The geological timeline of planet Earth is long…..and convoluted. Committing the whole lot to memory is no easy feat, especially in detail. However, having that information is incredibly valuable within the context of paleontology, since tracking the path that evolution has taken gives us greater context of what we study and where we currently are.
Don’t worry though, there are some tricks to remembering all of that info. The information we, as humans, tend to memorise the easiest are stories and the geological timeline is a story, after all.
Here’s a rhyme that one of my lecturers taught me in my first week of university:
Camels Ordinarily Sit Down Carefully, Perhaps Their Joints Creak, Painfully No-doubt.
COSDCPTJCPN
Cambrian Ordovician Silurian Devonian Carboniferous Permian Jurassic Cretaceous Paleogene Neogene
There you go. Each time period within the Phanerozoic condensed into a rhyme. Granted, it doesn’t include the Proterozoic, Archean or Hadean (also known as the Precambrian) but, in a paleontological context, we know rather little about that time. It also doesn’t include our current time period, the Quaternary. But that’s only one extra word to remember, right? Here it is as a table:
You’ll notice that the largest division is known as an eon. There are 4 eons, the Hadean, the Archean, the Proterozoic and the Phanerozoic. Only the Proterozoic and Phanerozoic are divided into eras, since the lines get very blurry before that. The three main eras we’ll be discussing in the Phanerozoic are the Paleozoic, the Mesozoic and the Cenozoic. These are then divided into the periods outlined above, with periods then divided further into epochs. The further we can divide a certain section, the greater the detail we have of it (since detail gets lost with the slow but constant movements of the Earth).
Still with me? Great.
I’ll be going through each of the periods, touching on the Precambrian before and Quaternary afterwards. The periods are probably the most important parts to remember, since they’re normally what are referred to.
Like any good story, there are chapters:
Chapter 1: The Precambrian
This is a large and vague time encompassing the previous three eons and spans from the birth of our planet, approximately 4.5 billion years ago up to 541 million years ago. The first eon is called the Hadean. In reference to the Greek god of the underworld, this was a time of hell on Earth. The Earth was young and very energetic, forming as nothing more than a lava planet. As denser elements sank into the middle and the outer layers began to cool and harden, the core, mantle and crust were soon formed. At this point, just 500 million years after the start of our planet, the Archean began. This is where things really started to cool and water was introduced to our planet (a whole discussion in itself), along with the start of plate tectonics. In this mix of fluids and gases, was what is known as a ‘primordial soup’, in which proteins began to form the first form of life on our planet: bacteria. This bacteria grew and grew, feeding by way of photosynthesis. Give bacteria a planet to photosynthesise on for 2 billion years and they’ll soon have an oxygenated atmosphere. Thus began the final era of the Precambrian, the Proterozoic. Like the other eons, I’m touching on it VERY briefly, but all you need to know right now is that there were several glaciations, oxygenation of the entire planet and the emergence of the first tiny shelled organisms (microscopic even).
Chapter 2: The Cambrian
Now the real fun begins. This is the first period in discussion and by this point, multicellular organisms were reproducing. Though no life whatsoever was found on land (which was still a rocky, barren wasteland), the oceans were soon teeming with life in what has been dubbed the ‘Cambrian explosion’. This is where fauna showed a rapid diversification, evolving new forms of life here, there and everywhere. Whether or not the Cambrian explosion was due to actual rapid diversification due to all niches being empty, or if it was just due to animals first evolving mineralised ‘hard parts’ (therefore fossilising much easier than anything completely soft-bodied) is unclear. Either way, the Cambrian is where we first see fossils in abundance. Things back then were small and very weird. This includes examples like Hallucegenia and Anomalocaris, which was a giant shrimp-like animal. At only 1 metre long, Anomalocaris was the blue whale of its day, being the biggest animal on the planet.
Chapter 3: The Ordivician
Soon, the Cambrian came to an end with a mass extinction (an event which acts as the divide between all periods). As to what caused this mass extinction, we are unsure of. What we are sure of is that life didn’t lose its momentum. The Great Ordovician Biodiversification Event (GOBE) saw another explosion of life, with many familiar faces starting to dominate, such as molluscs and arthropods. One group of animals decided to try something different. They decided “hey, we have this weird rod of nerves that we can move. Why not try and put our hard parts inside and attach it to this rod?”(you can find a more in depth explanation coming soon). This group were the chordates, soon giving rise to the first true vertebrates: fish. Most of these fish started off as jawless filter feeders, but the guys with jaws and teeth soon rocked up by the end of the Ordovician. Soon, this period came to an end with the first of the ‘big 5’ extinction events (the second worst one ever seen on this planet so far), when an initial glaciation acted as the first ‘pulse’ and rapid temperature increase acted as the second. This wiped out around 80% of life.
Chapter 4: The Silurian
444 million years ago, the Silurian began. Life began to recover quite quickly considering the severity of the last extinction, with marine vertebrates picking up almost where they left off with diversification. The supercontinent Gondwana was still hanging around and a second supercontinent known as Euramerica began forming. For the first time in a long time, the climate actually began to stabilise, remaining at a very warm temperature thanks to a high abundance of greenhouse gases. This stability meant that fauna had a better chance of adapting and thriving and so jawed fish became common along with, for the very first time, terrestrial multicellular life! The land began showing signs of vascular plant life as well as arthropods who were braving short bursts on land. It wasn’t hugely exciting, but it was still a massive step into the explosion that would happen on land. At this point, you wouldn’t have been able to breath very comfortably on land, if at all, due to the low oxygen and high carbon gases. Photosynthesising flora began converting these gases into oxygen, increasing the oxygen content of the atmosphere slowly but surely.
Chapter 5: The Devonian
No major mass extinction (at least not one classed in one of the big 5) happened at the end of the Silurian, rather a few smaller ones. 419 million years ago, the Devonian period began. Commonly and informally known as the ‘age of the fish’, the Devonian is also the defining period of tetrapods. Famous creatures such as Dunkleosteus and other placoderms were from this period, with marine vertebrates (which, at this point, only consisted of a wide variety of fish) flourishing.
It was during this period that the famous image of a fish first crawling onto land is from (though it probably didn’t happen quite as simply as that). Certain arthropods had successfully become fully terrestrial by this point and got first dibs on the land. Though vertebrates began their journey onto land, it isn’t quite known how they made the transition to amniotes (see below) thanks to Romer’s gap. The other very important addition to the land at this time was a great diversity of plants, seeing the earliest known vascular plants with true roots and wood-like structures. This larger influx of oxygen in the air led to the next period...
Chapter 6: The Carboniferous
Much like before, the Devonian mass extinction was actually a series of smaller extinctions that were clumped together as another one of the ‘big 5’. The cause of this one, however, is a mystery, despite many theories. So began the Carboniferous approximately 359 MYA. The name comes from the abundant coal-bearing rocks from this time, as vast forests, swamps and jungles had now become well established for the first time. Tetrapods had especially come a long way during this time, as they were simply semi-aquatic animals who still required water for reproduction (amphibians) during the Devonian, to now diverging into amniotes (fully terrestrial tetrapods who get around the wet reproduction issue by storing the embryonic eggs either in a hard shell or within themselves before giving live birth). It was here that the very distant ancestors of archosaurs and mammals diverged from each other (though they were still very much reptiles). What people typically think of with the Carboniferous, however, are the big-as-hell creepy-crawlies. The oxygen content was much higher at this time, with vast flora pumping oxygen into the air and not as much fauna breathing it back in. The arthropods gorged on the stuff and, due to how they breath using a sort of air duct system, were able to evolve contenders such as Meganeura, a dragonfly the size of a hawk and Arthropleura, a centipede reaching lengths of up to 8 feet!
Chapter 7: The Permian
Approximately 299 MYA, yet another extinction event occurred. This time it was due to climate change which affected mostly the land. The continents collided to form the supercontinent landmass known as Pangea, meaning that most of the land was now so far from the Ocean that it dried up and lost its rainforest-like humidity. Large glaciations also occurred around this time, leading to a much cooler and drier climate than most of life was used to. This drier climate led to only a few types of reptiles flourishing, namely the groups of synapsids. The two notable groups within the synapsids are the pelycosaurs, whose most famous member is Dimetrodon. The other group are the therapsids, which include all mammals (though there weren’t any around at the time) and all their ancestors. The other major group were the archosaurs, the group that would later include crocodilians, dinosaurs and pterosaurs, though these remained small and secondary compared to the synapsids. That’s right, not only was this guy:
more closely related to US than he was to dinosaurs (so next time you see them in a bag of dinosaur toys in a shop, go full Karen and ask to speak to the manager), but also the mammals(-ish) nearly took over much sooner than the dinosaurs were even around! But alas, yet another extinction hit, this time more disastrous than ever before….and more than we are yet to see.
Chapter 8: The Triassic
The great dying. It was named such because this was the time when life came unbearably close to being completely over on planet Earth. The end Permian extinction wiped out 98% of all life. Just imagine that. Think of everyone and everything, all the people, the annoying bugs, the birds in the trees, the fish in the sea, everything. All but 2% dead. Yet no one can agree on just what happened. As you can imagine, life took a very long time to recover. At the beginning of the Triassic, 251 MYA, virtually nothing was left, except for the odd tiny animal who would only venture from their burrow every now and then for scraps. It took a whole 10 million years for life to return to its levels of before, but this time with reptiles even more diversified in body plans than ever before. By the late Triassic, what was left of the therapsids had fully evolved into mammals as we know them today, whilst the archosaurs birthed the dinosaurs and the first ever vertebrates to ever fly: pterosaurs.
Chapter 9: The Jurassic
Climate change eventually brought around the end-Triassic mass extinction, mostly affecting marine life 201 million years ago, marking the start of the Jurassic. Here was where things started to look a little more recognisable. The supercontinent of Pangea begun to break into two (Gondwana to the south and Laurasia to the north). Crocodilians decided to make the transition back to a more aquatic life, kept company by the many aquatic invertebrates and reptiles such as plesiosaurs and ichthyosaurs. Meanwhile on the land, mammals trudged along whilst the dinosaurs got big...real big...like...sauropod big. The biggest sauropod from this period was literally called Supersaurus, with a length of 110 feet. Other familiar faces started to show up too, such as Diplodocus, Allosaurus and Stegosaurus, as well as true turtles. It was the mid-Jurassic that the arguably most important fossil discovery was deposited. The face of Darwinian evolution, Archaeopteryx, lived at this time, showing the missing link between dinosaurs and true birds!
Chapter 10: The Cretaceous
145 million years ago, the Jurassic ended and the Cretaceous begun. The warm climate from the previous two periods hung around, during which there were no polar ice caps. The landmasses were now very close to their modern placement, however looked fairly different thanks to sea levels. North America, as an example, had a shallow sea running straight through it:
Generally, mammals were still very small, but were starting to make headway in small steps. Some mammals even hunted smaller dinosaurs! The Cretaceous also marks the birth of flowering plants, along with many familiar looking insects. It was the time when dinosaurs reached their peak diversity and size! Despite the most famous dinosaurs being included in Jurassic park, most of them were in fact from the Cretaceous, including Tyrannosaurus rex, Triceratops, Velociraptor and Spinosaurus. Dinosaurs were also able to reach truly gigantic proportions, with Spinosaurus aegypticus being the largest terrestrial predator to ever exist and Argentinosaurus huinculensis being the biggest land animal to ever walk the Earth! Quetzalcoatlus northropi was also the largest animal to ever fly, being as big as a fighter jet!
The waters were also home to diverse and massive animals, such as mosasaurs and the monstrous Deinosuchus. In general, archosaurs were at their peak and many familiar families had begun to diversify. A famous disaster would soon strike though...
Chapter 11: The Paleogene
At this point, many paleontologists refer more to specific epochs within the periods as things are a little less hazy. The epochs within the Paleogene are the Paleocene, Eocene and Oliogene. 65 million years ago, a sizeable void of life was left in the wake of the Asteroid impact. The domino effect of such an impact meant that the dominant groups died off within a thousand years or less. Within this void was a large amount of ecological niches that needed filling once more, as well as a sharp increase in temperature across the globe. Reptiles got the spring back in their step pretty quick, with turtles and crocodylians diversifying rapidly during the global warming at the end of the Paleocene, as well as monstrous beings such as the Titanaboa:
Believe it or not, dinosaurs (or at least a certain group, namely the avian ones) recovered fairly well too! Familiar groups of birds begun to diversify, filling the empty niches in the sky left by the pterosaurs, along with unfamiliar ones, for example the aptly named 'terror birds' of South America:
But, of course, we are now in the Cenozoic era and it is, after all, named the 'time of the mammals'. Therefore it was the mammals that finally had their day. Though they remained unchanged at first, there was a rapid diversification and spreading of most mammal groups by the end of the Paleocene. Certain groups took to certain niches. Some stuck to more open plains and evolved into the large number of hoofed mammals (ungulates), some took to the trees and became primates, some went back to the sea and became cetaceans (the group that includes whales and dolphins), others specialised the claws and/or teeth and became the first cats and dogs.
Chapter 12: The Neogene
23 million years ago, the Neogene period began, consisting of the Miocene and then the Pliocene. North and South America had become connected (spelling doom for the terror birds since they couldn't compete with the mammalian predators from North America), the first humans began showing up in Africa and most mammal groups that we see today began to take more familiar forms. The Earth had also began cooling rapidly, bringing about the famous ice age! It is this period that most think of when they think of prehistoric animals that aren't dinosaurs, such as woolly mammoths and sabre toothed cats.
We also can't forget the monsters that evolved from the deep, with whales finally reached their collosal size, as well as the infamous Megalodon!
Now.
The current period is known as the Quaternary, with the epochs being called the Pleistocene and Holocene. The biggest thing to happen on this planet is...well... us. So much so that many paleontologists and geologists propose that the current epoch be named the 'Anthropocene', beginning at the time when humans began to have a real impact on the Earth and its atmosphere. Humans first popped up around 2 million years ago and there were lots of us, from Homo erectus, to Homo neanderthalensis, even to small hobbit-like species, Homo floresiensis:
Homo sapiens (us) are now the only species of human left. You'd think that would mean that the genus of Homo would be in trouble, but we've more than made up for being the last species. However, we might still be in trouble. Homo sapiens first showed up around 100,000 years ago, with modern interpretations of culture first appearing in around 4000 BCE. We started really affecting the planet in the last 300 years, forcing rates of climate change that would normally take hundreds of thousands of years and implementing, what some paleontologists argue, is the next big mass extinction. We are becoming more and more aware of the effect we are having on our planet and, hopefully, will soon be able to mitigate, even reverse it. One thing is for certain though, the effect we have had will forever be seen in the rock record, having happened in a geologic instant.