances of your environment, where elimination – death – might not be far away.

So, the selection of your traits is done by a very specific and sometimes brutal list of criteria. This is why people who say that they don’t understand how all animals could have “evolved by chance” don’t really understand how evolution works.

Here’s another phrase that doesn’t get it right: “evolution is just a theory”. In everyday speech, theory means guess; but in science, a theory is something that was tested time and time again, explains many different observations and is backed up by a mountain of evidence.

Evolution is a theory like gravity is a theory, and you don’t go jumping out your window because gravity is “just a theory”.

Why are we so certain? Emily knows:

Evolution is one of the most tested, most utilised and most widely accepted theories in science. It’s backed up by literally tonnes of fossil evidence which can show us shared traits with species that no longer exist, and help us map out lines of descent for creatures that are around today.

DNA sequencing further tells us about lines of descent and you can measure the commonality of the DNA possessed by two animals to tell how closely related they are, and when they may have split off from a common ancestor.

Radiometric dating allows us to assign dates to various fossils, further helping us map out lines of descent.

Then there’s the simple fact that extinct species are always found in the same rock layers you’d expect to find them. Which is why you don’t see a bunny skeleton in Cambrian rock layers from half a billion years ago. That’s also how we know that Dimetrodon is not a dinosaur.

Closely related species are often geographically distributed near one another. That’s not to mention that we can see evolution happening before our very eyes: whether it be the discovery of a new species that recently moved into a different environment, the development of newly adapted bacteria into superbugs, the evolution of new breeds of rapidly reproducing insects, or the almost constant changes in gene distribution in animal populations all over the world.’

– Green. H., Graslie. E. (2014, October 29) The Evolutionary Epic: Crash Course Big History #5