And this is segment 2 of learning module 11. So we were talking about primitive traits and how we're examining this chimpanzee skull in which we determined that having a large upper canine is a primitive trait. And large canines in general, large teeth in general, are primitive traits. Chimpanzees have much larger teeth than we do.
And they have a much larger mandible. than we do that lower jawbone the mandible and the question becomes why why do they have such large teeth such large mandible and it's also related to some another primitive trait oftentimes students identify right away this weird looking thing on the top of the skull it's a crest of bone it's called a sagittal crest sagittal referring to The bone up here on the top of the cranium is called the sagittal bone. And this sagittal crest traverses the cranium from front to back. It kind of looks a little bit like, I don't know, Marvin the Martian, right?
With that little crest on top there. And the question is, what's that crest for? Now, I want you to keep in mind that anytime you see crests or ridges on bones, it's... it always has something to do with muscle or ligament attachment, right?
That's how muscles are attached to the bone. The larger the ridge or attachment, or I'm sorry, the larger the ridge or crest, the larger the muscles that are attached to the cranium or to the bone. So looking at this sagittal crest, We can see that it is very pronounced, and if you feel the top of your own head, you'll feel that you do not have that.
You do not have a sagittal crest like that, so we don't have anything like that. So the question is, what's attached here at the top of the cranium? Well, you can see that actually it would be muscles that traverse each side of the cranium.
They would run down the side of the head and attach to the mandible. And that's because those are your jaw muscles, right? If you put your hands on either side of your head and you move your jaw up and down, you can feel the muscles on the side of your head moving. You can feel your jaw muscles. Those jaw muscles attach at the top of your cranium, come down the sides of your head, go past your ear, which by the way, if you ever have jaw...
issues that can result in ear pain or vice versa you have an ear infection that can lead to jaw pain because they're so close to each other so uh the the large sagittal crest means large jaw muscles so all together right we have large teeth large jaw muscles large mandible and the sagittal crest all indicators of doing some very heavy chewing action So what are they eating? No doubt they're not eating mashed potatoes, right? They're eating very, very coarse foods that require lots of heavy mastication, okay? What else do we notice?
We notice that the brow ridges, look at these huge ridges of bone on the top, above the eye sockets called brow ridges, very large. In humans, we do not have these large brow ridges. There are some human populations that have very, very slight brow ridges.
You might even have some very, very, very slight brow ridges, but not like this chimpanzee. And of course, students often ask, well, what's the purpose? What's the purpose of these large brow ridges?
And it's not so much that there is an adaptive purpose to the brow ridges, but rather it's because the cranium goes straight back. that you see this prominence of bone around the eye sockets right so it goes straight back it's not like vertical like we see in humans where we have a very vertical forehead it's it goes straight back and the reason why it goes straight back has a lot to do with those large jaw muscles that occupy this space at the top of the cranium okay so these brow ridges are really the result of this cranial you know deflation here basically where there's It's not elevated the way that ours is. So really, it's not so much that we don't have brow ridges, it's more that our brow ridges are absorbed into our forehead, which is an interesting image to imagine.
All right. So we've got large brow ridges, that's a primitive trait. You notice that a chimpanzee has a very large nasal cavity in comparison to ourselves. A lot of that has to do with another feature, which is called prognathism.
Prognathism has to do with the lower part of the face, in which you can see the snout area projects out a whole lot. In humans, if you feel the front of your face, your... Lower face area is pretty flat. It's pretty, pretty vertical.
You look at a side profile, it looks pretty in line, right? We don't have a lot of prognathism. There are some human populations that have really slight prognathism, but not like chimpanzees.
Chimpanzees have a huge snout area. Okay, so we talk about prognathism in terms of degrees. So chimpanzees have a high degree of prognathism.
So their lower snout area sticks out. quite a lot so that's a primitive trait then looking at the shape of the skull in general chimpanzees have a very ovular shape to ovular or oblong shape to their their cranium if you look at human crania which we will in just second here if we look at human crania the the human crania is very round in chimpanzees the cranium is is very oval very ovular Oblong it's it's a long cranium in general Alright, so we've identified some major primitive traits. Let's move on to looking at the Human skull and identifying some modern traits Okay, so in the human skull we can see in fact look how globular or rounded the human cranium is very globular in general the face is tucked under this cranial area so we have a very large cranial capacity a place in our crania that we have large brains so it all fits in this back area our faces are smaller in general our teeth are smaller our mandible is smaller we do not have a sagittal crest on top of our heads That does not mean we do not have jaw muscles.
We do, but our jaw muscles are significantly, well, how should I put this, puny compared to a chimpanzee, right? We have tiny jaw muscles in comparison. So of course, that's going to reflect on our diet too, right?
We have a small mandible, small teeth, small jaw muscles. What's that mean? Not a coarse diet.
We're looking at a soft diet. We'll be talking about why we see differences in the coarseness of diet between chimpanzees and humans when we get to talking specifically about tools. All right, so we see that there's little to no prognathism, very vertical face, the nasal cavity is smaller as a result of that. We have little to no brow ridges, you can actually see that the area around the eye sockets is absorbed into this large vertical forehead. Alright, so overall these are some modern traits.
Let's look at a list. Okay, I love lists. Alright, so modern versus primitive traits. Primitive traits, we established these are traits that are the most similar to the common ancestor.
And these include high degree of prognathism, projection of the snout area around the face, or snout area of the face, large brow ridges. Sagittal crest, a large mandible, large canines and teeth in general, small cranial capacity. We'll be talking about the differences between, let's say, chimpanzee brains and our own brain size. Large nasal cavity, flattened cranial base.
If you look at, let's see if I can go back here. If you look at a chimpanzee skull and you notice that the cranial base. or this area back here that sort of forms the base of the cranium is very flat and the reason for that is that if you imagine chimpanzees are quadrupeds right so if they're quadrupeds their spinal column comes out horizontally from the head at which point if the cranial base was rounded like ours like let's say it projected out this way it was rounded like this Every time they would try to lift their head, their head would literally hit the cervical vertebra, their neck vertebra.
of their vertebral column. Okay, so they have to have a flattened cranial base back here so that they can actually lift their head and not hit the cervical vertebra. On the other hand, in humans, we have a very rounded cranial base, and that's because we have this globular shape in general. A lot of this has to do with the fact that we're bipedal, in which our heads are very nicely bipedal.
balanced upon our cervical vertebra and so having a globular shape like this lends itself to balance at which point that's we also see the extension of the brain size in other words when we got larger brains it expanded down here in the bottom part of the cranium and up here in the top part of the cranium so that's how we got this sort of globular shape so when we talk about Modern versus primitive, we can see that primitive traits include a flattened cranial base, has to do with quadrupedality, and then this overall ovular skull shape. Modern traits, in comparison, we see little to no prognathism, little to no brow ridges, small mandibles, small teeth, smaller nasal cavity, large cranial capacity, that rounded back of the cranium, which the bone we're referring to is called the occipital bone. Just starting to familiarize ourselves with these bone terminology.
And then the globular skull shape and a chin, right? If you recall, looking at the chimpanzee skull, we do not see any sign of a chin in chimpanzees. It just goes straight back here.
And a lot of people ask me, well, why? Why is that? Why do we have a chin and they do not? and oftentimes there's the question of well is there some sort of adaptive reason why we would have a chin and they don't and it has to do really a lot more with the reduction of prognathism right I want you to imagine if we took that chimpanzee prognathism and it was sort of like clay and we smooshed that snout area down to the point where there was little to no prognathism the chin would be the result so sort of like an artifact you might say of such reduced prognathism and of course we have reduced prognathism because we have much smaller teeth all right so we'll talk a lot more about this when we get to talking about modern humans at the end of the course all right so here's a overall primitive and modern traits so when we refer to primitive and modern when we're looking at the fossil evidence you know we're talking about primitive traits having to do with the common ancestor modern traits being traits that are unique to modern humans alright let's get to talking about this earliest fossil evidence so the very earliest hominid we know about so far and we remember of course that new discoveries are being made all the time comes from a nearly complete cranium discovered in 2001 at a site called Toros Manala in northern Chad in Central Africa. So this cranium made a huge splash in the scientific world.
It's been dated to about 7 million years ago, which was a million years older than the earliest specimens scientists had found up until 2001. It's called Sahelanthropus chadensis. Now Sahel, named for the region of the southern Sahara of North Africa, and then Anthropos meaning human, the Charensis is an honor of it being found in the country of Chad. The morphology, ooh, remember that term morphology, referring to physical characteristics, the anatomy of the skeleton. The morphology of the fossils are quite unusual with a combination of characteristics that are unlike those found in any other early hominid. Okay.
So let's take a look at this fossil. Actually, this is a teaser. Let's move on to looking at Sahelanthropus in the next section.
So please follow me to segment three.