Lecture 5.2: Earliest Fossil Hominins PDF

Lecture 5.2, the Earliest Fossil Hominins. At this point, we've moved past general primate evolution and can start zeroing in on our own direct evolutionary lineage. In this lecture, we'll examine the earliest hominins. These are extinct or fossil species that are from our side of the human ape evol...

Lecture 5.2, the Earliest Fossil Hominins. At this point, we've moved past general primate evolution and can start zeroing in on our own direct evolutionary lineage. In this lecture, we'll examine the earliest hominins. These are extinct or fossil species that are from our side of the human ape evolutionary split. While the list of early hominins is slowly expanding as every few years more fossils are encumbered that might be from a new species we've not known about before. In this course, we'll just look at the major ones, the ones that we have pretty good fossil evidence for and are well established in the human evolution literature. But first, let's look at a couple of more important evolutionary terms. Plesiomorphic trait. Plesiomorphic traits are what in lecture 4.2, we were calling primitive traits. These are traits or characteristics inherited by species because their ancestors had it. It's not a new trait. It had been around for a while and is not first appearing in the species we are looking at. Prehensile feet is a plesiumorphyr in the living apes because they evolved from ancestors who had this trait. Chimpanzees inherited their prehensile feet from their ancient ape ancestors who inherited it from even earlier primates. Another example is us humans lacking a tail. This is a plesiosomorphy because tails disappeared in our ape ancestors. We can compare this to apomorphic traits. These are new traits that first appear in the species we are looking at. They did not inherit the trait from an ancestor. A good example is the lack of a tail among the earliest fossil ape species from around 20000000 years ago. Early ape ancestors evolved from monkey ancestors who had a tail, but early on in ape evolution, tails disappeared, likely because apes were larger and spent less time in the trees where a tail can be very useful. So among these earliest apes with no tail, this was an apomorphy because they evolved from species which had tails. Another example is the loss of prehensile feet among early hominins around 4000000 years ago. This is an apomorphy because these early hominins evolved from species with prehensile feet. Another example would be the loss of body hair coverage in the human lineage likely with Homo erectus who evolved from an earlier hominin with a furry body like an ape. We'll cover this in more detail in unit 8. Today, a lot of time is spent hunting for new fossil hominins. This is especially the case in Africa, although some very old hominin remains are found in Asia and Europe as well. And every year, new bits and pieces are found and from time to time, something really exciting is uncovered, maybe an especially complete and well preserved fossil. Whenever this happens, whenever a new fossil is found, the first task is to try to determine where it fits in our evolutionary tree. Does it fit into an existing genus and species? Is it just another example of a species that we're already familiar with, or is it different enough for previous fossils that it should be given its own species or even genus designation? This typically involves studying the collection of plesiomorphic and anthropomorphic features that the new fossil might have. A lot of time goes into this process called evolutionary systematics. Evolutionary systematics is the process of trying to reconstruct the evolutionary relationships of different species, whether they are living or extinct fossil species. 2 important terms associated with this are phylogeny and taxonomy. Phylogeny refers to the actual evolutionary relationships between different organisms. How are they actually related to each other in their evolutionary pasts? Taxonomy is the process of classifying organisms based on our best guesses about phylogenetic relationships. Using whatever data we have at hand to do our best to figure out the phylogenetic relationships of different species. Now the different groups in the taxonomic system, different species, or different genera are referred to as different taxa. The singular term is taxon. With living organisms and a few fossil species, we can use their DNA to get a much more accurate idea of actual evolutionary relationships. DNA can give us an almost 100% confidence in phylogenetic relationships. However, for most extinct species, like most of our hominin ancestors, we don't have DNA. So our best guesses about their phylogenetic relationships is mainly based on how different or similar two fossils are in their morphology, how many plesomorphic traits they share, and what apomorphic traits distinguish them. Evolutionary systematics produces phylogenetic trees like this one for the hominins. Phylogenetic trees like this show the order and timing in which different taxa split apart, and they indicate possible ancestor descendant relationships. As you can see this tree has a timeline along the bottom edge in 1000000 of years. Phylogenetic trees indicate when we think new species appeared and how much time passed between divergent events by taking into consideration the dating of fossils. Different researchers will have different ideas about how a tree like this should be drawn, but this version is a very common one. From here on through the rest of the course, we're going to be looking at the species in this tree, so it's worth you spending a bit of time familiarizing yourself with it. I know it looks like there are a lot of species for you to learn, but we will tackle this very systematically, which will make it a lot easier and you'll easily become familiar with all these species, especially the later ones. In the lecture on primate evolution, we were introduced to some geologic periods that you didn't need to learn. You'll be happy to know that you still don't have to learn most of these. However, the most recent ones are important to the emergence of the hominin lineage and eventually to the emergence of us humans, in particular, the Miocene epic from 23 to 5,300,000 years ago. This is when we see the emergence of the hominids, the apes. However, now we are really only interested in the very end of the Miocene between 8,000,005,000,000 years ago as this is when our own hominin lineage appears and diverges from the other ape lineages. The next geologic period is the Pliocene Epoch from 5000000 to 2,600,000 years ago. In this period, we see other important stages along human evolution. It's right at the end of the Pliocene at the beginning of the next, period, the Pleistocene, that we see the appearance of our own genus, Homo. And during the Pleistocene, we see our genus, Homo, evolve into a number of early direct ancestors of ours, like Homo erectus, and eventually the emergence of our own species, homo sapiens, and some close cousins like the neanderthals. The final geologic period, the one we're in today, is the Holocene epoch, which only started 12000 years ago. It's only in this very recent period that we see people switch from hunting and gathering to farming, raising domesticated animals and plants, and starting to settle down in villages and then towns and, eventually, cities. It's also just in the last half of the Holocene that writing is invented and advances in technology really start to take off at an exponential rate. You should learn the names of these epics and their start and end dates. The Miocene, the Pliocene, 5,000,000 to 2,600,000 years ago, the Pleistocene, 2,600,000 to 12000 years ago, and the Holocene Epoch, the last 12000 years, and, also, the quaternary period that is made up of the Pleistocene and Holocene Epochs. We were introduced to the terms hominid and hominin in lecture 4.3 on modern primate taxonomy. Hominid comes from Hominidai, the family that includes humans and the great apes. However, at this point in the course, we are no longer interested in the apes. We want to find out about the earliest hominins. Hominin comes from Hominini, the tribe level at which humans are separated from the great apes. We share a common ancestor with modern chimps and bonobos, the Panini lineage. That common ancestor lived sometime between 68000000 years ago. Chimps and bonobos have been undergoing their own evolution since that split, but the fossil and genetic evidence suggests that, in fact, unlike us hominins, the Panini lineage has not changed much over this time. That common ancestor probably looks something like a chimp or a bonobo, and so we typically use modern chimps as an analog for what the earliest hominin might have looked like. Therefore, it's worth looking at some of the major characteristics of chimpanzee skeletal anatomy to help us know what the earliest hominin fossils might look like. I've included a small picture of a modern human skeleton here for comparison. Starting with their body, everything below the neck, what an anatomist called the postcranial skeleton, chimps differ from us in a number of important respects. Unlike us, compared to the rest of their body, chimps have relatively long arms and relatively short legs. And they have a long, narrow pelvis, and their legs are widely spaced and tend to be parallel to each other. They have long, narrow fingers and toes, and both hands and feet are prehensile. And unlike us, they have a rib cage that is narrow from side to side and deep from front to back. Looking at their cranium, the skull and mandible, they have a brain size typically between 304 100 cubic centimeters with a mean or average of around 350 cubic centimeters. They have a very prominent growth of bone above their eyes called a brow ridge, though we'll learn a more technical name for this later. Their face exhibits what is called alveolar prognathism, That is their lower face protrudes. Most animals are prognathic, which just means the face sticks out. But in most animals, their whole face protrudes, not just the lower half. They have a body structure in their lower phase called canine pillars. This provides structural support for their large front teeth and powerful bite. If we were to look up at the roof of their mouth or down onto their lower jaw, we would see that they have a u shaped tooth row. Their cheek teeth on either side are parallel to each other. They have relatively small cheek teeth. This is the molars and premolars. They are only slightly larger than ours. But they have quite large incisors, the flat teeth at the very front of their mouth. And they have large canines, especially the upper canines and especially in the males. In fact, the upper canine has a feature common in carnivores, a canine third premolar honing complex. When they close their mouth, the rear cutting edge of the upper canine is sharpened against the lower premolar. So when we start looking for the earliest hominin, we are generally looking for fossils that might have a lot in common with chimps. They might share a significant number of plesomorphic traits inherited from our common ancestor with chimps, but we'll also show at least some epomorphic or advanced characteristics that have evolved since hominins split from this common ancestor. In terms of specifically hominin evolution, the last 6 to 8000000 years are the most important. This is when we split off from the ape lineage, so this is when the first hominins appear, and this happened in Africa. For the 2 oldest fossils we will look at, it is currently unclear where they sit in the evolutionary tree relative to hominins. We don't know for sure yet whether they predate the split and are therefore just hominids, or if they're actually pennies and ancestral to chimps and bonobos, or whether they are actually very early hominids. The first fossil species is called Sahelanthropus charensis. Its nickname is Tumai, which means hope of life in the language of the region. This fossil was found in 2,001 in the South Saharan Desert in Chad, far from East and South Africa. It's dated to around 7000000 years ago. So right around the time of the hominin chimp split. So far, we have just this one cranium, 2 lower arm bones, the ulna, and a portion of the left femur. This is a virtual reconstruction of the skull filling in a few lost bits. And this is an artist's reconstruction of what Sahelanthropus charensis might have looked like. Overall, it has a very ape like face and a relatively small brain, 350 cubic centimeters, comparable to chimpanzees. But unlike apes, it has relatively small canines, though not much smaller than some female chimps. Thumai could be a chimp ancestor, but the question is how bipedal was it? Was it a habitual biped like chimps? Was there evidence it was better adapted to bipedal walking and possibly an ancestor to us? In fact, we can tell something about tummy's posture from the location of the nuclear line on the back of the skull where the neck muscles are attached to the back of the cranium, and from the location of the foramen magnum, a large hole in the base of the skull where the spinal cord exits from the brain. This is a view of the nuchal muscle location on modern humans. Our muscles extend from our shoulders to the back of our skull. They keep our head from falling forward and can lift our head up. Our skull is balanced nicely on our spinal cord, so our nuchal muscles are rather small and attached near the base. This is where chimps' nuchal muscles attach to their skull, high up on the back. This is because chimps walk on all fours much of the time, which requires their neck muscles to hold their head up so that they can see where they're going. As is the case with most terrestrial animals. Here we see an X-ray of the location inside view of the nuclear line in modern humans and an X-ray of a chimp and Thumai. Discoverers of Thumai argued the location of the nuclear line indicates Thumai stood upright more than apes do, that Chu Mai was more bipedal, so an early hominin. However, some researchers argue that the location of Chu Mai's foramen magnum does not reflect full bifidality. The position of this hole in the base of the skull where the spinal cord exits is also directly related to posture. Ours is in the exact center of the base of our skull because we stand upright and our skull is well balanced on the top of our spinal cord. A typical terrestrial quadruped like this dog has a foramen magnum right at the very back of their skull because their head sticks out in front of their body. A chimps is roughly halfway between, which seems to reflect their partially quadrupedal and partially bipedal adaptation. Some researchers point out that tummy's foramir magnum is positioned more to the rear of the skull, although it is clearly further forward than a chimps. So it is currently unclear which side of the human ape split the species is on, but tumai is certainly a prime candidate for the earliest hominin that we have discovered so far. Our second candidate for an early hominin is Ouroentuganensis. This fossil was discovered in Western Kenya in 2000 in deposits that are about 6000000 years old. So like satanthanthus jadensis, it lived right around the time of the ape human split. The fossil remains are pretty scant and currently only include fragmentary arm and thigh bones, lower jaw pieces, and isolated teeth. No cranium. The incisors and canines are ape like close to those of a female chimpanzee, but the molars are smaller like us. The limb bones are about the size of a female chimpanzee, but there are some important distinctions. For example, the muscle attachment point on the femur where the large gluteus maximus muscles attach are on the back of the femur like us rather than on the side like in chimps and bonobos. This is associated with legs that swing forward and back like us obligate bipeds rather than legs that can be moved easily to the side to grab branches like in chimps. This strongly suggests the species was more bipedal than the chimps. Orin was likely a good tree climber, but appears to have been adapted to habitual bipedalism when they were on the ground. However, we need a lot more fossils of this species before we can really say anything for sure. And for the moment, they simply remain a good candidate for being an early hominin. For the period between 5,000,000 and 1,000,000 years ago, there's fossil evidence for at least 20 species that have been proposed over the years. Some recently proposed and some old and no longer used. We'll look at just 11 of the better known ones. These are placed in 4 different genera. Ardipithecus, which includes one species we'll look at, ramidus. Australopithecus, we'll look at 4 of the currently 7 proposed species. Paranthropus, the 3 species. And our genus homo, which includes 16 proposed species. In this lecture, we'll look at just 3 that date to this time period and another 4 that appear after 1,000,000 years ago. The oldest of all of these fossil species is Ardipithecus ramidus, which is dated to 4,500,000 years ago. Ardipithecus ramidus was first discovered in 1992 in the Afar Depression in Ethiopia. We don't have any one complete cranium, but we have large portions of several different individuals. So we know what they look like, and this is a reconstruction of their cranium. I will include a chimpanzee cranium in the lower right for easy comparison. Ardipithecus has several plesiomorphic traits, traits that were common among earlier species of apes and that we still see in chimps today. Their brain is similar in size to chimps between 304 100 cubic centimeters. They have relatively large front teeth, their incisors. Enthear molars have a more rectangular shape, which is very ape like trait. But Ardipithecus also exhibits more apomorphic traits, traits that show up first with this species but become common in later hominin species. Their canines are a bit large, but they are also smaller than modern chimps. And their incisors are larger than later australopithecines, but smaller than those of chimps. And they have reduced alveolar prognathism, that is a flatter, more vertical face. And their their forearm and magnum has moved more forward suggesting very upright posture and likely obligate bipedalism, at least when they were on the ground and not climbing trees. Although we have no complete skeletons, we have pieces from at least 36 diff different individuals. We can tell that the adults were about 120 centimeters tall and weighed around 40 kilograms. This is about the size of a 7 year old boy. They lived in heavily forested woodland and seemed seemed to be well adapted to bipedalism when they were on the ground. But they had relatively long arms and short legs and long fingers and toes, and their feet were still prehensile, like an apes, so they could still grasp branches with their feet. These are obvious adaptations to living in the trees, and this species is clearly still strongly arboreal. Currently, it seems that most researchers accept the species as a hominin. So for the moment, this is our earliest widely accepted hominin ancestor. For our next genus, Australopithecus, we have found many more fossils that have been assigned to several different species. But we'll just look at the 4 best known species. Australopithecus anamensis, Australopithecus afarensis, Australopithecus africanus, and Australopithecus sediba. Though we might have some reservations about this last one as we'll see. The oldest species in this genus is Australopithecus anemensis, which dates to around 4000000 years ago. As we'll see, some early hominins seem to have been found only in South Africa and some only in East Africa. Animensis is found only in East Africa. Animensis fossils were first discovered in Western Canada in 1965 by Maeve Leakey, the daughter-in-law of Lewis and Mary Leakey that we were introduced to in lecture 5. More fossils were found in Western Canada are in the 19 nineties and some in Ethiopia as well. Currently we have bone fragments from at least 20 different individuals, but no complete complete crania until 2019 when a well preserved skull was found in Ethiopia. They have a few primitive plesiomorphic features that include a relatively small brain by hominin standards. It's only 370 cubic centimeters. This is well within the range of chimpanzees. They have a generally apelike cranium and dentition. Their face and teeth still generally resemble chimps, and they still have relatively large canines with significant sexual dimorphism. They also have apelike parallel tooth rows that reflect a significant degree of alveolar prognathism. However, they do show some really important evolutionary advances. Their epimorphic features include a postcranial skeleton that is their torso and limbs that is typical of the later hominins. They have slightly longer legs and slightly shorter arms. They're also a bit larger than Ardipithecus, around 50 kilograms and a 130 to a 140 centimeters tall for males and a bit smaller for females. Most importantly, their leg bones like this tibia have features, features present in modern humans that indicate that it was fully bipedal, and its feet were not prehensile. Animenses is our earliest for sure obligate biped, and all the hominins we'll cover from here on were as well. The australopithecines like Animenses lived in a range of types of environments, but these were all treated to some extent, either wooded grassland or full on forest. And they likely regularly climbed trees to escape predators or to sleep. But obviously, they spent considerable time on the ground walking around on 2 feet foraging for food. I'm going to talk about the next two species together since the differences between them are not that dramatic. Australopithecus afarensis and Australopithecus africanus are 2 of the better known fossil hominins, and afarensis in particular is a likely candidate to be a direct ancestor for our genus Homo. A. Afarensis fossils date from between roughly 4000000000 years ago, and the africanus fossils are between 3000000 and 2,400,000 years old. Afarensis fossils are only found in East Africa, primarily in the Haddar region of Ethiopia, but also Lytole, Tanzania and Cubipora, Kenya. Africanus is a South African species, and its fossils come from cave sites like Styrpfontein and Tong. While there are some slight differences between these two species that I'll touch on briefly, in general, they are very similar. Appearances and Africanus plesomorphic features include a relatively small body. Females are about 110 centimeters tall and weigh 30 kilograms. Males are about 140 centimeters tall and weigh 45 kilograms. This shows a degree of sexual dimorphism greater than with modern humans. With us, there is a roughly 1.1 to 1 ratio. Men are on average perhaps 10% larger than women. With gorillas, orangutans, and baboons, the ratio is about 2 to 1, males twice the size of females. With apheresis napricanus, the males are on average perhaps 30% larger than females. As we have seen with modern nonhuman primates, sexual dimorphism is strongly associated with social structure. Increased sexual dimorphism seems to reflect increased male male competition for females. Perhaps with afarensis and africanus, competition for females was less than less than with gorillas, but still significant. One difference between the two species is that Africanus has slightly longer arms than afarensis. Afarensis and africanus crania also exhibits some obvious plesiosomorphic traits. While Africanus brains are slightly larger than afarensis, collectively, their mean cranial capacity was only about 425 cubic centimeters. This is not much above the upper range for chimps and still less than than a third the size of our brains. Their incisors are still quite large, similar to female chimps. And while they do have smaller canines than chimps, they're still quite prominent and do exhibit a significant degree of sexual dimorphism. Though not as much as modern apes, the male's teeth are notably larger than the females. And in fact, 75% of afference's crania have a canine premolar diastema, a gap between the lower canine and the first premolar where the upper canine can fit when the mouth is closed. Apes have a large canine diastema, you'll be more familiar with the canine diastema that dogs and cats have. These are the last hominins to have canine diastema, which disappear in all hominins after this. Here we can compare the dental arcades of chimpanzees on the left, afarensis and africanus in the middle, and a modern human on the right. Dental arcade refers simply to the shape of the tooth row. The red arrows indicate the canines. You can see the chimp has very large ones, and the australopithecines are still relatively large, while ours are quite tiny. You can also see the diastema, the gap in the teeth between the canine and the incisors that we completely lack. Afarensis and Africanus also still had a u shaped dental arcade with parallel tooth rows similar to Australopithecus anemensis that we saw earlier and in apes, whereas we modern humans have a parabolic shaped arcade. You can also see one of aparensis and africanus' important apomorphic traits, very large molars compared to us and chimps. This increase in the size of the molars is even more obvious in Africanus than in the afarensis. And Africanus also has a more robust and strongly built lower jaw or mandible. These are traits that become even more important in the parenthesis genus that we'll get to shortly. Another apomorphic feature in aparensis and africanus is more human like hands, shorter fingers mainly and a slightly larger thumb, and wrist bones that look more human and less apelike. Well, it seems that obligate bipedalism had already appeared with earlier species, at least with Australopithecus anemensis. There are a number of characteristics of the afarensis and africanus skeletons that really illustrate their obligate bipelism. First, the femur, the upper leg bone, clearly indicates that these two species were obligate bipeds. The femur neck shaft is significantly elongated, pushing the ball of the femur that articulates with the pelvis away from the femur shaft. And the femur neck has an increased angle, which results in the femur shaft and the legs in general angling in towards each other and towards the centerline of the body. While these differences may seem slight in this illustration, they are really important for how these species' legs worked. Also had a more modern looking pelvis, quite similar to ours in fact, which has evolved to serve mainly as a stable anchor from which our legs can suspend like pendulums. Chimpanzee's pelvis plays more of a role in their need to be able to move their legs through a much greater range of motion, which is important for climbing trees. Chimps have to be able to move their legs and feet in a huge range of motion to be able to grab onto branches. This collection of features we see in afarensis and africanus, a more stable pelvis, and femur heads with longer necks and a more open neck angle result in the legs being angled in toward the the center of the body with the knees close together. This allows these 2 species legs like ours to swing freely and easily beneath the pelvis, forward and back like 2 pendulums. Also, as with anemensis and us, the attachment location for the gluteus maximus muscles is on the back of the upper leg bone, the femur, rather than on the side like chimps. This also facilitates the legs being able to swing easily forward and back. On chimps, these muscles are attached on the side of the femur, which is important for them because as we noted above, they need to be able to move their legs around a lot to be able to grasp branches with their feet. But because chimps legs are widely spaced, when they walk bipedally, their center of gravity shifts from one leg to the other, which makes them waddle and sway and results in an awkward walking gait. Having your knees close together under your body gives you a single center of gravity as you walk, and having femurs that swing smoothly from back to front results in results in a smooth easy walking gait with no major swaying from side to side. We also have the well preserved afferentsus footprints at Laetoli that we were introduced to earlier. These are also clearly the footprints of an obligate biped. Researchers carried out a high resolution analysis of the form and weight distribution in a modern footprint compared to the Lytole footprints. The Lytole footprint here is a bit less clear than the modern one because the modern one's made in very controlled conditions. However, the results indicate that while there are slight differences in weight distribution across the bottom of the foot as each step was taken, afferentes had an almost modern foot and an easy walking gait much like ours. There's no doubt that afferentes was well adapted by pelism. This was their only mode of locomotion around the ground which makes them all the good bipeds. We should also add here that this is the case starting with australopithecus anemensis and continuing with all the later hominins. It's not important for this course that you learn about lots of different examples of afarensis and africanus fossils, but it's worth covering a few of the important ones. This fossil named Lucy is a 3,200,000 year old partial skeleton of a female Australopithecus afarensis found in 1974 at Hadar in Ethiopia. For many years, this was the most complete fossil we had for afarensis. You can see how fragmentary these fossils typically are. And this is a very optimistic reconstruction of Lucy by a French artist named Elizabeth Daemus. And this fossil is called Catanumu, which means big man in the local language. It's a 3.6000000 year old adult male, Australopithecus afarensis, discovered in 2005, also in the Hadar region. While it is also far from complete, there's enough of his skeleton along with many other fragmentary ones to reconstruct the stature and proportions of both Australopithecus afarensis and africanus. Here we can see comparison of male and female modern humans, a male chimp, and a male and female afarensis or africanus. You can see how much smaller they were. This is the case with all these early hominins and also how much more sexual dimorphism there was compared to us modern humans. Our third fossil is called salam, the Arabic word for peace. Also discovered in Hadar in 2000, it's the partial body of a 3000000 year old Australopithecus afarensis child dated to 3,300,000 years ago. While we like most of the lower body, there are some important parts of the upper body that were very well preserved that have shed new light on Australopithecus adaptations. This is a virtual reconstruction of the cranium that corrects the distortions that occurred to the skull after being buried for so long. And this is an artist's reconstruction of what Salaam may have looked like when it was alive. Specifically, this fossil provided new evidence of climbing ability. It had already been noted in other references fossils that their finger bones, the phalanges, are a bit curved, a feature that would make them better tree climbers. Here we can see Salem's finger bone compared to a chimp and to a modern human. While the differences seem slight in the photos here, the difference in curvature translates into significant differences in tree climbing ability. We also have Salem's scapula and shoulder blade. You can see how similar it is to a modern humans like yours or mine. The scapula is an important part of the upper arm and shoulder structure, and Salem's scapula has features that allowed a greater range of arm movement, also useful when climbing trees and much like an ape's. Selim's neck is also a bit shorter and thick like an ape's. And finally, Selim's inner ear bones were also preserved. These tiny bones, not pictured here, play a role in keeping your balance. For us, this is important when balancing on your 2 hind legs. In apes, these bones are shaped a bit differently and play a role in their staying balanced when climbing through the trees. So lambs' inner ear bones are a bit ape like, also indicating an adaptation to climbing trees. Fossil species include the type specimen, the Tong child, Raymond Dark's missing link from the 19 twenties, and a fossil called missus Pleas, a 2 and a half 1000000 year old crewnium of an adult male. In missus Plas, we can see some other apomorphic traits in Africanus, traits we don't see in anemensis or afferentis. Along with especially large molars and a robust mandible, we also see the appearance of bone structures specifically related to chewing. There's a forward shift of the zygomatic arch, the cheekbone, creating very prominent and extruded cheek and making the face look wide and flat. There are also slightly more prominent canine pillars. These buttress the chewing structure of the upper jaw. These are structural changes in the face and jaw that translate into increased use of their cheek teeth. It gave them a more powerful bite. And as we'll see shortly, this makes Africanus a good candidate first step on the branch that leads to the next genus we'll look at, the parenthipenes. Originally, Africanus was seen as simply a South African variation of afarensis, but a more common view is that afarensis is the direct ancestor of africanus. Our final member of the genus Australopithecus, Australopithecus sediba, was only announced as a new species in 2010. The fossils were discovered in Malapa Cave in South Africa in 2008 and date to about 2,000,000 years ago. So far, only 2 individuals, 1, a young boy, and 2, an adult female, have been recovered. Both are incomplete, but between the 2, most body parts are represented. There's no cranium for the adult female, but the boy's skull is really well preserved. His cranial capacity was 420 cubic centimeters. Even though he was not an adult when he died, this is likely as large as his brain would have gotten. So it's right around the same average per afferents as an Africanus. Both individuals were about 130 centimeters tall. However, there are potential problems with the collection and interpretation of these fossils. Most of these bones were not found in anatomical connection. They're spread out higgledy piggledy in the Breccia. The adult female's arm and hand were the only parts that were in connection. Therefore, we don't really know for sure if all these bones even belong to just these 2 individuals. They could be from 3, 4, or even 10 different individuals. And some researchers have pointed out that the Malapa 1 cranium looks remarkably similar to that of a young female Africanus, And the Malapa 2 mandible looks very similar to mandibles from early species from our own genus, Homo. To some researchers, it really looks like these two skeletons are just a mixture of bones from at least 2 different species, which explains why when you put them together as one, they look like a brand new species. For the time being, we'll put a big question mark next to Australopithecus sediba. So some conclusions about the australopithecines. These were definitely obligate bipeds when they were on the ground, and they likely spent most of their time during the day on the ground walking around foraging for food. However, though not as arboreal as Ardipithecus, they did retain some anatomical features that are adapted to spending significant time in the trees. They probably slept in trees at night like modern apes, maybe even making leaf nests like chimps do, and it is very likely that they escaped into the trees when predators attacked. Many researchers think amphorhensis in particular could be a direct ancestor for our genus homo. The next and last genus that we'll look at is peranthropus, and the first of 3 species is Aethiopicus. Only a single cranium has been attributed to the peranthropus Aethiopicus species, an adult male cranium called the black skull, because it was buried in sediments that stained it with a dark mineral. It was discovered in Western Kanaquina in 1985 and is dated to 2 and a half 1000000 years ago. It is a mixture of features that make it a good transitional species between Australopithecus africanus and the pranthus line. It is similar in some ways to male afarensis or africanus, but has significant differences. It has a relatively small cranial capacity, 410 cubic centimeters, similar to the afarensis average. It also has a more highly developed masqueratory apparatus or chewing structure, which includes a large thick bony palate in the roof of the mouth, even larger, more flaring cheekbones than we saw in Africanus, a large sagittal crest like a gorilla. None of the hominins we looked at have this feature. And while no teeth remain in the cranium, the size of the tooth sockets indicates Ethiopia's molars were really large. All these features are closely related to chewing muscles and a strong bite force. To illustrate this, we can compare the masticatory apparatus of a modern human and a gorilla. Both we and gorillas and all hominids have 2 major groups of muscles that provide our bite force, the temporalis muscle and the masseter muscle. The cheekbone or zygomatic arch supports the masseter muscle, and by pushing the zygomatic arch forward, leverage is created for increased bite power. Then the temporalis muscle attaches from the lower jawbone up high onto the side of the skull. If you clench your jaw, you can feel both the masseter muscle and the temporalis muscle, and you can tell where the temporalis muscle ends at the temporalis line on the side of your head. Like gorillas, Ethiopicus had a very large temporalis muscle which which extended all the way to the very top of their cranium where the left and right temporalis muscles met and were anchored there by a large bony sagittal crest that you can see clearly on this modern gorilla. These changes in Ethiopia's chewing anatomy would have given them a very powerful bite force. We modern humans can produce a bite force of about £150 per square inch. Compare this to a gorilla's £1300 per square inch. This is higher than a lion's bite force, though low compared to a crocodile or a tyrannosaurus rex. Ethiopia would have been somewhere at the top end of the range between us and gorillas in terms of the bite force. Because we don't have any postcranial bones from Ethiopia, very little else is known about the species. But as we'll see, it's a logical ancestor for the next two species of paramphibines. Here is one possible but popular version of the hominin phylogenetic tree. Many see Australopithecus afarensis as a good candidate for the root for our line and the parenthipine line with Australopithecus africanus simply fitting on the line between ampharensis and aethiopicus. An aethiopicus is a likely ancestor for the next two species, boisei and robustus. As I did with Australopithecus afarensis nafricanus, I'm going to lump the next two species together since the differences between them are also rather slight. Like Australopithecus afarensis nafricanus, and Paranthropus robustus are 2 very similar species that are found in 2 different regions. Paranthropus boisei fossils are only found in East Africa, primarily in Tanzania and Kenya, and places like Olduvai Gorge and Koobi Fora, and date from 2.5 to 1,200,000 years ago. Ampharanthobus robustus fossils come from caves in South Africa, like Swartkrans and Dremelan, and date to between 2,000,001,000,000 years ago. Brathabis boisei and robustus had both similar body proportions and similar statures to the australopithecines, approximately a 120 centimeters tall and 35 kilograms for females and a 140 centimeters and 50 kilograms for males. But they had some significant differences, several important epimorphies that distinguish them from the australopithecines. These are almost entirely in their cranium. Note the remnants of a sagittal crest like we saw in. The males of both Boisii and Robustus had large sagittal crests, and they had larger canine pillars than Africanus or aethiopicus. While difficult to see in this fossil, Boisii and Robustus also had widely flaring zygomatic arches like we saw starting in Africanus and developing further in Ethiopia. This particular cranium, nicknamed Zinge, is actually the type fossil of this species and was discovered in 1959 in Olduvai Gorge by Mary Leakey. This is a particularly important discovery for the history of biological anthropology because it vindicated Lewis and Mary Leakey's 30 previous years of work, which had produced very little. The discovery of this fossil led to a huge increase in interest in fossil hunting in East Africa. Other apomorphic features include a further reduction in the size of the front teeth, the canines and incisors, smaller than Africanus, and a continued increase in molar size. Boisii and Robustus had huge molars, had a huge and very strongly built mandible. In fact, Boisii was nicknamed nutcracker man because of its huge molars and powerful jaws. I'll talk more about this below. And they have one other major apomorphic trait, a main cranial capacity of 520 cubic centimeters, 100 cubic centimeters above the australopithecines. This is a big increase considering that their body is no larger than any of the hominins we've looked at so far. There are 2 particularly well preserved robustus crania from the breccia at the site of Drimelin in the Cradle of Humankind area of South Africa. They both date to around 2,000,000 years ago. Fossil 7 is a mostly complete cranium and mandible of a female Paranthropus robustus. Her skull is significantly smaller than the males, and she lacks the sagittal crest. And fossil 155 is a male with a large sagittal crest and significantly larger brain. These and other fossils indicate that, as with the australopithecines, there is a significant degree of sexual dimorphism in both boisii and robustus. The males had notably larger skulls and brains than the females, and the males had a sagittal crest, whereas the females did not. The most striking feature of the parenthesis lineage, especially with Boisii and robustus, is the huge size of their molars and premolars. The molars are indicated here with red arrows and the premolars with blue arrows. Paranthropus molars and premolars are so big they're called megadontia, that is mega sized teeth. Boaceae had the largest molars in any hominin, similar to gorillas who wept 10 times as much, and even their p molars are larger. This increase in molar size, starting with Australopithecus afarensis and continuing through the parenthesis, has been interpreted in various ways. In most species, molars are used for grinding food up, and the original theory was that these peranthrophy species subsisted entirely on hard, gritty nuts and plants. This seems to be in serious doubt. Recent bone chemistry analysis indicates a broad omnivorous diet, even including some meat. But the bulk of their diet was probably very poor quality fibrous plant food, much like the diet of gorillas today. While boisei appear before robustus, they overlap a lot in time and both are found in layers of stone tools. Both species made and used simple stone tools. We'll learn more about these in the next lecture. However, the parenthesis branch seems to be an evolutionary dead end. No hominins descended from them. These are classic examples of end species of lineage that had become too specialized. Species can be either generalized or specialized in their adaptation. General species have the ability to live in a wider range of environments and eat a wider range of foods. Because they're not perfectly or specifically adapted to any specific conditions, they may not thrive in any one environment, but they tend to be more broadly adaptable. They can adapt more readily to changes in their circumstances. Specialists, on the other hand, are adapted to very specific conditions and have very specific diets. They will thrive as long as the environment they specialize in continues to exist. But when things inevitably change, they find it very difficult to adapt, and often they die out. The evolutionary tree of life is full of dead end branches that were the result of lineages becoming too specialized to adapt to changing circumstances.