The Science of Everything Podcast - Episode 162: Human Evolution

Episode Date: June 30, 2026

Beginning with primitive Cynodonts in the late Permian, we our evolutionary development through Mesozoic mammaliaforms, the divergence of monotremes from marsupials and placentals, and culiminating in the emergence of primates in the early Cenozoic. Along the way we discuss the emergence of important traits such as the mammalian ear and various primate adaptations for arboreal life. We then discuss the sequence of hominin species in the lead up to to humans, including Ardipithecus, Australopithecus, Homo erectus, and Neanderthals. We conclude with an analysis of the emergence of distinctive human traits, including bipedalism, encephalisation, and extensive tool use. Recommended pre-listening is Episode 161: Dinosaurs and Other Ancient Reptiles. 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

Hello, you're listening to The Science of Everything podcast, episode 162, human evolution. I'm your host, James Fodor. So this will be the concluding episode in our series on the evolution of animals. So recommended pre-listening here will be the previous episode, 161, dinosaurs and other ancient reptiles. And unsurprisingly, we're going to be talking about the evolution of homo sapiens, beginning with the synodonts of the early Mesozoic, and, surveying their evolution through the development of mammals, primates, and then humans. And I'll also focus on some of the most distinctive traits of homo sapiens, including bipedalism

and our large brains, as well as talking a bit about tool usage. With that being said, let's get started and pick up where we left off with the synapsid line in the previous episode. So I talked previously about the split of the amniote line into three. The diacids, which are the reptiles, the synapsids, which gave rise to the mammalian line, and then the paraphyptiles, which became extinct at the end of the Triassic. We're picking up from where we left off there. I talked about in the previous episode how there were many different types of synapsids called Pelagosaurs, which lived in the Permian and into the Triassic, which are sometimes called mammal-like reptiles, although they're actually, they're not reptiles. And then I also talked

about the Therapsids, which was a line of kind of transitional forms, which branched off in the late Permian and eventually gave rise to the mammalian line. We'll talk about that trajectory. I also talked about synodont, so the synodons are a branch of the Therapsids, in turn a branch of the synapsids, which gave rise to the mammalian line. And in the previous episode, I mentioned how during most of the Mesozoic, the terrestrial ecosystem were dominated by large, mostly reptile forms, so particularly the dinosaurs. And there were still synodonts living during the Mesozoic period, but most of them were fairly small, sort of insectivores, shrew-like animals. And it wasn't until the end of the Cretaceous with the mass extinction of the

dinosaurs and other life forms that the mammalian line then became dominant. So we're going to trace that history out a bit and then focus on primate and then human evolution. So let's pick up with the synodons. So the synodons are a clade of therapsids, which are in turn are a clade of synapsids, as I mentioned. They first appeared in the late Permian and diversified during the Triassic, but stayed fairly small throughout the Mesozoic period in terms of diversity and number of species. It's thought that synodonts had quite a lot of characteristics of modern mammals, although they acquired those sort of progressively, becoming more mammal-like over the course of the Mesozoa. So cytonans are thought to have had fur. There's evidence for nerve passages for sensory whiskers, and whiskers are thought to be associated with fur.

By the early Triassic, they had developed a number of other mammalian characteristics, including long but slender tails compared to the typically thicker tails of many reptiles. A distinct lumbar vertebrae as compared to the thoracic vertebrae, as well as satital skull crest. So the shape of the skull is one thing that differentiates the synodons quite noticeably from their synapsid ancestors, like Dimetrodotron, for example, which still have fairly typical reptilian characteristics of their skull. So a number of the important changes are the development of differentiated teeth,

so heterodontism, it's called. And that's the thought to be associated with the diversification of diets of the synodonts, which, so originally the ancestral forms ate meat, and the teeth are mostly just for sort of grabbing, gripping, and tearing, not actually for chewing. Most of the large synapsids, like early synapsids, as well as the carnivorous dinosaurs, didn't chew their food very much. The development of chewing was something that happened, well, it involved multiple times, but in the synodont lineage, it's associated with heterodontism, so a wider range of dietary options, such as insects, fruits, and eventually as well, other plant matter. and that then necessitating the development of different types of teeth, which are specialized for different types of chewing and different diets. Chewing also necessitates stronger jaw muscles, and so you see the progressive development in the transition of the early synapsid skull right through to the synodont and then later mammalian skulls, where you have an increase in the robustness of the jaw muscles, which are adapted for chewing.

And you also see a characteristic development called the Sagital Skull Crest, which is essentially like a ridge that runs along the center of the skull, found in many mammals. And that's thought to be primarily for the attachment of jaw muscles for chewing. So this skull crest is sort of a sign that we're seeing a development of a wider range of diets and a more adaptation for chewing of food. So this is something that's characteristic of the sign of dot line. I think I also mentioned in the previous episode there were also changes in the vertebrae and the pelvis associated with allowing synodonts to adapt their posture from the sprawling to erect.

This also happened in the archa so talked about that previously, the movement from kind of the lizard-like sprawling posture to the posture where the legs are straight down under the torso, which allows for more efficient motion and also allows for breathing at the same time as walking, which is obviously an important adaptation for terrestrial life and a more active lifestyle. Cynodonts are also thought to have been warm-blooded. That developed progressively, I think, during the Triassic, around the same time that also the archa-saws were developing warm-bloodedness. And again, that's associated with the more active lifestyle. The evolution of fur is also thought to be linked to that because fur obviously provides for insulation, which helps to reduce heat loss and therefore reduce caloric requirements

when you're maintaining a body temperature that's higher than the ambient temperatures. Another adaptation associated with chewing is the development of dental occlusion, which refers to the fact that the teeth properly link up and interface with each other when the jaw is closed. This is not something that you actually see in most reptiles. It was also developed in non-efficient dinosaurs, which were also adapted for, many of them were adapted for a wider range of diets, plant-based diets as well. and this is an adaption associated for chewing. It allows for much more efficient crushing of the food particles in between the teeth. Again, it's not seen in earlier synapsid and earlier reptile. Jaws, which are just, the teeth really just adapted to biting and tearing, not really for chewing.

Another related adaptation was loss of the continual lifetime replacement of teeth, which is still seen in many reptiles. Again, this doesn't really make sense if you are adapted for dental occlusion because the teeth need to be sort of specialized to fit into each other's grooves and to interface well for most efficient chewing. That doesn't really work with continual teeth replacement throughout the life. Continual teeth replacement is great if you don't need the teeth to fit together very carefully and you just use them for biting and tearing. but when you need highly specialized interfacing teeth for chewing, then the continual teeth replacement becomes a problem, and so that was lost. So instead, in sign of nonce and then also in contemporary mammals, you only have two sets of teeth, so the baby teeth and then the adult teeth that come in later. The transition to two sets of teeth is also

associated with, it's not entirely clear when this evolved, but it's also associated with the evolution of mammary glands. And, and the feeding of milk to the young, which obviously is easier when you don't have teeth. Now, one of the really interesting adaptations that, again, occurred progressively over the Triassic and into the Mesozoic as well, of synodonts showing the transition

from the early reptile-like synapsid forms to the mammalian-like forms by the Cretaceous is the development of mammalian ear. So I didn't know this until recently, but reptiles don't have ears, or at least they don't have external ears in the same way that mammals do. I think I mentioned this in the previous episode. They can hear, but the external hearing apparatus is different, and in particular the inner bones of the ear, mammals having three bones in the inner ear, are different. The reptiles only have one bone in the inner ear. And two of these bones, the two extra ones that mammals have, the Malleus and the Inchus, they actually,

originated from two jaw bones in reptiles or ancestor synapsids, which are ancestral, so the two bones are also found in reptiles. But they gradually evolved into much smaller bones that then became part of the mammalian inner ear. What happened, and this is documented in progressive skulls that have been found for different organisms, the jaw joint moved in terms of which bones were at the base of the joint. So primitive synodont forms still have the reptilian jaw joint, and more derived forms actually have double-jointed jaw. And then once you get to mammals, you see the loss of the older jaw joint and then the conversion of the associated bones into the mammalian bones of the ear. So that's a very interesting sequence of adaptations, which sort of showcases the transition from

the early reptile-like synapsid forms right up to the through the synodons and up to the mammals. So these are some of the annotations and developments that were taking place during late Permian through the Triassic and also the Mesozoic, but a lot of this happened roughly around the Triassic. By the early Triassic, there were only two clades left of the synodons. Many of them went extinct at the end of the Permian. Some of them survived partway into the Triassic, but many of them went extinct before the end. So by the sort of early Middle Triassic, there were only two clades of synodonts. left. There were the mammaliforms, which is basically mammals plus a few closely related

species that aren't technically mammals. And one other clade called the the tritulodons. So the tritulodons are quite interesting. They're an extinct family of small, specialized, mammal-like synodons. They're fairly closely related to the mammals, but distinct enough to warrant their own clade. They are the last known grouping of non-mammaliform synapsids, so they persisted right up to the early Cretaceous. Most of them, were thought to have been herbivorous and shared many traits in common with mammals, but there were some differences in the skeleton. So during most of the Mesozoic era, the only synodonts that were around were mammals and a couple of closely related groups, so the mammiforms, and then the

Tritolinons. And neither of these were particularly diverse, as I said mostly, they were fairly small insectivores or small predators, a kind of roughly shrew-like in their overall appearance. And they sort of played sick and fiddle, if you like, to the dominant land animals at the time, which were the dinosaurs, as well as some other reptile forms like the pseudosukians, for example, the crocodilian lineage. The Tridilodonts did die out in the early Cretaceous. So by the end of the Cretaceous, it was only the mammiforms left. And in fact, there were only a couple of species that were not part of crown mammals. So essentially by the end of the Cretaceous, the, the, the, the, the, The mammals were all that was left of the synapsid lineage.

So all of pelicosaurs had gone extinct long ago. The non-synodont therapsids all went extinct by the end of the Triassic. And then the surviving non-momomeliiform cyanidons also went extinct by the early Cretaceous. So it was just the mammals left. You know, and if you looked at the time a little bit before the end of the Cretaceous, it might have looked like that the whole synapsid lineage was on the path to extinction because there were so few species left and so little diversity left. But as it turns out, mammals started diversifying in the last five or ten million years of the Cretaceous quite significantly. You see a big

uptick in the number of different species and the sort of morphological variation. Many of the major orders within mammals are thought to have originated in the late Cretaceous. There's still some dispute as to how many of these orders originated in the late Cretaceous versus the early Paleocene, but it does seem like quite a few of them were, had already split from each other during the, by the late Cretaceous, like before the mass extinction. One other important characteristic of these Mesozoic mammals is that many of them were nocturnal, which is probably one reason they were able to compete with the dominant dinosaurs is because they were nocturnal. I think most dinosaurs were not. I'm not actually 100% sure about that,

but that would explain why they were able to, why they had slightly different niches. And we know that they were nocturnal because they possessed only two instead of four cone cells in the retina, suggesting a loss of colour vision, which is obviously not really needed at night. Instead, mammals had more rods, which allow for better vision in low light.

Primates have actually regained or essentially evolved a new cone, so they have three cones instead of the two that many other mammals have. So now we've talked about the synodons and how the non-mammalian synodons sort of petered out and eventually all went extinct by the end of the Cretaceous, but over the Mesozoic era, we did have the progressive accumulation of more mammaliform characteristics. Already by the early Triassic, cyanidons would have looked like mammals, although like sort of odd ones, they wouldn't have looked quite right, you know, to a non-expert. But they would have had fur, and, you know, they had the slimmer tails and the mammal-like jaw. not all of the adaptations were there yet. So they would have looked broadly mammal-like, as I said, but by the end of the Cretaceous, they were definitive mammals and would have looked similar to species today. So it was a slow transition, but broadly speaking throughout the Mesozoic era,

most of the synodons that existed were already fairly mammal-like, certainly by the end of the Triassic. But when we actually come to crown mammals, which is the technical class of mammalia, one way to define that is that it consists of all animals descended from the most recent common ancestor of the platypus and human beings. There's nothing actually special about human beings that you could put any placental mammal there, and it would define the same grouping of animals because they share the same common ancestor with the platypus. But basically those are the two most distantly related types of mammals. You've got the monotremes, platypus and echinol, and then the placental mammals, which is the vast majority of mammals. We'll talk more about that later, but that's one way of defining mammals is in terms of all the animals descended from the most recent common ancestor of planet pus and humans.

Under this definition, mammals evolved in the late Triassic about 230 million years ago and gradually diversified slowly at first, but then with increasing pace in the late Cretaceous, over the course of the Mesozoic era. Mammals are divided into three major clades, the monotremes, the marsupials, and the placentals. So they're defined, I mean, there's other differences, but one of the most obvious differences is through their reproduction. So monotremes lay eggs, marsupials are nursed their young in pouches, and placentals give birth to live young who are nursed externally. So the vast majority of mammals are placental mammals, including humans, or at least most placental mammals originated in the northern hemisphere, whereas the monotremes and the marsupials, they also seem to have originated in the northern hemisphere as well, but then subsequently migrated to South. via South America to Australia and Antarctica. So we often think of monotremes and marsupils as the southern mammals

and then the placentals as the northern mammals. Monotremes diverged from all of the other living mammals a very long time ago, almost as far, well, monotremes diverge from all of the other living mammals about 230 million years ago. I mean, that sort of defines the beginning of crown mammalia. Whereas the Therian mammals, so that's placentals plus marsupials. they're called the Therians. They diverged from each other only about 170 million years ago. So there was a long time between the divergence of the monotremes and then divergence of placentals and marsupials.

Or in other words, monotremes are much less closely related to any other mammals than any other types of mammals are related to each other. In fact, it's likely that if the monotrems had happened to go extinct, there's only a few species left. We wouldn't even count them as mammals. We'd probably define the mammals as just the placentals and the marsupials. just because of how far back in evolution the monotrems are compared to everything else. So that's our three major clades of living mammals, monotremes, placentals, and marsupials. There is a fourth major mammalian clade. I mean, there's a number of smaller ones, but a fourth big one, which was intermediate between the monotremes and the therians. But they became extinct in the paleo gene.

They're called the allotheria. Many of these animals had large protruding incisors, so sort of front teeth, if you you want to look that up, it's kind of weird. They seem to have had sort of specialized diets and they have these very large protruding front teeth. But they went extinct in the paleo gene, so tens of millions of years ago. And so now we don't see any sort of transition between the monotremes and the marsupials and placentals. There's a sort of big gap in evolutionary time and morphology between them. But there were many other species there. I just mentioned the allothera, but there were many other smaller branches as well, but all of them went extinct before modern times.

So that gives us some background about the evolution of mammals more generally. Now we're going to specialize a bit and talk about the evolution of primates, obviously on the way to talking about human evolution. So the picture at the moment is that we have this diversification of placental mammals, which happened in the late Cretaceous. They'd already long split off from the marsupials and even longer go split off from the monotremes, and you've got all of these different branches of mammals quietly divestinal. developing during the Mesozoic era and sort of increased pace in the late Cretaceous.

And then they really explode after the extinction of the dinosaurs, opens up a lot of terrestrial niches that they could develop into. And one of those key groups of mammals were the primates. The earliest primates likely resembled tree shrews, and in fact they're closely related to the tree shrews. Primates are relatively closely related to rodents as well, not as close is the tree shrews, but they split off relatively more recently from rodents compared to other mammals. So one way of thinking about the origin of the mammals is that it's sort of a squirrel-like organism that started living in trees and particularly began to develop its forearms to swing from branches or to grasp onto branches and then later down the road, brachiate, so swing from branches.

So that's what defines primates as distinct from other arboreal animals, tree dwelling animals. So they originated from something like tree shrews, but then the organisms began to be adapted for brachiation and different types of gripping of branches, and so thence originated the primates. So primates are all possess adaptations for climbing trees, and are all descended from tree dwellers,

even if they don't currently live in trees. And this is one of the, this adaptation for brachiation is one of the reasons why primates have distinct, upper from lower limbs. I mean, obviously other animals do as well, but many animals, many quadrupedes don't have much of a differentiation between the front and the rear limbs. Primates, most of them aren't habitually bipedal in the way the humans are. Even monkeys and chimps and other types of primates have a differentiation between the four limbs and the rear limbs, and this is a large part because of the adaptations for brachiation, climbing and swinging,

where it's useful to have essentially different types of grippers to grab and maneuver along the branches, which is just not quite as useful if you're a quadruped on the land, right? So that's one of the key adaptations of the primates. One of the very earliest primates was called Pleziadipus. I may have mispronounce that, but it is interesting because they lived in the early paleogen, very small animals. They kind of look a little bit like a squirrel, but with sort of longer legs. A bit hard to explain, but it's sort of interesting how they clearly resemble a

a primate but they're not quite there yet. So that's sort of an indication of some of the early primates that they may have looked a bit like squirrels, a bit like tree shrews, but then gradually became more adapted to brachiation. So over time, those adaptations included longer tails. So this plesiacopus organism had quite a long tail, which helps with balance and also for gripping. Toes and fingers become more differentiated for grabbing branches and fruits. And, And another primate adaptation is improved to aeroscopic vision, which is obviously especially useful when you have to judge distances and make complex maneuverings in three-dimensional space,

which again is less necessary if you're just a terrestrial organism and only walk on the ground. Now, I want to mention here an interesting theory about primate evolution called the primate angiosperm co-evolution theory, which is the idea that early primate adaptations were influenced by their reliance or increasing reliance on fruits and or insects as sources of food. And so the idea is that they needed to be very visually oriented to locate the fruit, which many of them are brightly colored and need to be sort of differentiated from other parts of plants.

And so adaptations to visually locating and then grasping for and collecting fruits on slender branches led to adaptation such as, well, that I mentioned before, the toes and the fingers, the slender tails, longer tails, better limbs for gripping, and the improved stereoscopic vision. And so this kind of led to a co-evolutionary process with angiosperms, which are flowering plants, which developed in the Cretaceous and then became more and more dominant in the Cenozoic era. And so the idea is that angiosperms became more and more successful as more of their fruits are eaten and dispersed by primates, and primates become more successful as they have more a source of nutrition which is less available to other animals which are not as adapted to

an arboreal life and can't reach all of the fruits that they can and then insects of course are a part of that as well because insects helped up with pollination and dispersal of the pollen of plants and help them to spread so there's a sort of an interesting process there i don't know exactly how well is substantiated this theory is but i think it's an interesting idea which probably played at least some role in the development of the primates. So there are today many different types of primates, and they, as like many other animals, branched off progressively from their ancestral forms. So the first type of primates to branch off of the lemurs, about 63 million years ago, then the Tarsia's about 58 million years ago, and the next major branch to split off of the New World

Monkeys. So New World Monkeys are quite interesting. interesting because it seems that during most of this time, most of the primates were living in Africa. So primates are adapted to a arboreal existence. So they live in trees. So they typically live in forest areas, mostly tropical rainforest, it seems. So many of them lived in Africa, and some then spread elsewhere to Southeast Asia. But I don't think there were any in the Americas until the New World Monkeys arrived there. And it's interesting because we know that the New World Monkeys diverged about 40 million years ago from ancestral forms. of monkeys that were living in Africa, and the question is how they got there. So 40 million

years ago, the ocean levels were different to how they are now, and the continents were a bit differentially placed. But Africa was still separated from South America by a very large ocean, and they don't appear to have gone around, like, north through Europe and then across the Arctic or something like that. I mean, they couldn't have survived anyway. In fact, the evidence is that they, or the ancestors of the New World Monkeys actually arrived in South America directly from Africa, possibly through some intermediary islands that may not exist anymore, but through land rafts. So these are rafts of vegetation that occasionally are eroded away on like river banks

or similar locations where a large rainfall can dislodge them. And entire floating rafts of like logs with some vegetation and sometimes animals can be dislodged and washed down the river and out to sea. This has actually been observed to occur, and animals have been observed to be transported hundreds of kilometers on these. So this is a rare event, but it does happen, and occasionally some animals will be on one of these, and even more occasionally, some of them will actually survive the journey to a distant location. And it seems that this is how monkey, the ancestors of New World Monkeys got from Africa to the new world, where they diversified and formed their own primate branch there.

So you've got the new world monkeys which are evolving in the new world, but most of the action we're interested in happens back in Africa by about 25 million years ago, so around the measine. We have many different species of essentially what we might call monkeys, although not contemporary species of monkeys, but what we broadly call monkeys lived in the rainforests of Africa. And then we had a progressive further branching of those into a few different forms. So the main split that occurred around 25 million years ago was between the old world monkeys, which still live in the rainforests and jungles of Africa and Southeast Asia, and then the apes. So the apes are fairly distinctive morphologically from the monkeys that they split from. What distinguishes apes from monkeys is that they are typically larger and also tailless.

So apes are generally adapted for a slightly, different style of living. Many apes don't live directly in the trees. They live on the ground or spend at least a fair amount of time on the ground. So the apes are split into kind of two main groups, the so-called lesser apes and the great apes. The lesser apes are the gibbons, of which there's quite a few different species. And they live exclusively in Southeast Asia. Then we have the great apes, of which they're four sort of main kinds. Humans are one of them. Then there's the chimpanzees and bonobos, often just called chimpanzees, but there's two species of those. Then there's gorillas and orangutans. So those four groupings together constitute the great apes,

and they're the most, those species are the most closely related primates to humans. So the order of splitting is fairly well established, so although the exact dates are still a bit in dispute. We'll talk about that in a moment. So the gibbons were the first to split off, maybe 20, 18 or so million years ago. Orangetans, maybe 15 million years ago, then gorilla and then chimpanzees. We'll talk about the Danes of those last two in a little bit because there's some controversy around those.

But we do know that chimpanzees are the most closely related species to humans and gorilla the second closest. And that's probably what you'd sort of intuitively think in terms of their morphology. And all of these species of great apes are omnivorous. The chimpanzees and orangutans mostly eat fruit. You know, it harkens back to what we were talking about with an angiosperm co-evolution theory.

And as I mentioned, all of the great apes share a common ancestor approximately 15 million years ago and then progressively split off from there. Gorillas and chimpanzees as well as early forms of humans lived and involved in Africa. So that's where we'll be focusing most of our attention from now on. So now we're going to transition from talking about primates to talking about human evolution or ancestral forms of humans leading into the development of modern humans. So the location, as I said, where this happened is in Africa, specifically East Africa. So there's sort of a strip of the continent from the south right up through to Tanzania, Kenya, Ethiopia, and also parts of the Sudan near the Nile, and extending a little bit inland as well into parts of Chad.

So one of the big questions in human evolution is when did humans or the ancestor of humans split from the ancestors of chimpanzee? When is our most recent common ancestor with the chimpanzees? And as I was indicating before, the answer is it's still a bit contested. So it's sort of gone back and forth a little bit at different times. People arguing for a slightly older recent common ancestor, people arguing for a newer recent common ancestor. I think for a lot of the 20th century, it was thought that humans split from the chimps more like 10 to 15 million years ago.

So that would be around the time that the other great apes, split off as well, or at least orangutans split off around that time. So the thought was that we were sort of more distantly related from chimpanzeism. But recent work with sequencing has, now that gene technology is much more widely available, has indicated that probably, and see, these all rely on a number of assumptions about rates of genetic drift and mutation and things like this and sampling biases and other issues. So it's difficult to get one precise number. But the scientific literature that I'm seeing at the moment indicates probable dates of around 6 to 7 million years ago was when our most recent common ancestors that live with chimpanzees. You will see a slightly different numbers given in different sources.

I've seen as low as 5 million years, and sometimes you do still see the 10 million years mentioned. Personally, it's looking like 6 to 7 is sort of the best guess as to when we diverged from the chimpanzees. And so guerrillas would have been slightly before that. That's why I didn't give a precise figure before, because it depends on when you think we diverged from chimpanzees. It's clear that gorillas were before, but exactly how long before, exactly when that was, is sort of less clear. So figures that I'll give here is if we think that chimpanzees diverged about six million years ago, then gorillas were maybe seven million years ago, and then orangutans, about double that, about 14 million years ago. So gorillas and chimpanzees are quite a lot more closely related than orangutans. And that also fits their location because Gibbons, the lesser apes and orangutans, both live in Southeast Asia, whereas chimpanzees and gorillas live in Africa, although more sort of Western Africa, whereas humans evolve more in Eastern Africa, although that's probably, historically, they probably overlap more.

So how do we get from our ancestor with chimpanzees about six million years ago to modern humans? one of the most important things to realize about the evolution of humans is that there is thought to have been a relative change in habitats that humans or the human ancestors mostly lived in compared to our ancestors with chimpanzees. This is still a little bit controversial. I'll talk a bit more about this as we go forth. But the basic idea is that our ancestor with chimpanzees as well as probably guerrillas around six, seven million years ago, those animals mostly, lived in tropical rainforest around sort of central Africa and were adapted to like heavily forested regions. What happened with at least some of the more eastern parts of the African continent is that there was a climate change talking around 10 to 5 million years ago over this

sort of time period in the Miocene. There was a cooling and drying of this region so that some areas, not all areas, but some areas, particularly in the east of the African continent, transitioned from being dense tropical forests to drier and more open savannah, which has a mixture of vegetation, which includes some forested areas, but also some open grasslands and some intermediate transitional regions. One of the things that I want to emphasize here is that there's been some controversy about this in literature, the so-called savanna hypothesis that humans evolved to live on the open savannah and with open grasslands instead of the densely forested areas where chimpanzees live today and

where our ancestors lived, you know, before the split with chimps. This is the Savannah hypothesis. So this has been contested. And one of the issues here is how Savannah is defined because there have been different usages of the term historically. So when we talk about Savannah, we shouldn't imagine that this means just open grassland with little to no vegetation, like other vegetation. Savannah is not the same as open grassland, like a complete open grassland.

Savannah means a word often used is mosaic. It's a mixture of vegetation types. There is some grassland, but there's also trees, and it varies in different regions, obviously, like locally at a larger scale. So the technical definition is savanna is a partly forested area where the vegetation is not so dense that it prevents the growth of grass. So in canopy forest, like tropical rainforests, that two little sunlight hits the, ground for grass to grow. And so that would not be a savanna. But when there is no full canopy layer and the vegetation is sparser such that grass is able to grow, then that can count as a savannah. Encompassing in that is, however, quite a broad range of environments, ranging from mostly grass with some isolated trees and bushes here and there to quite highly forested regions, but nevertheless not with a full canopy overlay where grass can't grow. So we need to understand that you there when we're thinking about the so-called savanna hypothesis, the idea that there was climactic change in eastern Africa around the Miocesan like 10 to 5 to 10 million years ago, which

led to the specialization of human ancestors to a savannah as opposed to a rainforest mode of existence. I think that broadly that is what happened, that is what the evidence supports, but we have to understand the nuanced understanding of it, not as the idea that there was just a sort of a strict moving from trees to grass. That's too simple. And we know that human ancestors did live in a range of different environments, but we should be thinking in terms of one of the reasons that we did diversify and become more morphologically distinct from our chimp and guerrilla ancestors, shared ancestors with those modern species, is because we, our ancestors became more adapted to a different form of environment, which was less heavily forested compared to where the gorilla

and chimpanzee forms continue to live, which are in the more forested regions of central and western Africa. So that's the broad context in which we're thinking of how humans split from gorillas and chimps. And so this occurred in 5 to 10 million years ago, let's say 7 for gorilla and 6 for chimpanzees. So one of the key adaptations for this new environment, obviously was bipedalism. So gorillas famously engage in knuckle walking, chimpanzees mostly walk on all fours.

They can walk on their hind limbs. They don't do so. They don't always do so. They don't do so most of the time, but they can sometimes. Whereas humans are habitual bipeds. So we essentially always walk on two legs. So that's a significant adaptation and a difference from all of the other great apes. So it's naturally thought that that bipedalism arose as an adaptation to the environmental change,

the different habitats that we lived in relative to the chimps and gorilla ancestors. exactly why we evolved bipedalism specifically is sort of less clear. We'll talk about that in a moment. But what we can say is that fairly early on in our split with chimpanzees, we already see evidence of more extensive bipedalism. And again, we shouldn't think of this as something that was a strict binary. That sort of one day the chimps decided to walk on two legs and then, you know, it was a straight road to human evolution after that.

That's obviously not what happened. and we shouldn't get a false picture on our heads. Chimpanzees already use bipedal forms of locomotion sometimes. What we should be picturing is a progressive increase in the number of circumstances and the amount of time that human ancestors spent using bipedal forms of locomotion and therefore a progressive increase in the adaptations for that mode. So we should think of progressive, ancestors becoming progressively more bipedal over a period of probably a few million years.

Now there's something else that I need to mention about the fossil record here. Unfortunately in tropical rainforests in particular, the fossilization is very rare in those conditions and so the fossil record of our ancestors going back through like with the great chimps right through the other great apes is extremely poor. As far as I know we don't really have any fossils in this period of like the eight the seven to 20-ish million years ago probably even even further back than that. Very limited to no fossils at all showing either ancestral forms of these organisms or are split with those. So we don't know exactly when, that's one of the reasons why we don't know exactly when we split from these organisms because the fossil record just isn't

there. The flip side to that is that where humans lived or human ancestors began to live in more grassland savannah type areas, fossilization occurs more readily in those conditions and we have quite a good, at least relatively good, fossil record for the evolution of humans. So the trick then is to know how it connects to gorillas and chimpanzees and the rest of the great apes for whom we have very poor fossil records. So we kind of have a good picture of human development, but we don't know very well as to where chimpanzees fit into that. That's one of the reasons why we don't know exactly when we split with chimpanzees. It seems to have been around six to seven million years ago, and we do have some fossils from that time, but we don't quite know where they fit in. So

I mentioned a couple of these early fossils. The oldest that I'm aware of that is relevant here is called Sahel Anthropos. So this was an extinct hominid that dates to about seven million years ago. So this is around the time that we're thought to have split with chimpanzees, maybe on the older end of that. When it was discovered, it was thought that it was potentially ancestral to both humans and chimpanzees. So this is sort of the missing link, quote unquote, between chimpanzees and humans, the last common ancestor that we'd love to find. But it now increasingly seems that it probably wasn't extremely close to the last common ancestor. It seems like it was a branch outside of that.

But still, it did live around the time of the split. So we know that it exhibits a combination of traits, a sort of intermediate between chimpanzees and humans. It may have engaged in knuckle walking. There's controversy about whether it was a biped. From what I've seen, the newer studies indicate it probably was not a habitual biped, but may have been a knucklewalker, may have sometimes engaged in bipedalism. So this is sort of, although it's probably not the most recent common ancestor with chimpanzees, it may be even more closely related to gorillas.

It's sort of unclear. It does highlight that there were a variety of other forms of great apes that lived around this time. And we sort of know that there must have been, but prior to the, I think this was only discovered about 20 years ago, so quite a recent find. We hadn't seen many fossils with this. So Sahanthropists is interesting. we don't quite know where it fits in. The next major fossil is

Ardipithecus. So this is, we're jumping forward a little bit, this looked about 5 million years ago. So this is thought to have dated after the split with chimpanzees. Again, we don't know exactly how it fits in. In particular, we don't know whether it's a human ancestor or whether it's a branch off.

As with Sahal Anthropis, it seems to show an intermediate morphology, so an intermediate traits between chimpanzees and humans. As with Sahal Anthropis, it's also a bit controversial as to whether Artipithecus was bipedal. There's conflicting studies on this. It seems like it was at least sometimes bipedal, but it's not entirely clear as to exactly how often it engaged in bipedalism. So again, with Artipithecus, we see a form that is clearly intermediary, transitional between the common ancestor with chimpanzees and modern humans. we're not exactly sure what's to where it fits in.

The next fossil evidence then is the Australopithecus. So you may well have heard of this. The famous Lucy fossil is a species of Australopithecus. Australopithecus is quite a well-studied genus. There are many different species, but I'll just talk about them together. So these early hominins lived in East Africa around 4 to 2 million years ago. It's not entirely clear, as I said, how they are related to artipithecus or Sahal Anthropos. my guess would be that artopithecus is probably not an ancestor of australopithecus but may be

sort of related to the ancestors of australopithecus but australopithecus is clearly much more human-like than either of those two earlier forms the really fascinating about australopithecus is that they have very clear adaptations for bipedalism but still have quite small brains so prior to the discovery of australopithecus so sort of early 20th century the pretext The prevailing theory was that human ancestors first evolved large brain, so intelligence came first, and then they became bipedal. But with increasing discoveries of Australopithecus, it became clear that it was the other way around.

Bipedalism very clearly came first, intelligence then developed, or increased intelligence developed after that. Of course, chimpanzees are already fairly intelligent, so we're starting from a fairly high baseline, but in terms of the difference between chimpanzee brain size and human brain size, Osteropithecus is only their brain size as measured by the volume of the skull is only slightly above that of the chimpanzee. They also have fairly small stature, so their body size and shape is more similar to chimpanzees in some ways, except for the fact that they have pelvis and feet which are very clearly adapted for bipedalism. I recommend looking this up if you haven't seen it yourself.

A chimpanzee, pelvis is sort of quite elongated relative to humans. Their feet are also differentially shaped, like they have a sort of a more spread toe, which is more adapted to arboreal living compared to humans where our feet are adapted to, you know, for walking. If you look at the feet and pelvis of the Australopithecus, they look very, very similar to homo sapiens skulls, to modern humans. sorry, they look very, very similar to modern Homo sapiens pelvis and feet, with the much sort of shortened shape of the pelvis in particular. So that and a wide range of other

adaptations with bipedalism are very clearly found in Australopithecus. So it's very clear that by four million years ago, human ancestors, and it's thought that Australopithecus almost certainly was ancestral to modern humans, that they had become predominantly bipedal. So that's why it's a little bit unclear as to like where artopithecus, for example, fits in that probably was bipedal to some extent, but it's not clear whether artapithecus was an ancestor of Australopithecus or was branched off on the side a bit. But Australopithecus is pretty clearly a human ancestor, and it's quite clear that it was habitually bipedal. However, its intelligence was probably not much different to contemporary chimpanzees, which are still quite intelligent,

but a long way from human level. The next genus that we're going to talk about is peranthropus. Now, this is another genus of extinct hominin, which contains only two, species, so it's not as widely studied as Australopithecus, which I think has like six species or something. Now, Paranthropus I did want to mention, because it has been fairly well studied, not as well as Australopithecus, but it's more contested because some authorities class Paranthropus as a subset, effectively, of Australopithecus. They're sometimes called robust Australopithecines, as opposed to the other Australopithecines, which are called gracile. Now, this is a term that you sometimes see used in sort of physical anthropology, so I wanted to comment on this. This seems to mostly refer to the

shape and size of the skull, as well as the overall build of the body. Basically, robust means larger and grace-hout means smaller, although it's a little bit more complicated than that. So skulls in particular that are called robust tend to have a very pronounced sagittal crest. Remember I talked about that before? It's a mammalian characteristic, which is where sort of the jaw muscles attach. That's found on the top of the skull. So robust has a pronounced sagittal crest. they have a very thick, heavy jaw, often a protuberant, protuberant and large teeth and cheeks, and a very sort of long, deep face. Whereas skulls that are described as more graysile, they have a much less prominent or even no

obvious sagittal crest. They have much smaller cheekbones and jaws. They tend to have smaller, less protuberant teeth, and a shorter face as opposed to the very, very deep face that's found in the robust forms. So that terminology is used to differentiate robust from graysal osteopithecines, but you will also see it in other early hominids as well. So I just thought I'd mention that. So some people define peranthropists as its own genus.

Some people say that they're really just Australopithecines that have a more robust physiology. But regardless, the paranthropists live from about three to one million years ago and were otherwise sort of similar to Australopithecines. They are not thought to be human ancestors, so they thought to have branched off from earlier Australopithecines. So basically you've got your earlier Australopithecines. They develop around 4 million years ago, bipedal, but otherwise more similar to chimpanzees.

They thought to have around 3 million years ago branched into the robust form of australopithesis, or peranthropus, if you prefer, on the one hand, and then on the other hand, into Homo. So Homo is our genus. It's comprised of humans, plus our closely related ancestors. So henceforth we're going to be talking about the homogenes, so fairly closely related to modern humans now. Not all of these are direct ancestors of modern humans. In fact, it's still a bit controversial as to which are sort of more cousins and which are ancestors, so I'll talk about that as we go.

So let's start with Homo Habilis. So this is some of the oldest Homo species. They evolved around 2.5 million years ago and lived until about 1.6 million years ago. Again, all of these species lived in East Africa, so from like southern Sudan, right down to South Africa, had a range of around those areas, some more local than others, but all of them lived in this area until we, until Homo erectus left Africa, but we'll get to that. So the Homo habilis is clearly a sort of a step towards modern humans, so they are physically, they're still bipedal, they're physically a bit larger and have substantially larger brains than Australopithecus. and they also made the first stone tools, or at least the first widely accepted. There's some debate as to whether some later Australopithesis informs made some stone tools, but generally, at least the traditional classification, is to say that the first stone tools were made by Homo habilis,

and they're called Older One Tools. They're quite primitive, but still very useful. We'll talk more about the different types of stone tools later, so I'll just mention the names here. So Older one stone tools from about two million years ago associated with Homo habilis. after that came Homo erectus. So Homo erectus may have evolved from Habilos or maybe from a related species that lived around the same time. Another one that sometimes mentioned is Homo Rudolf Ennis. That's sort of less well studied.

Some people grouped that in with other organisms, with other species. But Homo erectus is one of the best studied species within the homogenous. They lived for a very long time. They evolved about 2 million years ago and only became extinct about 100,000 years. ago. So they became extinct quite recently and overlapped for some time with modern Homo sapiens near the end there. They are the first human species to evolve a human-like body plan and gait. Overall, so they essentially look very close to modern humans. If you look at Homo habilis and Australopithecus, they are noticed a bit different. Australopithecus in many ways is more like a chimp,

but bipedal. Homo habilis is sort of intermediate. Eractus looks very modern. The face is a bit different. It's a little bit smaller, but overall, it's very human-like. And Homo Habalus was a step-up in brain size from the osteopithecus. Erectus is another step up again from Homo habilis. Still not up to the modern human level, but a significant increase. Homo erectus are the first known humans to use fire. They also made a more sophisticated type of tools called Shulian tools, stone tools. So they're a step up from the older one. It is thought that Homo erectus is a direct ancestor of modern humans, as well as some other species as well, the Neanderthals and the De Nosevans. We'll talk about those in a little bit.

Homo erectus are also the first known hominins to leave Africa. There's some other disputed forms as well, like Homo antecessor, which has been found in Spain, although that probably still post dates when Erectus first left Africa. Homo erectus first expanded out of Africa around 2 million years. ago probably, maybe a little more recently than that, and they spread throughout the Middle East into India and Southeast Asia. There were probably many different migrations, and some of them were sort of successful, some died out fairly quickly and then were replaced by other ones, so it's a sort of a complicated process. But we definitely know that Homework Reck just left Africa and lived in large parts of the old world for millions of years, or for over a million years at least.

And they definitely have been found throughout India, many parts of the Middle East, possibly parts of Europe. They didn't live throughout Europe, it seems, but they may have inhabited parts of Europe, either Homo erectus or a similar species, and also throughout Southeast Asia and China. It seems that Homo erectus did not inhabit most of Europe, was probably still too cold during this time, because remember the Ice Age with the Pleistocene Ice Age, starts about 2.5 million years ago. So the emergence of the homogeness pretty much coincides exactly with the Pleistocene and the beginning of the Ice Age. So throughout most of this period, much of the northern hemisphere is very difficult to live in, and certainly for Homo

erectus, it would have been too cold. They don't seem to have lived in very northern climates or in much of Central Asia, but everything sort of south of that and all of Africa, in the old world, they seem to have spread to most of that. They don't seem to have reached Australia or the new world. Now, one thing about Homo erectus is that although they were a significant step up in intelligence and tool use capabilities from Homo habilis, they'd don't seem to have possessed a very rich or robust culture. I'm not aware of any burials found from Homo erectus. Their tools, surely on tools, were a big step up from the old one of Habilis, but they remained static for almost two million years. There was very little development.

And despite being very long lived and living across a wide range of environments, they just doesn't seem to have been very much change in Homo erectus populations over that time. And this is in sharp contrast to what we see with Homo sapiens later. So this leads a lot of people to think that Homo erectus hadn't quite reached that level of intelligence to have a very rich, adaptive and changing culture that contemporary humans do. And this also leads me to think that Homo erectus probably did not have language, although they may have had more sophisticated means of communication than modern chimps do, but probably not full language. We'll talk a bit more about that later. Some commentators distinguish between erectus populations that stayed in Africa,

which they call Homo Agasta, and then the erectus populations that spread throughout the rest of the world, which they call Homo erectus. But others just use the same word for both. So this leads to confusions. Now, I mentioned very early southern European populations found in Spain called Homo Antecesor.

Those branched off maybe 1.5 million years ago. There were probably other branches as well. One very interesting early branch, which is one of the most puzzling in human evolution, is the homo Floriances. So you may have heard of this. This was discovered only about 20 years ago made a lot of press coverage.

So this is a species of small archaic humans that inhabited only a single island, the island of Flores in Indonesia. They lived for a very long time. It seems that they branched off over one million years ago, maybe even two million years ago. And they survived until the arrival of modern humans about 50,000 years ago.

So they lived a very long time, but in a fairly small geographic location, they're thought to have been, like they're very small, and this is thought to be the result of island dwarfism, which is a common phenomena around many animals found isolated to islands. They become smaller. What's interesting about them is that they don't even seem to be descended from Homo erectus. So one hypothesis was that they're an offshoot of Homo erectus that became smaller,

but with DNA sequencing, we actually know that they predate the arrival of Homo erectus in that part of the world. They may have lived alongside Homo erectus at times, but it seems that they at least arrived before Erectus even did. So we don't even know where they came from. They may have been, they may have diverged from an even older group of unknown hominin species which left Africa perhaps even before Erectus. So that's a very strange one. We don't really know where they came from or how they branched off so much earlier than seemingly

all of the others. But they do seem to have been a fairly isolated population. So maybe there were many populations like this that we still haven't discovered yet. Who knows? It seems to be a very complicated picture. What we do know is that Homo Floriancer certainly weren't ancestors of modern humans. They lived on their island separately from the rest of humans for a very long time, and then only becoming extinct with modern humans arriving about 50,000 years ago. So putting aside Homo Florianzus, the picture we have now is that we have a population of Homo

Erectus, sometimes called Homo Oigaster, living in Africa and continuing to evolve. we then have Homo Erectus across much of the rest of the old world, apart from the northerly areas, living in Southeast Asia and India and the Middle East and parts of Europe as well, diversifying somewhat but also sort of intermingling to each other with some extent. Then what happened around 800,000 years ago, 600,000 years ago, you see these sort of different dates, is that a subset of Erectus evolved into a new form called Homo Heidelbergensius. And Homo Hidal Bligenius seems to have left Africa, or some of them migrated out of Africa, maybe half a million years ago or so.

Again, very hard to say. Some of these migrated out of Africa and into the Middle East and Europe and evolved into Neanderthals. Neanderthals are probably the most widely known form of early humans. They are an extinct group of archaic humans. They lived in Europe and Western Central Asia from about maybe 300,000 to 40,000 years ago. They became extinct with the arrival of modern humans. Neanderthals are famous for having very flat and broad skulls, heavy brow ridges and very wide noses. So they sort of look quite different in the face to modern humans, but in other ways they were very similar.

It's thought that many of these physical adaptations were responses to cold climates where many of them lived. Neanderthals maintained quite a low population. They lived in quite harsh environments and they seem to have suffered a lot of inbreeding. and this may have inhibited their ability to evolve technologically because they seem to have lagged behind modern humans in some ways. Now this is despite the fact that Neanderthals had, in fact, even larger brains than modern humans. This doesn't mean that they were more intelligent.

Maybe it was just because of other bodily adaptations, but there certainly seems to be evidence that they were at least as intelligent as modern humans, and we find evidence of more sophisticated tools. So they produced the Malssyrian stone tools, which was a significant development on the Ashulian tools previously used by Erectus. And we also find evidence of burials of Neanderthals. And so they seem to have been quite sophisticated, but very low populations. And so this may have inhibited their progress, as we'll talk about later,

that larger populations and greater ability for cultural development is probably very important for the evolution of modern humans. Another group that branched off from these early Homo Hidal Borgensius groups that left Africa are called the Denisovans. This is a fairly poorly documented group that doesn't even have a proper species name because it hasn't been agreed upon yet. But generally these fossils are found around sort of China, parts of Central Asia. They seem to have diverged from Hidal Borgensius around the same time as Neanderthal,

so maybe 300,000-ish years ago, but they're not very well characterized yet. So the picture we have circa 300,000 years ago is that we still have erectus living in parts of, Homo erectus living in parts of Eurasia, Middle East, South and Southeast Asia. We have populations of Homoidal Borgensius living in Africa and probably other early hominids there as all. Homo erectus is still living in Africa at this time. There are probably other species too. Some Homo Hidal Borgensius left Africa around half a million years ago and then evolved into the Neanderthals and the Denisovans. Nianthal's more Europe and Middle East, whereas Denisovins more East Asia.

And then we finally, around 300,000 years ago, get to Homo sapiens. So they evolved from a homo-hydrogyzegensius in East Africa around 300,000 years ago. It's thought that they resulted from a sort of merging intermingling of populations in the east and the south of the continent. These modern humans migrated out of Africa for the first time into Egypt to Israel around 200,000 years ago. There seemed to have been a series of sort of small-scale migrations. Not all of which were successful. But then around 70,000 years ago, there was a massive migration wave, which seems to have actually gone south across the Red Sea,

which at the time was lower than it is now, and across southern Arabia, and then around the Persian Gulf, through Iran, India, and out to China. And so those migrations took place around 70,000 years ago quite quickly. And then by 50,000 or 60,000 years ago, they'd crossed into Southeast Asia, Indonesia and into Australia that entered Europe by around 40,000 years ago,

crossed into Northern Asia, maybe 20 or 30,000 years ago, and reached the Americas by 15 to 20,000 years ago. So very quickly, within the space of a few tens of thousands of years, modern humans went from living exclusively in southern and eastern Africa to living effectively across the entire planet. So we spread very, very quickly. And with the spread of modern homeroyalmers, sapiens out from Africa beginning 200,000 years ago, but most intensely about 100,000 years ago,

we see the progressive extinction of all of the other early hominin species. It's not known how many hominin species existed at the time that humans evolved around 300,000 years ago, but it's likely that there were quite a few. We know that there were at least the homofluerensis, homo erectus, Denisovans, Neanderthals, and other populations that lived in Africa. Hydebligences and probably other forms as well. So there were probably at least five or six other forms and maybe more species that lived in different parts of the world at the time of the emergence of modern Homo sapiens. As Homo sapiens spread, particularly around 100 to 50,000 years ago, all of these other forms went extinct. So the Homo Florianus went extinct about 50,000 years ago. Neanderthals, about 40,000

years ago. The Denisovins, I'm not so sure about around the same time as Neanderthals, probably 40,000 years ago or so. Erectus, they went extinct a little earlier, maybe 100,000 years ago. They had competition with other archaic homo forms as well, but the emergence of fully modern humans probably was a factor in their final extinction as well. So one thing that's very clear is that once fully modern humans evolved and began spreading outside of Africa, they very quickly out-competed all of the other archaic human forms that existed. And that points to probably a fairly clear, adaptive benefits or greater adaptation for the similar environments that these other forms of humans were living in. And almost certainly this is greater intelligence. So humans, modern humans,

homo sapiensians had similar skull sizes to Neanderthals, Neanderthals actually a bit larger, but homo sapiens had noticeably larger brains, certainly than homofluerensis and homo erectus and other archaic forms as well. So they were likely much more intelligent than most of the other forms of of early humans that existed. It's less clear in why we out-competed than Neanderthals, because although Neanderthals have a reputation of being sort of backward and sluggish, that's probably not deserved, whether or not there were significant differences in intelligence is unclear.

It does seem that the nanotals really struggled with low populations and inbreeding, so that may have been a factor, but we still don't really know why they were out-competed by modern humans. So we've already talked over this discussion about some of the major adaptations that humans have compared to other great apes. We've talked about bipedalism. So I mentioned that it's, although we know roughly when bipedalism emerged, sometime around 5 to 6 million years ago, probably Ardipithecus was the first biped. Definitely by 4 million years ago, Australopithecus was habitually bipedal. What we don't know is why bipedalism developed.

It does seem to be associated with the change in habitats from fully forested to more open savannah, but remember that doesn't mean completely open. But exactly why that's benefits bipedalism is less clear. There are many, many different hypotheses that have been postulated over the last couple of centuries. The three main ones that are still discussed today is that being bipedal allows you to see further, which is useful for spotting food and predators in an open savanna environment, less useful in a heavily forested environment because the trees are in the way. So there's better vision. Second, it allows us to use hands for carrying tools and or food. And third, it reduces the amount of sun falling on our body, compared to if we're

walking on all falls, which promotes for cooling, again, particularly when you have a sunny environment, like the savannah as opposed to the forest where there's obviously much more shade. So is it vision? Is it freeing up the hands, or is it for cooling? There's actually no reason to choose any one of these. It's likely that all of these played a role. So my guess would be that initially bipedalists have developed for better vision,

but then the freeing up the hands became more and more important with later species of hominins. But that's just a guess. We don't really know which of them were most important. Now, the next major adaptation that distinguishes us from the other great apes is increased brain size or encephalization is the technical term. We see a very clear trend of increased brain size from osteopithecus to homo habilis, from homohabelous to erectus and from homo erectus to Homo sapiens. Each step shows a very clear increase in brain size. The biggest single step is

from homo habilis to erectus, where you see a nearly doubling in brain size, very substantial increase in intelligence. As I said though, that step from Homo erectus to modern humans, which is only about 30% or so, seems to have been very important because we don't see the same cultural richness or vitality in Homo erectus that we do in Homo sapiens, or Neanderthals, for that matter. Now, there are no unique human brain regions that no other primates have. What we do have are differently sized brain regions, in particular the cerebral cortex, the outermost layer of the brain, is four times the size of a chimpanzee. Humans are capable of types of cognition that either other animals, other mammals in particular cannot perform,

or only perform to a much lesser extent. So language is probably the most clear of these. So other animals do have forms of communication. And some animals like chimpanzees and corvids can be taught symbols, so an abstract pattern that stands for something else. But only humans have the ability to learn full language, a full language which comprises a generative syntax that allows for a combinatorial recombinations of symbols to be used in novel ways in accordance with the preset rules. I've talked about this in previous episodes where I discussed

language, so you can go back there if you're interested. But language is more than just communication. So people often sort of sort of naively talk about animal language, but technically speaking, it's generally thought that only humans have language. Other animals do have different forms of communication, but not as sophisticated and rich as human language is. So that's obviously one thing that distinguishes humans. I mentioned earlier that we don't know when language evolved. Language, unfortunately, doesn't fossilize. But what does fossilize are the cultural products that almost certainly would have required

language in order to generate or in order to have seen these. So burials is one thing that's thought to be indicative of a level of sophistication that would have required language because burials are indicative of sort of symbolic, culturally shared beliefs, which almost certainly would have required language in order to pass on. It's a bit controversial as to exactly when the oldest burials are because it's sort of hard to establish. In some early cases, what counts as a burial and what doesn't, but it does seem like the earliest burials can be dated to archaic homo sapiens and Neanderthals, so 200,000 years

ago, 100,000 years ago, something like that, not before, so not Homo erectus or previous to that. Other adaptations, including not just more sophisticated tools, but tool culture that changes significantly over time. We don't really see that in Erectus. Erectus developed the Echulian tools and then kind of there was very little change for a million plus years. Whereas when we have the emergence of modern Homo sapiens, we see the development of much more sophisticated tools. And then particularly after about 100,000 years or so, many more sophisticated forms of tools, of bone tools, fishing tools, mining tools, barbed points, pigments, and so forth are progressively found.

And then later on we see like bees and images, like cave paintings from about 50,000 years ago and so forth. So this sort of rapid progression of cultural forms is just not seen in any other homin species other than modern homo sapiens and to some extent Neanderthals. And this to me points to a sort of cultural learning and development that required language. So certainly modern humans had language. I suspect that Neanderthals also had language. It may have been a little different to human language, but I suspect it was probably quite similar. Homo erectus I don't think had language. They may have had some sort of proto-language, but I don't think it would have counted as full human language.

Because I think if they had had that, they probably would have shown much more cultural development over time than we do see. That's just my opinion. We don't really know. It's thought that probably full language developed around 300,000, 200,000 years ago with modern humans. Or maybe just before the split with modern humans, from modern humans with us, other types of Heidelbergensis, so maybe also the ancestors of Neanderthals also had language. Or it's possible that they both subsequently developed language. Other aspects of human thought and intelligence that are fairly distinctive is our ability

to perform abstract generative thinking. So this is essentially like recombining existing ideas in novel ways. Obviously other animals can do this, but the human ability is just much greater than other animals. Our ability to use symbols, words and images. also our ability to apply something that we've learned in one domain to a different domain. Many other types of animals really struggle with this. To be honest, humans still often struggle with this to take a lesson you learn in one case and apply it to a different case where there's similar principles, like drawing analogies essentially.

But this is something humans can do to a much greater extent than other animals. Humans excel in very in flexibility, and we engage in planning to a much greater extent than other animals. Most animals don't really plan at all. They just act sort of in the moment on the basis. of stimuli that they're presented with immediately. Some animals can do limited levels of planning. I should say many behaviors that look like planning are actually instinctual, building nests and things like this.

They're not formed by the animal consciously thinking through a sequence of stages of achieving this goal to achieve that goal to achieve the final goal. That's what I mean by planning. And the extent of that is unique to humans. Obviously, again, there's degrees, but human extent of planning are much in excess of any other animals. Another thing is social learning, so most animals, their behaviors are instinctual or learnt only to a limited extent.

For example, bird song is instinctual, but the specific form of song is learned and does differ between different groups of birds depending on the environment. So there are degrees of social learning that you see in other animals. But again, the extent to which that happens in humans is much, much greater than other animals. Human infants are born very altrucial, so very poorly developed, and they have a very long period before they reach. physical maturity, it takes like 20 years to reach physical and intellectual maturity. And throughout all of that time, humans are engaged in social learning. I mean, hopefully we continue learning after that, but particularly during that time of development, we learn the language, we learn customs, and we learn our role in society.

And that prolonged period and extent of social learning is much, much greater than found in any other animals. There are obviously animals like bees and ants that have very extensive social interactions, but those are almost all instinctual with only a very small extent of learning that modifies the specific form of behaviours but doesn't really change it in any fundamental way, whereas the flexibility of human behaviours is extreme. And you can see that with the range of environments that humans can live in. So modern humans within tens of thousands of years occupied essentially all areas of the world. there were a few islands that they took a bit longer to get to.

But everywhere outside of Antarctica was inhabited by modern humans, you know, before the voyages of discoveries, like pre-1,500. So everywhere from deserts to tropical rainforests to temperate regions to Arctic regions, humans adapted to all of these environments. And that's just a much greater adaptation than we see in most other non-domesticated animals. There are certain animals like rats that can kind of live wherever humans do, but that's sort of because they're living off human culture. But generally speaking, human adaptability to the environment into different diets and things is very high.

And we do that through social learning, through adapting collectively and using tools and other things like this. So use of clothing and fire, for example, are very distinctive to humans. And those allow us to adapt to a wider range of environments than would otherwise be possible. Now, this combination of traits that we have, social learning, greater communication, ability to plan, flexible behaviours, thinking abstractly. All of these suite of traits and the particular adaptations that they allow for has been called a cognitive niche. So we think of a niche as like a particular an aspect of the environment or a way in which the animal lives in that environment that allows them to survive. Like, for example, pollinating insects have a niche that allows them to get

food from collecting from pollen of flowering plants, take them back to the nest, where they use that energy to feed their young. And flowering insects produce that the nectar that goes with the pollen because it benefits them to have insects spread their pollen around. So that's a niche that pollinating insects have developed that allows them to survive in a particular context of environments. Nitches are dynamic. So pollinating insects can't live without flowering, plants, so they couldn't have lived prior to the development of angiosper the evolution of angiosperms. So niches are not just static, they develop over time and they're highly dependent on climate and geography and the other set of organisms that exist in a given place at a given time.

Humans have evolved what some call this into a cognitive niche where our specialty is being generalists, and we can be generalists because we're very flexible, and we work together using tools and planning and communication in order to just kind of deal with whatever environment where we encounter. So this is thought to be why humans developed increased brain size is that we, after we sort of of quote-unquote came down from the trees, we evolved bipedalism in a more open environment. We specialized into this cognitive niche, which is sort of being generalists, allowing us to have this degree of flexibility and learning through culture.

One question is why haven't other animals developed this cognitive nature or developed greater intelligence? Obviously there are animals that are quite intelligent. There's chimpanzees, dolphins, pigs, and corvids are all very intelligent, as well as cephalopols, although we understand those less because they're so distant from us. So there are other animals that have developed greater intelligent and more flexible cognition, but none of them have reached quite the level that humans have. And my personal view on this is that there's nothing special about primates that makes it such that humans were the first. It's just sort of a bit of a historical accident, like kind of someone had to be the first, and it happened to be humans. One of the major benefits of being bipedal, obviously, is that we have the use of our hands to make tools. And that's very important for flexible behavior, because we can augment the limited biological capacities we have of like our fingernails and teeth and so forth and our muscles with tools that allow us to,

perform important adaptive behaviors in a much wider range of environments. So having the hands free to make tools, I think is probably crucial for the evolution of intelligence. And then once you're making a wide range of tools, it's very useful to be able to communicate this to other people because you can teach your children how to make those tools. Often in other animals, this sort of knowledge would become instinctual and become encoded in the genome. But the downside of that is that it takes a long time to change, and it's not flexible

to changes in the environment. The cognitive niche that humans developed for was one in which, there's rapid change and flexibility. And so these behaviors need to be taught through culture, not encoded in the genome. So there's definitely, I think, a link there between tool use, bipedalism, and encephalization, increase in brain size. It's possible that other animals, in different historical trajectories could have developed this as well. But again, the challenge for, say, birds or pigs is that they don't have the hands to use to make tools. And so that might have been one constraint. And the benefit of primates is that they'd already developed,

differentiated four limbs compared to hind limbs because of the need for brachiation. And so then when humans came down from the trees and began to walk, their ancestors began to use bipedal locomotion more often, then they had that freed up their hands to do other things with. So that might be one reason why primates were particularly well suited for this. One interesting speculation is whether what sort of levels of intelligence could be achieved by an octopus, which has multiple arms, which it can use to manipulate things. Of course, their disadvantage is that they live underwater, and so there's a greater limitation for using things like fire to cook food or use for making tools. But who knows? We don't even know how intelligent octopuses are,

let alone what hypothetical other evolutionary pathways could have delivered. But anyway, that's enough on human intelligence. One last adaptation that I wanted to mention is the so-called obstetrical dilemma, which is a hypothesis about why humans have such a difficult childbirth. Most primates give birth without assistance easily, and it's not a major, like a traumatic experience, which it can often be for humans. The hypothesis to explain this is that there's a trade-off between an increased skull size, which allows for an increased brain size, which allows for greater intelligence, on the one hand. But on the other hand, there's only so why, the pelvis can become for human females allowing them to fit the skull through it during birth

and thereby, you know, deliver the baby. So the human females need to stay bipedal, but they also, we also want to have as larger skulls as possible. So there are a number of responses to that. One is that the human skull, the human brain has become incredibly folded. Other mammals have folded brains as well because it allows for more surface area to fit into a smaller volume. but the human brain is particularly heavily folded. That's one adaptation. Another adaptation is that humans' babies are born very prematurely compared to other primates.

So this is called altrucial births. And it's thought that that's in part an adaptation to allowing the, if the baby is born earlier, then it can still achieve a larger adult brain size, but fitting through a relatively smaller pelvis size. So that's one of the reasons we think that human births are so distinctively different. Before finishing up, I wanted to say a final word on tool use. Here we're just talking about stone tool.

The development of more sophisticated types of tools like pottery and then metallurgy really is only dated to the Holocene like last 10,000 years, maybe a little bit before that with some of the pottery. So here we're just talking about the Stone Age prior to the inventive agriculture. Within that period, there is often categorized into four main modes of tools. We've already talked about three of these. So the first mode is the older one tools that were used by homo habilis. They are quite really primitive tools which involve taking a rock of flint and then chipping

off some of the flakes, which you can then use as cutting tools. They're not very substantially worked to any extent, but they were thought to be used as sort of primitive axes or chopping utensils. So those were used by homohabelus from about 2 million years ago. Maybe some Australopithecines also used, some later austroplethicines also. produced old one tools, but it seems to be mostly homo habilis. Homo erectus started off using Old One tools, but then developed the Ashulian tools. So this is Mo2, and these are hand axes that are much more significantly shaped.

So the idea here is that instead of just chipping some flakes off of a piece of flint, you actually shape the core of the flint so that you remove many pieces and then shape it on both sides into into an axe. So it's a much more heavily worked piece than the older one tools. But as I said, there doesn't seem to have been much development in this shulian tool industry, as it's called, for over a million years. These were worked by Homo erectus from about two million years, right up to about 100,000 years ago. The next mode, mode three, are the Mousderian tools. These are flake tools that are produced by much more intense working of the initial core of flint. So you sort of work it in a similar way to the Ashulian tools, chipping off parts of the edge

to give it a sharp edge, and then you sort of cut a larger flake off is then further shaped and makes for much smaller and sharper edges. The Mouse Deerian tool industry was mostly used by Neanderthals, but there's also similar industry that's used in Africa at the time, so it seems like other archaic homo species were also developing more sophisticated forms of industry. The final two modes, 4 and 5, I can't pronounce the name for Mode 5, so I won't try to say it, and Mode 5 is often called Microliths, very small blades. Both of these, Mode 4 was like long blades, microlifts, very small blades. They were more complex, heavily worked pieces that are associated with modern Homo sapiens,

so about 100,000 years ago or so, I think. And particularly the microlists are thought to have been used in composite tools like harpoons and arrows and things like this. We see a progressive increase in the sophistication of stone tools alongside an increase in brain size. So this is sort of encouraging that the increased brain size is associated with more sophisticated behaviors. And in particular, a much greater increase in the sophistication and rapidity of development of stone tools when we see both Neanderthals and modern Homo sapiens from around 200,000 years ago or so. One last concept that I want to mention is the term called behavioral modernity. So this is an idea that a suite of behavioral and cognitive traits emerged in relatively short period of time.

People give different dates around 50,000, 60,000, 70,000 years ago. These traits are thought to include things like planning, abstract culture, complex social learning, full language, music and dance, art, things like this. whether this is a good way to think about the development of monomotomersapians is sort of less clear. So it is true that as you get closer to the present, you see more and more of these more complex behaviors and more abstract behaviors. What's less clear is whether this is the result of any particular, I mean, what some have said, it's like single or a couple of gene mutations, or whether this is really just sort of an accumulation of greater and greater cultural sophistication over time with learning and cultural accumulation and also larger populations, more trade and interfacing between them. I tend to think more the latter. I think that probably there was no real cognitive difference

between modern homo sapiens 150,000 years ago or 50,000 years ago. The differences were largely cultural and just due to sort of greater cultural adaptation. I mean, we certainly know that there was no intelligence difference between humans 10,000 years ago and humans today, even though obviously our technology is much more sophisticated and our culture has become more sophisticated and intricate as well, but there's just been a lot more time for cultural accumulation and building on the past. So I tend to think that there was no sort of key evolution of any sort of key mutations around 60,000 years ago that led to a big bang of cultural adaptations. There was just an increasing prevalence and greater sophistication of many of these types of

behaviors, so toolmaking, artwork, burial, and other complex traits that we can see evidence of that we see more and more of over time. So the idea that there was a sort of a relatively short time in which this originated, which we can call behavioral modernity, you know, it's still a bit debated, but I think it's a bit of an older idea and not particularly well supported, because we see a gradual emergence of these behaviors, and then they become more and more common. And there's not really strong evidence to think that there was any singular mutations that led to these. And I think that's sort of implausible for other grounds on other grounds as well. So anyway, we conclude about 10,000 years ago with modern human beings living essentially

all over the planet and the very beginnings of agriculture beginning to emerge in certain places like in the Near East. So perhaps in a future episode we'll talk about agriculture, including its origins and some of the related science. But for now, we're going to end our story here. So thanks very much for listening. I hope you found this episode interesting. That concludes also our sort of history of life series that we've done for the past few episodes. Also, just a brief announcement before finishing up, so if you're listening to this in 2026 when it was released, we have coming up from the 15th through 23rd of August, National Science Week. So there are over 2,500 events that will be held across Australia to celebrate

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