The Science of Everything Podcast - Episode 157: The Geologic Time Scale

Episode Date: January 17, 2026

A survey of the Earth's history through the lens of the geologic timescale. We begin with a discussion of the terminology of geochronologic units and how they are specified using 'golden spikes'. We then review the development of Earth's atmosphere, geosphere, and biosphere through the Hadean, Archean, Proterozoic, and Phanerozoic eons. We cover many topics including the supercontinent cycle, the great oxidation event, the evolution of eukaryotes, the Cambrian explosion, and mass extinctions. Recommended pre-listening is Episode 156: Fossils and Dating Methods. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

listening to The Science of Everything podcast, episode 157, the geologic timescale. I'm your host, James Fodor. Today we're going to talk about the history of Earth, focusing, of course, on the geologic timescale, which is a way of assigning dates to particular segments of rock and corresponding periods of history throughout the development of Earth. And we're going to talk about the main periods in the geologic timescale and what major events carry. characterize those. We'll talk about the changing composition of the atmosphere, the supercontinent cycle, the different phases in the evolution of life, and also mass extinction events. Recommended pre-listening is the previous episode 156 fossils and dating methods. So just picking up from some of the things we discussed in that episode, let's begin by talking about some terminology.

So you may recall from the previous episode that we distinguished between geochronology and chronology. stratigraphy, which is, again, quite confusing because both of them seem to be about dating of rocks. But the difference is, effectively, whether we're referring to rock segments or rock layers or periods of time. And that's the main distinction that we introduced last time. So a geochronologic unit is a period of time. It's a period of time in which a particular unit of rock was formed. A stratigraphic unit is the unit of rock. It's the layers of rock that are identifiable and have a particular age and a distinctive in some way so that they can be mapped and so forth. So you've got your rocks and they've got your period of time in which the rock was formed.

Here in this episode we're mostly interested in the geochronology. So that's the periods of time in which rock units are formed. So the geologic timescale is a division of the times over the history of earth based on geochronologic unit. So it's based on identifying different segments of rock and when they were formed and mapping that over the course of history and also the course of the service of the earth and different rock formations there. And these units of time and corresponding rock segments are all specified and agreed upon by an international scientific body

called the International Union of Geological Sciences. And so the way this is done, at least for more recent periods, as we'll talk about in a moment, is that they identify a... particular boundary which will mark the temporal boundary between two geochronologic units so periods of time effectively and this is identified with reference to a particular stratigraphic unit so that's a particular unit of rock which will be located at a specific location at a particular point like on earth and and vertically in layers of rock and so once a particular boundary point has been agreed upon

based on numerous factors such as its significance in the in the history of earth its ability to be readily identified and how relevant how useful it is for like extrapolating across different regions of earth because it's more useful if it's a some sort of formation that has that's more extensive across space or is at least identifiable with like correlates elsewhere that's more useful than one that's like highly localized so there's many factors that they consider when deciding on these boundary points, what points will mark the boundary between geochronologic units. But once they've identified that, they actually physically insert a, what's called a golden spike, it's basically a piece of metal, which is driven into a geologic section, like

physically into the rock at the relevant vertical position, like between certain layers. And that serves as the marker, a precise marker for current and future geologists, of the boundary between stratigraphic units. and therefore the boundary between the corresponding geochronological units, like the periods of time. So that's what this phrase golden spike refers to. It's sort of like used in an abstract sense, but more literally it is, it's not made of gold, it's made of metal, but it's a physical marker that's driven into a particular stratigraphic units, a region of rock, which marks the boundary between geochronologic units after extensive discussion and agreement by relevant scientists.

So that's how this is all determined. Now, let's talk about the terminology for different units of time. Now, I've been a bit loose here and just using the word period so far, but I should be a bit more precise because different units in geochronology have different names, and period is actually one of those. So we need to be a bit careful here. So let's start from the highest level. The highest level, the longest unit, is called an Eon. There are four of these, and they last for often billions of years, at least like more than half a billion years. So they're very, very long units of time. And we'll talk about them in more detail, but they're exactly four, and that's unlikely to be changed anytime soon. It's the Hadean, the Archaeoan, the Proterozoic, and the Phanozoic.

Now the next level down is called an era. There are 12 of these. So they're usually hundreds of millions of years long. The Archeon and the Proterozoic eons are divided into several of these that I've actually been changed recently into exactly how those are subcategorized. But those are less important. The most important three eras are those of the phanaerozoic eons. So the phanaerozoic ion is the is the ion in which we're currently live. And that's where most of our knowledge of Earth's past takes place. It's about the last 540 million years or so. Again, we'll go through this in more detail in a moment, but just to situate yourself, the Phanozoic Eon roughly corresponds to the period in which macroscopic animal life and plant life as well.

Animal and plant life has existed either in the oceans or on the surface of the earth. Prior to that, there was life of various forms, as we'll discuss, but it did not, there was not macroscopic animal or plant life, at least not to any significant scale. So roughly we're talking the last 540 million years, the Phanerozoic Eon, pretty much all fossils that you think of as fossils, like scale. skeletons and preserve large organisms and things like that. They all date from this time. And that's how the boundary between the proto-ozoic and the fanerozoic is defined. It's defined by essentially the beginning of the fossil record of hard fossils and macroscopic, like large organisms. So the point there is that the three eras that matter most are those of the fanerozoic-eon.

And they're called the Paleozoic, the Mesozoic, and the Cenozoic. So it's kind of easy to remember. It's like old, middle, and new. One way to think about this is that the Paleozoic is pre-dinosaurs and the origin of organisms like fish, amphibians, and the first tetrapods to walk on land. The Mesozoic is the period of dinosaurs and other large reptiles. And the Cenozoic is the most recent era in which mammals have been dominant, and that's where we're currently living today. So the level down from era is called a period.

There's about 20 of these. We're currently living in what's called the Quartanary Period. and each of the errors, or at least the more recent ones, are divided into sort of two or more of these periods. Next level down is called an epoch. So periods last for maybe dozens of millions of years. An epoch usually lasts for a few million years. There's about 40 to 50 epochs. It depends on exactly which.

The numbers differ between source because it depends on exactly which version of the timescale you're talking about because it's regularly updated. But the exact numbers don't matter here. The point is that the epochs last for some number of millions of years, and there's 40 to 50 of these. The current epoch we're living in is called the Holocene, so that's a term you may have heard before. That's roughly the last 10,000 years, the times in which agriculture has existed,

and the time since the beginning of the most recent interglacial period, which is where we're currently living. And the lowest geochronological unit is called an age, and this isn't used very often, at least in the readings that I've done. And these will last some various number of millions of years, and there's more than 100 of these to find. So the units of time that we're going to be talking about mostly are going to be eras and periods, because that's a convenient level of resolution for what we're going to be discussing. Epoch is a bit too specific for most of our purposes, and Eon is too general. So the terminology

is unfortunately confusing because of the terms for units of time, three of them begin with E. and it's sort of not obvious initially what is going to be longer and what's going to be shorter. But I'll do my best here to try to make things as clear as possible. So now that we've introduced some terminology, let's go through important units of geological time in a little bit more detail, and we'll talk about some of the major events that have happened there. So I'll sort of do a bit of repetitions to try to help you to remember the different units of time and how they relate to each other. So we'll go through these things sort of an increasingly more detail.

details. So don't worry if you don't pick it up on the first go through. So you recall that the longest unit of geochronology is the eon. There's exactly four of those. So let's go through the four eons again. So the first is the Hadean eon. So that began with the formation of Earth about 4.6 billion years ago. And it ends with the earliest known rock formations, roughly, about 4 billion years ago. So this is the early time of, like after the formation of Earth, but there's still a lot of, but there's still a lot of changes happening

in terms of the Earth's composition of the atmosphere and in the, with the differentiation of the internal layers of Earth and asteroid bombardments and things like that. So it's a very high energy. Life is thought to have emerged just at the end of the Hadean eon. So the next ion is the Archaeon, and that lasts from 4 to 2.5. billion years ago. It begins with the formation of the first rocks, as I mentioned, and it ends

2.5 billion years ago. So I've not actually been able to find a very clear reason as to why the Archaeon ends at 2.5 billion years ago when the proto-ozoic starts then. Like, what is it that demarcates the two? With the Archaeon and the Phanozoic, Phanerozoic being the most recent eon, it's very clear that the Hadean ends with the formation of the first rocks, and the fanerozoic begins with the time in which the first hard-bodied like large fossils are found with the Cambrian explosion, which we'll talk about more in a bit. So those are very clear. In between those two are the archaeon and the proto-ozoic eons. The archaic ends 2.5 billion years ago and the proto-ozoic begins at that time. The question is, what is the significance of 2.5 billion years ago? As far as I can

tell, this was originally defined basically just by dividing the boundary between the end of the Hadesian, and then the beginning of the phanerozoic into roughly two parts. The duration of time from the end of the Hadesian to the beginning of the phanaerozoic is about 3.5 billion years. So I think that this was effectively just divided roughly into two. The first 1.5 billion years and then the second 2 billion years being divided into the archaeon and the protozoic. I'm not 100% sure about that.

If anyone knows the details here, you can send me an email. However, there are a few important events that did occur around 2.5 billion years ago, which can be roughly thought to demarcate the boundary between Archaon and Protoozoic, even though I'm not sure that that was the original basis of this. So the most important thing that happened approximately 2.5 billion years ago, although it didn't happen at any precise time. It was a long process. It was something called the Great Oxidation Event.

This was the time in which oxygen began to accumulate in significant levels in the Earth's atmosphere as a result of photosynthetic activity of microorganisms, bacteria. And I'll talk more about this a bit later, but basically this resulted in probably one of the earliest mass extinction events and substantial change in the atmospheric composition as well as effects on life as it existed at the time, which was just microbial life. So 2.5 billion years ago approximately is the time when the great oxidation event occurred. And so that's sort of a useful boundary between the archaean and the protozoic. Another thing that happened around this time, although again, give or take a few hundred million years,

is the beginning of the supercontinent cycle. Today, Earth's landmasses are divided into five or seven continents, depending on how you count them exactly. But historically, there have been times in which the continents have been all or mostly joined up together to form what's called a supercontinent. I think that that's defined as something like when 70% or more of Earth's landmass is united into a single continent. Again, precise definitions vary a bit. But looking back historically, what we find is that this process is cyclical.

So there's periods in which the continents are all joined up together to form a supercontinent, and then there's periods of time in which the continents are separated, as is the case today. And there have been several of these cycles throughout Earth history. It takes hundreds of millions of years to go through the cycle. And this is ultimately driven by plague tectonics. I've discussed that in previous episodes. If you're interested, I won't get into the details too much here. But basically convection in the mantle results in movements, a creation of new oceanic crust,

which moves the continents relative to each other. The very first supercontinent is thought to have been formed approximately 2.7-ish billion years ago. Now this is still uncertain, and particularly because going back that far, the evidence is fairly limited. But anyway, the point is that before that, there was relatively little continental crust, certainly for the first billion years or so after the formation of the first rocks. The earth was actually mostly a water world. There was some continental crust, but much less than today. And so most of the surface of the earth was just covered by water.

In fact, very early on, at the end of the Haiti, and there was likely no continental crust that was visible at the earth. surface so the earth was completely covered by oceans and then gradually more continental crust has formed over time. The point there is that 2.5 billion years ago is a sort of convenient boundary because it roughly marks the time of the great oxidation event as well as roughly the beginning of the supercontinent cycle. Although I think that 2.5 billion years is just kind of a convenient demarcation. I don't know that it was chosen for either of those specific reasons. So a little bit messy there than we might like. Now, a few other things to note.

So I think I mentioned that at the roughly at the end of the Hadean eon, about 4.1 billion years ago, it's thought that life emerged at this time. So quite early in the history of Earth, this would have been, well, initially even very primitive forms of microbial life, which eventually developed greater complexity, but essentially archaea and bacteria. I've talked to many years ago I did an episode on The Origin of Life, so you can check that out if you're interested. I won't talk about those processes here, but it's, as I said, the thought to have occurred probably in shallow regions of the oceans where there was geothermal springs that provided energy that facilitated the formation of organic molecules and the catalysis

required to join them together. There's many other aspects to that, but that's thought to be where life originated, so in the oceans, then the shallow oceans, and very early in the earth's formation. Now, for the next two billion years or so, maybe even a bit longer, two to two and a half billion years, the only life that existed on Earth was bacterial life or archaea and bacterial life. So no macroscopic life. I mean, there were mats of bacteria that existed, so they would grow over a surface in some of these fossilized, now called stromatolites. These are sort of fossilized mats of bacteria.

So they could be macroscopic in that sense, but that's just because there's many bacteria growing on a surface or growing near each other and forming a sort of a film that covers the surface and is identifiable. But that's not a single organism. And there's no specialisation of tissue of cells into different tissues or anything like that. So we're still talking about individual microscopic organisms. And they were all the life that existed on Earth for at least the first two billion years. That's definitely into the protozoic eon. And it may have even been longer than that.

But it's not really known when eukaryotic life first developed. Remember, prokaryotic life is essentially bacterial life. They're much smaller and simpler. Eukaryotic life are larger cells, or many of them are larger, and have much more complex internal cellular machinery. It's thought that eukaryotic life probably originated around the time of the great oxidation event or slightly afterwards. They didn't cause the great oxidation event.

Remember, that was about 2.5 billion years ago. That was caused by photosynthetic bacteria that were pumping out more and more oxygen, it's thought that that may have changed up the ecological space and basically opened up a niche for the development of more complex microscopic organisms, which became eukaryotes. But these were still single cellular. So don't care if you're eukaryotic is a more complex type of organism and multicellular organisms are all eukaryotic organisms. But the first eukaryotic organisms were not multicellular. They were still single cellular and remain so for for hundreds of million years after that.

The first eukaryotic land organisms appeared only about one billion years ago, and these were likely photosynthetic protests that were the ancestors of modern plants. So probably early plants came onto land before, or at least the ancestors of modern plants, I should say, came onto land before animals did. So the point that I'm making here is that throughout the vast majority of Earth's history, there was no life on the land at all, apart from mats of bacteria, essentially. Most life lived in the sea, and there was some bacterial life on land, and a bit later, some microscopic eukaryotic life as well, but still nothing that we would really recognize. So there was no plants, there was no animals, nothing like that.

So it wouldn't have been sort of very interesting, from our point of view, there wouldn't have been very much to see. It would have just been sort of barren rock mostly, not even soil, because soil has a large organic content. There would have been bacteria in the soil, but nothing else. So it would have just been like barren rock and sand and such. Now that all changed with the Cambrian explosion, which marks the beginning of the Phanerozoic Eon. Actually, probably a little bit before that, as we'll see. But the Cambrian explosion is particularly the time when hard-bodied complex fossils first appear, and that marks the beginning of complex life in the oceans first and then later on land.

Okay, so I've talked a bit in a bit more detail about the major divisions of geologic time, so the eons, the Hadesian, the archaeon, the proto-ozoic, and the phanozoic. And so we have a bit of an understanding about the major course of Earth's history, and particularly that there's not really much to say until the last maybe 700 million years or so, because prior to that, the only life that existed on Earth was microscopic life. For the first maybe 2 billion years or so, it was all bacterial and archaeological life. And then after the emergence of eukaryotic life, for another billion years, it was still just microscopic life, but now with eukaryotic organisms there as well. We know that throughout this time there were changes in the Earth's temperature as well as the composition of the atmosphere.

So I'll talk a little bit about that. We don't have much knowledge of Earth's temperature prior to the Phanozoic, but we know that it did change. There were periods of glaciation. We know that, so times in which there were significant ice covering the earth, and in fact, likely times in which the entire surface of the earth was covered in ice. That's called a snowball earth scenario. We know that that has happened. The most recent time in which we know that happened was during the cryogenian period, which I'll talk about more momentarily. But that's likely happened at previous points in Earth history as well.

We just don't have good records of this. What we do know, and I've discussed this in previous episodes when we looked at natural climate change, certainly in the last 500 million years, so essentially during the Fanozoic ion, we've seen temperatures, global average temperatures fluctuate over a range of about 20 degrees Celsius, so a huge range. Remember that a single degree change in average global temperatures is large and makes a significant difference. 20 degrees is huge. Now of course this isn't over short periods of time, these 20 degree changes occur over tens of millions of years, and they fluctuate going up and down depending on various factors. One of those is how much carbon dioxide is released

as a result of volcanic activity. So there's times in which there's more volcanic activity, releases more carbon dioxide, that results in greater global warming and increases global temperatures. And then there's times in which progressively carbon dioxide is pulled out of the atmosphere by geologic processes, resulting in global cooling, and potentially even ice ages or snowball earth scenarios if there's sufficient cooling. So that process is happening sort of consistently over Earth's history. We don't have detailed data about that going back more than about 500 million.

years, but that's definitely been happening. Another thing that's been happening related to this, as I said, is changes in atmospheric composition. So the early Earth's atmosphere during the Hadesian and also into the Arcan as well, had very little, if any, free oxygen. It was mostly thought to the atmosphere. Initially, during the Hadean consisted of a very thick hydrogen-rich atmosphere. That likely resembled, not as thick, but resembled the atmosphere of the gas giants. So a lot of water vapor, methane, and hydrogen. Over time, as Earth cooled, a lot of that atmospheric water condensed into liquid and formed a super ocean that I mentioned before, so that during the Hadesian and early Archaean eons covered the majority or even all of the planet. Later on during the Archaean,

more of the hydrogen was lost, water vapor condensed, and so it's thought that the atmosphere mostly consisted of methane, although there's still, I think, some debate about exactly what the composition was. It wasn't until the protozoic ion, as I mentioned, with the great oxidation event, that free oxygen began to accumulate in any significant quantities in the atmosphere. So that happened about 2.5 billion years ago, but oxygen has continued to accumulate over time since then. So different studies have different hypotheses about exactly how rapid the accumulation of oxygen

was, whether it was very sudden or whether it took place over over like tens or hundreds of millions of years. But at least one hypothesis is that there were small amounts of free oxygen produced, obviously in the billion years or so, 1.5 billion years or so after the origin of life, as photosynthetic organisms developed and released oxygen. Just remember a photosynthetic organism of the type we're talking about, it takes in carbon dioxide from the atmosphere and sunlight and uses that to produce sugar, so glucose that it can consume as energy.

as well as oxygen, so O2, that's dieoxygen molecule, which it then releases essentially as a waste product. We don't normally think of oxygen as a waste product, but from the perspective of these organisms it was. I mean, they didn't breathe it, they didn't use it for anything. It was just a waste product, and it was actually toxic. Free oxygen is highly reactive. That's what oxidation is, right? Things rust or combust when they get too hot. Those are forms of reaction with oxygen.

So it's actually very dangerous and very toxic to many forms of life, not forms of life that we're familiar with, but many microscopic forms of life and older forms of life that don't exist anymore. There are, of course, many anoxic organisms that exist today that live in environments that are poor in oxygen. But the point is that early life was all like this. There was no early life that was able to, or at least very little early life that was able to withstand large concentrations of oxygen. That's why the great oxidation event was so significant, is that it probably killed off the vast majority of life that existed on Earth, other than some of them that were a bit more tolerant to oxygen and managed to survive, and then others that lived in environments that were protected from the oxygen that was progressively released. So what was happening effectively over the course of the Archeon is that more and more of this oxygen was released,

but initially it was taken up by geological processes. So in particular, or it oxidized like free iron and other metals that existed on Earth, as well as sulfur, hydrogen, and other molecules that exist or other elements that existed, which were able to react with the oxygen to form either other compounds or chemical species, which removed that oxygen from the atmosphere. So for a long time, any free oxygen that was produced was depleted fairly quickly. But at some point, the capacity of the atmosphere and the ocean and the geosphere to absorb that free oxygen was saturated. And at that point, the oxygen just accumulated probably quite rapidly in the atmosphere, like over a period of, you know, maybe a million years or something like that or a few million years, but like relatively, relatively quickly, not hundreds of millions of years. And this is the great oxidation event. there was a massive increase in the proportion of free oxygen in the atmosphere probably went from effectively zero to maybe like 10% of the atmosphere by pressure like by partial pressures.

Might not have quite been that much. Maybe it was 5%. Different sources say different things, but given that it's going from effectively zero, that is just an enormous increase, like many orders of magnitude in proportion. It seems that then after that point about two and a half billion years ago, or maybe slightly more recently than that, when the great oxygen event happened, the concentration of free oxygen in the atmosphere stayed roughly constant until about a billion years ago. And then it started to increase again.

Again, effectively as more and more organisms, including more eukaryotic organisms, began to photosynthesize and release more and more oxygen into the atmosphere. So that's been an important trend over the past two and a half billion years or so as progressively increasing oxygen concentration in Earth's atmosphere. Today, the partial pressure of oxygen is about 0.2 atmosphere, so it's about 20%

of the atmosphere. The highest it's reached in Earth's history is significantly more than that, about 1 third, I think, about 0.33 atmospheres, which occurred a few hundred million years ago. So it's not at the highest it's ever been, but it's still very high relative to most of Earth's history. And again, that's largely because of all of the photosynthesis that occurs. We now do have animals which breathe in free oxygen and emit carbon dioxide. So they're doing the reverse of photosynthetic organisms. But it's only possible for those organisms to exist because of the free oxygen that exists on Earth. I mean, no animals would have been able to exist even one billion years ago, and certainly not two billion years ago, because there wasn't enough free oxygen in the

Earth atmosphere to sustain their metabolism, to sustain respiration. So this is one of the instances. I mean, there are many of these where in order for certain forms of life to exist, certain more complex forms, there have to have been many sort of prerequisite steps in evolutionary history. And one of these is that there needs to be enough free oxygen in the atmosphere to sustain their respiration. And that can't occur until you have enough organisms that are photosynthesizing and that have been doing so for a long enough time to saturate all of the various oxygen sinks and lead to accumulation of oxygen in the atmosphere. So this is an important dynamic, which allows us to see that complex animal life existing in the oceans and on the surface of the planet really wouldn't have been possible until the last billion years at the absolute most and probably even less than that. Another thing that's important that I touched on before, but I wanted to go through it a bit more detail is the supercontinent cycle. So as I said, for the first billion years or so, Earth was largely a water planet from about four to three billion years ago.

There were some continents, but there was relatively little continental crust. One of the factors here is essentially that it takes time for Earth's crust, what mantle and crust, to differentiate. So basically the continental crust is the least dense material, more felsic material is the technical term. And it's less dense, and so it tends to sit higher relative to the oceanic crust, which sits a bit lower on the mantle. And that's why, you know, because obviously the water, you know, spreads to a common level, the denser oceanic crust which sits lower is covered by water and then the less dense continental crust sits higher. And much of it, not all, some continental crust is still covered by water, but a lot of it is above sea levels. But there was relatively little continental crust initially because it took time to differentiate from the mantle.

and it also took time for the plate tectonic cycle to begin. It's not really known when plate tectonics first began. I've seen very different estimates for this. It probably started up sometime in the Archaean Eon, but it sort of took time to get going and to reach the current, to reach its current form. It sort of changed. There's a lot of details there,

which I won't get into here about how it's developed over time, the exact mechanisms. But the point is initially there was relatively little continental crust. The first supercontinent that it's thought to have existed is called Keniland, and that formed about 2.7 billion years ago. So prior to that, there was relatively little continental crust. And then we've had a new supercontinent roughly every 500 million to 1 billion years after that. So about 2 billion years ago, there was Noonah, or also called Columbia. About 1 billion years ago, there was Redinia.

and then about 500 million years ago, there was Panetia, and about 300 million years ago there was Pangae, which is probably the one you've most heard of, because that's the most recent supercontinent, and also the one that existed during the time in which land animals first evolved, or roughly that time, and Pangia was progressively breaking up during the time of the dinosaurs as well, so that's also relatively well known.

So there have been half dozen or so of these different supercontinent cycles dating to, you know, roughly 2.5 billion years ago, give or take, roughly the transition from archaeon to protozoic eons. And the earliest of these would have had no life on them other than some bacterial life. It's really Panatia, which was the supercontinent that existed around the Cambrian explosion, that would have been the first supercontinent to have any significant, life, macroscopic life, to exist on it. First, probably some proto-plants and then later on were the first tetrapods that moved on to land, as well as other non-vertebrates as well. We've got to think about the early arthropods, which were some of the first organisms to come to land as well. I'll talk about that in a future episode where I go through the evolution of animals specifically from the very beginnings up until the first vertebrates to walk on land. But that all took quite a long

time. The first evolution, life, as I said, originated in the oceans. Animals also evolved in the oceans. And even from the Cambrian explosion onwards, most animal life still existed in the oceans up until maybe 300 million years ago or so. It took a long time for life to adapt to land. And so, you know, only the last approximately 300 million-ish years, do we see significant amounts of complex multicellular life on land, and it's only during this time, roughly last 300 million years, that land would look anything recognizable in terms of having plants and animals and things like that on it. So very interesting that that's less than 10% of the history of the Earth, in which Earth looks sort of anything like we imagine it today with plants and animals

and greenery and all of this stuff. None of that would have existed before then. Okay, so I've talked about the supercontinent cycle, we've talked about changes in the atmosphere, and we've gone through and discussed the major eons and what demarcates them, the great oxidation event and stuff like that. I'm now going to go through and talk in a bit more detail, beginning with the protozoic, there's not too much more to say about the Hadean or Archaean eons. I want to talk a little bit more about the later parts of the protozoic and then also give a bit more detail of the Phanaerozoic. So as I said, the protoozoic, that begins about 2.5 billion years ago, with the first supercontinants and great oxidation event. I've already discussed that. I also mentioned before, the first eukaryotic organisms are thought to have evolved maybe just after the great oxidation event, maybe around 2 billion years ago, but there's a big uncertainty there.

But these were, although eukaryotic, there was still single cellular and still marine. The first eukaryotic organisms probably didn't develop on land until about 1 billion years ago, and they were likely photosynthetic protests that were the, ancestors of modern plants, but actual plants wouldn't have didn't evolve until only a few hundred million years ago. Now, I want to discuss the final two periods of the of the proto-ozoic eon. Remember, ion is the largest unit of time. The next largest under that is era. So the eras of the proto-ozoic are not really very interesting. They're just called paleo-pro-ozoic, meso-proto-ozoic, and neoprotozoic. So basically the protozoic is just divided into the old middle and the new errors. And they're just defined based on chopping it up into thirds anyway.

So there's something very interesting to that demarcation effectively because there's just not enough rocks surviving from this time to use this process of the golden spikes and the identifying stratigraphic units and things like that. In fact, that's really only possible going up to the beginning of the phanozoic. They're pushing it a little bit earlier than that, as we'll discuss now, to the end of the proto-ozoic eon. But that process really only works going back 5 to 600 million years. Prior to that, the demarcations of geochronologic units are largely just based on arbitrary durations of time. Hence, while we have the paleo-prozoic, mesoproto-ozoic, and neoprotozoic, just dividing it up into three. At any rate, the neoproto-ozoic is the era in which we're interested in here. That is effectively about from one billion years ago to the beginning of the phanozoic.

So that's about 1 billion to 540 million years ago. And the neoprotozoic era is a very interesting one because it is during this time that animal life as well as plant life evolved. So the neoprotozoic era is divided into three periods, the Tonian, the cryogenian, and the ediocaron. And these are all very interesting periods of time. So the Tonian period is probably the time in which animals first evolved. But at that time, they were sort of just splitting off from other single-celled eukary out. So they wouldn't be animals in the way we think about them, just single-celled organisms. But were the ancestors of bond animals.

That likely occurred during the Tonian period. The next period after that was the Crygenian. This is interesting because it was a time most of... this period was a time in which the earth was completely covered in ice or almost completely covered in ice, a snowball earth scenario. And this lasted for over 100 million years, so very, very long duration. But it's actually thought that this global freezing event may have been important for the, not the very first origination of animals, that probably happened in the Tonian period, But the development of animals and early diversification, for example, the earliest things that look like sponges, there's some sort of faint signs of fossils of what appear to be sponges date to the Crygenian period.

So there's already some macroscopic animal forms that seem to have developed in this period here over 600 million years ago, although the details are still very fuzzy because we don't know very much about. this time. So the cryogenian period lasted from 720 to 635 million years ago. The as I mentioned it was the time in which the earth was covered mostly with ice and snow and interestingly one consequence of this is that the ice sheets blocked off light in most of the shallow seas which led to a mass extinction of marine life and that's thought to have as I said opened up new niches which were in part filled by early animals which began to diversify into more complex macroscopic forms, including the earliest sponges and some other forms that are harder to identify. Now, the period after that, the ediocaran lasted from 635 to 540 million years ago.

It's the final period in the protozoic, the very end of the protozoic. The ediocaran is the time in which we find the first large numbers of macroscopic eukaryotic organisms in the fossil record. There's a few hints in the cryogenian, but substantial fossils only begin to be found in the idiocharan. However, these are all soft-bodied fossils, so they generally trace fossils. The first hard-bodied fossils are only found in the Cambrian, so that's what marks the beginning of the Phanozoic Eon, the Cambrian being the first period in the Phanerozoic Eon. But the Edioccaron still has a lot of interesting soft-bodied trace fossils or trace fossils of soft-bodied organisms. Many of them are very

hard to classify, we don't know what types of life they were. They appear to be some forms of sponges or ancestors of sponges as well as forms that would become or perhaps a class of viable as niderian so like jellyfish and sea an enemies and things like that but but and there's others that may be some sort of ancestor of segmented organisms like even arthropods or something like that but we don't really know i'll talk about this in more detail when i do the episode on the evolution of animals because we'll cover all of these um interesting ediacaran biota they're called so there was definitely macroscopic animal life with differentiated tissues and including mobile organisms, so organisms that moved, not just sessal organisms. That definitely existed during the Ediacaran,

so at least by 600 million years ago, so at least 50 or 60 million years before the Cambrian explosion. So the Cambrian explosion was not very origins of multicellular animal life. It's the time in which it began to fossilize in hard-bodied fossil. with shells and skeletons and things that we can see in the fossil record. So this marks now the transition from the proto-ozoic ion to the fanerozoic ion. Recall that the fanerozoic ion is divided into three eras, so the paleozoic, the mesozoic, and the Cenozoic. Easy way to remember that is paleozoic is like fishes and amphibians and first tetrapods. Mesozoic is like dinosaurs. Cenozoic is the period in which mammals were dominant.

So that's how I remember them. So I'm just going to talk about a few of the important things that happened during the phanozoic. I'll discuss this in much more detail in some future episodes when we talk about the evolution of animals. I'll also do an episode on dinosaurs and other large reptiles that existed, and I'll do an episode on the evolution of humans. So we're going to cover many of these key events in greater detail later.

We're just going to quickly discuss and review a few major milestones to help understand what happened. So the Cambrian period, that's the very first one in the Phanozoic, marks the beginning of the time in which we see the first hard-bodied organisms preserved in the fossil record, so the first organisms that excreted shells or had endoskeletons or exoskeletons or something like that that could be fossilized directly. Previously, there were only soft-bodied organisms so they could leave trace fossils but didn't fossilize directly, or very rarely was there anything that fossilized more directly. Subsequent to the Cambrian there was the Ordovician period, then the Silurian, Devonian, Carboniferous, and the Permian. Now I don't expect you to remember

all those. They're largely just names at this point. Just a few things to keep your bearing. So they're all approximately 50 million years long. The Silurian's a bit shorter, the Devonian's a bit longer. But you can think of it as that the Paleozoic is about 200 million years long. It's divided into six roughly 50 million year-long periods. And each of these periods marks a time in which a certain type of life was more dominant. So the first half, roughly, of the Paleozoic was dominated, so that's the Cambrian, the Ordovician, and the Silirian. It was dominated by arthropons like trilobites and sea scorpions, mollusks like snails, and non-vertebrate chordates like jawless fish. So for roughly the first half of the Paleozoic, we don't have any vertebrates, anything that we

would recognize as type of animals that we're most familiar with today. Those only began to evolve during the Devonian period, which is sometimes called the age of fishes, because bony fish, you know, with a skeleton first developed during this time. The carboniferous was, the period after that was sometimes called the age of amphibians, because amphibians first developed during this time. The carboniferous was, this was about the time when animal life first moved onto land. And then the Permian period, the final period, the final period, of the Paleozoic era was the time in which early reptiles first developed. So what we see over the Paleozoic is a progressive increase in the complexity of organisms

and also increase in biodiversity, some more different types of organisms, more niches filled. And we also see the progressive development of adaptations required for land. I mean, initially, it's not like life was trying to get onto land. We should not see it that way. I'll discuss this more in future episodes when we go through this. But in retrospect, we can see that these developments were necessary. So the development of a skeleton is necessary to support the weight of the organism on land because you're not buoyant in the same way as you are in water.

So that was necessary first with the first fish. And then development of amphibians first, and the first tetrapods as they moved onto land, and then further developments that helped, like, the early reptiles and then early mammals as well to survive without going back to the water all the time as amphibians need to. So like the hard-shelled egg in the case of reptiles or internal gestation of young, as it goes in most mammals. So these developments were progressively occurring over these hundreds of millions of years in the Paleozoic. 250 million years ago, we transitioned from the Paleozoic to the Mesozoic, and this was marked by a mass extinction event.

I'll discuss that in a moment, as we'll finish on mass extinctions. The Mizzoic era is divided into three periods, Triassic, Jurassic, and Cretaceous. The Mesozoic is probably one of the most famous times in the Earth's history because it's when the dinosaurs were dominant. So dinosaurs developed and diversified during the Triassic, and they were very dominant during the Jurassic and Cretaceous periods. So it wasn't just the dinosaurs that was dominant during this period. This was also a time when the first... So the Mesozoic was not just the time in which dinosaurs were dominant, but there were many other types of what are called archisauran reptiles, basically large reptiles that are

look like dinosaurs, but technically not dinosaurs. This includes the dinosaurs, but also things like mesosaurs and terosaurs and such. Again, we'll look at those in much more detail when I do an episode on the dinosaurs. One interesting thing is that although by this time, the surface of Earth was covered by plants, including trees, flowering plants did not exist, or flowering plants began to evolve during the Cretaceous period,

so like the end of this time. For most of the Mesozoic era, gymnosperms were dominant. So these are non-flowering plants like conifers. Grasses also didn't exist during the Mesozoic era. So the Earth is more recognizable than it was previously, but there's still many differences. No flowering plants for the most part, no grasses. The climate was also much, much warmer. It was a very hot climate, much higher levels of carbon dioxide in the atmosphere.

The Pangaea, the supercontinent, was progressively breaking up and dividing up into smaller units. So a very interesting time. We'll discuss it more in a future episode. The end of the Mesozoic era is marked, the end of the Cretaceous is marked by another mass extinction event, which brought about the beginning of the Cenozoic era. This is divided into the paleogene, the neogene and the quaternary periods. The Cenozoic era was characterized by the dominance of mammals. So reptiles still existed, but much less dominant than they were previously, the main branch of reptiles that diversified in this period of the birds. Insects and angios burns, so flowering plants, also significantly diversified during this time.

The most recent period, the quaternary period, began about 2.5 million years ago with the onset of what's called the Quartanary glaciation. So this is an ice age. Earth has periodically gone through different ice ages, times when there are large, permanent ice caps. at the poles. And throughout a lot of the Phanozoic eon, there were no ice caps at the poles. So the Earth was not in an ice age. There were some that took place, but particularly during the Mesozoic, for example, the Earth was generally much warmer than it is now. But the past 50 million years or so has seen a progressive cooling trend up until about 2.5 million years ago where we actually entered an ice age. And as I mentioned, we're currently in an interglacial time of that ice age. So we're still

technically in an ice age because there are permanent ice caps. But that's what marks the beginning of the quaternary period, which is where we now are. The Holocene, I mentioned before, is the smallest, is an epoch, so it's the smallest subdivision of time. And that is the current epoch in which we are living. And it's marked by the beginning of the interglacial period, so the less severe part of the ice age, but still in the ice age. So we're currently in the Holocene epoch of the quaternary period of the Cenozoic era of the Phanozoic Eon. And as I mentioned before, I will discuss the evolution of mammals and other humans in more detail in a future episode. Before finishing up, I want to just mention a little bit more, discuss in a little bit more detail,

the mass extinctions that have occurred periodically throughout Earth's history. There probably have been more of these that occurred prior to the Cambrian period, prior to the Cambrian explosion, but we don't have enough evidence in terms of the fossil record to really say much about these. there was a mass extinction event. I mean, we know that there was a mass extinction event of some kind with the great oxidation event, but because it was only microbial organisms, we can't really say very much about it. The phanozoic eon, at the time in which we have enough fossil evidence in order to assess these things, has seen five major mass extinctions. The way that a mass

extinction is defined is in terms of a certain deviation above the natural rate of extinction. Organisms are constantly going extinct, and there's some sort of rough. rate at which that occurs but during mass extinctions the rate of um extinctions dramatically increases relative to that background level and so this big five these big five mass extinctions as they're called who are identified in the 1980s and um there's not any specific reason why these five are the big five because there have been many other extinction events as well of similar magnitudes these appear to have been the the largest five extinction events but again that it's not like that they stand out in any very special way they just happen to be the biggest five i mean you

you could identify the biggest three or the biggest 10 you know it and it wouldn't really be any different but but this is sort of a convenient grouping that's often discussed and so i'll go through them here so a mass extinction event is defined as a time in which biodiversity of earth decreases and there's a much greater than normal rate of extinctions of species and even genre and whole families The largest five extinctions that have occurred in the past 500 million years or so are called the Big Five, and I'll just go through them briefly. So the first of these was the late Ordovician mass extinction, which occurred, not surprisingly, at the end of the Ordovician. And it resulted in the extinction of about 85% of all species. It's thought to have been caused by volcanic activity, which led to climactic changes.

But because this was so long ago, we don't know a lot of the details there. but this resulted in major changes, particularly in Earth's Ocean, so at this time there was not any macroscopic life on land, so this was just a marine event. We then have the Lake Devonian mass extinction, which occurred over the latter parts of the Devonian. This one, to be honest, I don't even know if it belongs with the other four on this list, because all of the other four extinctions took place fairly rapidly within probably hundreds of thousands of years, maybe even even less than that. But the late Devonian mass extinction took place over millions of years, and it's really a series of events or like prolonged time in which the rate of extinction was

elevated. It doesn't jump out as clearly as the other mass extinctions do. So it probably didn't have any single cause. Like with volcanism and asteroid impacts and probably changes in ocean levels resulting from continental movements all contributed to this. So it's much less clear as a singular event and may not really belong on the same list. But certainly there was substantial extinctions that occurred around this time. But this time is like over 10 million years. The next three mass extinction events are all much clearer.

So the Permian Triassic extinction event, about 250 million years ago, this marked the transition from the Paleozoic to the Mesozoic eras. This is the largest mass extinction event that we know about. It resulted in the extinction of over 95% of all species, including the trilobites. So they were a very dominant type of marine arthropod that is very widely attested in the fossil record prior to this, but they were made extinct during this time. The huge range of species that went extinct during the Permian Triassic extinction event, it opened up wide range of niches, which allowed for the radiation of many different organisms, including the reptiles that, then proliferated during the Triassic period, as I discussed earlier.

So the Permian extinction was probably important for the rise of the dinosaurs. It's generally thought that the Permian Triassic extinction was caused by massive flood basalt volcanic eruptions, which created the Siberian traps, so essentially in Mondays Siberia. This released huge quantities of carbon dioxide, which raised global temperatures, acidified the oceans, and thereby substantially changed the climate, leading to mass extinction. Then only 50 million years later, there was another major mass extinction event, the Triassic Jurassic Extinction event. So this marks the transition from the Triassic to the Jurassic periods, and about 80% of species

became extinct during this time, so not quite as severe as the Permian Triassic, but still very severe. It's thought to have been caused by volcanic activity leading to climactic changes. That's the cause of most of these, as you may have seen. Importantly, this is not the event that brought about the end of the dinosaurs. In fact, this was near the beginning of the time when the dinosaurs were around. The final of the big five mass extinction events, the Cretaceous Paleogene extinction event, formerly called the KT extinction event or the KT boundary, because the Cretaceous period is abbreviated K,

and the paleogene was previously referred to as the tertiary period, or, I mean, the tertiary period overlapped with a paleo gene, but the tertiary period is not used anymore. It's now called the Paleogean, so we call it the Cretaceous Paleogene extinction event, which occurred 66 million years ago. And this is the one that most people know about. This is when an asteroid, about 10 kilometers across, collided with the Earth in what's now Mexico and formed the Chicksilhub crater, and the ash and debris that was thrown up into the atmosphere, blocked out the sun for years and caused the extinction of about 75% of all species, including

all of the non-avian dinosaurs and other large reptiles that existed at that time. This extinction event is unique in pretty clearly being caused by a singular asteroid strike. There is some hypotheses that there may have been volcanic activity that contributed to this. There's some hypotheses that maybe some of the dinosaurs were already struggling at this time, or already some of them some species have been going extinct at a bit more than background rates. But from what I can tell, that really does seem to be pretty minor in importance. Overwhelmingly, what caused this extinction event was the asteroid impact. And that's unusual because although there have been many asteroid impacts over the course of Earth's history,

and they have contributed to extinction events, the other four mass extinction events that don't seem to have been caused directly by any specific asteroid collision. There may have been asteroid collisions that contributed in particular to the late Devonian, mass extinction, but remember that took millions of years, so it's not really a singular event. But the other ones, particularly the late Ordovician, the Permian, and the Triassic-Jurassic, seem to have been caused by volcanic activity and not asteroid strikes. But some combination of massive volcanic activity throwing up carbon dioxide into the atmosphere and changing global temperatures, asteroid strikes, blocking out sunlight, and also changes

in the level of the ocean, disrupting ecosystems, particularly in the shallow ocean, areas, which is where much life is focused. A combination of these three factors are the main, thought to be the main causes behind mass extinctions, although there's many other factors that probably contribute as well and complex interplays between them. Okay, so that brings us to a conclusion of the mass extinction section and a conclusion of the episode as a whole. So let me just give a very brief reminder of what we talked about.

We talked about the geologic timescale, and in particular we went through and discussed the major units of Earth's history. the largest of those units being the ion. So I remember there was the Hadesian, the Archaeoan, the Protozoic, and then the Phanozoic ions. The Phanozoic Eon being the time in which we have hard-body fossils and macroscopic life that first developed in the ocean and then moved on to land maybe 300-ish million years ago. Throughout most of Earth's history, from about 4 billion years ago to about maybe 600 million

years ago, just a bit before the Cambrian explosion. So during this three and a half billion year period, life existed on Earth, but it was all microscopic. At first, it was just bacterial and archaea, and later on eukaryotes evolved, but there were still single cellular microscopic organisms. So there was no life on land other than some bacteria and a bit later some eukaryotes, but no animals, no plants, no fungi, nothing like that. So Earth was quite barren and largely not many features that we would recognize. Much of the history of Earth that we know about in any detail is the that took place in the Phanozoic Eon, the past 450 million years, which saw the emergence first of various non-vertebrate

animals like trilobites, snails, and jawless fish, and then later on we saw the development of bony fish and then amphibians and then early reptiles, and then with the Permian extinction, we see the end of the Paleozoic era, the beginning of the Mesozoic era, the rise of the dinosaurs and other large reptiles, which dominated Earth for nearly 200 million years, until the asteroid strike ended the Cretaceous, brought about the extinction of all non-avian dinosaurs and other large reptiles, and brought about the beginning of the Cenozoic era, which is where we currently are, and the rise of the mammals.

And most recently, with the beginning of the Quartanary period, about two and a half million years ago, with the onset of the Quaternary Glaciation, we see a climate that's sort of roughly similar to what we live in now. prior to that was much warmer generally. And also, interestingly, 2.5 million years ago also marks roughly the time of the evolution of early hominids. We'll talk about that in further detail in a future episode because it sort of depends how far back

you want to go. But, you know, roughly speaking, we can think of as the 2.5 million years ago as a rough time when, just as the glaciation period was beginning with the quaternary period, so too was early hominids evolving that were fairly recognizable as ancestors. of modern humans. So that concludes our summary and hopefully you've found this episode interesting. I know it's a little bit hard to follow with all of the names and dates, but hopefully I did an all right job of trying to summarize that for you to give you an overview of Earth's history and the geologic timescale. As I mentioned, we will be delving into more detail, all of the periods

in which we have more to say, so essentially the cryogeny and onwards. We'll be talking about in future episodes, the evolution of animals and their development from the marine to walking on land. and I'll talk about dinosaurs, I'll talk about all of the interesting large reptiles that existed during the Mesozoic era, and we'll also do an episode on the evolution of mammals during the Cenozoic era, and particularly focusing on the evolution of humans. So stay tuned for those future episodes. If you would like to support the podcast, there's a number of ways you can do so. You can put up a favorable rating or review on Spotify, iTunes, or whatever aggregator you use.

You can also check out the podcast on YouTube. I'm progressively putting up more of the other. episodes on YouTube with visual accompaniments. So the audio will be the same, but there is now visual guides, which will, I think, adds quite a bit to the episode. So check those out if you're interested. Just Science of Everything podcast, search that on YouTube, and you better find it there. If you can give individual episodes, likes and leave comments, that really does help to push that in the algorithm. So I appreciate that a lot. I'm hoping to reach a new audience there. So if you want to help the show, that's a very easy way you can do so. If you'd like to make a financial contribution,

which helps to cover the cost of editing the visual content for YouTube, as well as I'm hoping to develop a new website and I'd like to devote more time to and maybe hire some people to help me with other tasks to get more episodes out. So if you'd like to make a contribution to those costs, you can send a donation through PayPal. Fodd's 12 at gml.com is my email address.

You can send that to. Or you can become a regular Patreon supporter through my Patreon. Just Google Science of Everything, Patreon. You'll be able to find that. I really appreciate all of my backers there who've helped to fund a lot of the work that I've been doing with the YouTube

and keep the show more regular. So I'm really grateful for that. If you'd like to send me an email with questions, suggestions or any other feedback, I'd love to hear from listeners. You can email me, as I said before, Fods12 at gmail.com, F-O-D-S-1-2 at g-mell.com. Thanks very much for listening. I'll talk to you next time. By the way, just as a short post-script, I wanted to apologize. This episode is about a month later than I'd hoped it would be. There have been a few big life events that have happened, that have contributed to that.

So I actually had an accident recently. I fell off my bicycle on a patch of ice. and broke my elbow and left hip. So I was in hospital for over a week, and I'm still recovering from that. So that's been a bit of an ordeal. But then also, just the other week, I got married. So obviously that's quite a big change

and lots of organizing around there. So just a few updates on my side and a bit of an apology for why the episode is late, but I'm doing well now and hoping to get back on a bit of a more regular schedule. So thanks for your patience and take care.