Lecture 6: Emergence of Homo PDF

Lecture 6 The emergence of the genus Homo. The genus Homo is the genus to which we modern humans belong, which means we are now talking about hominin species that we know are well within our own ancestral line. Some might argue that the first members of our genus Homo were the first humans, though t...

Lecture 6 The emergence of the genus Homo. The genus Homo is the genus to which we modern humans belong, which means we are now talking about hominin species that we know are well within our own ancestral line. Some might argue that the first members of our genus Homo were the first humans, though the term human is not well defined. While many researchers think that Australopithecus afarensis is a good candidate for a direct ancestor to our genus Homo, there is certainly no consensus on this, and in fact we can't really know for sure whether the genus Homo emerged from the genus Australopithecus or not. Although currently, there are no other reasonable options. Which fossils we decide to include in the genus Homo is a particularly important decision, since the implication is that we think that these are on our own direct evolutionary line. There are currently two very early species that most researchers assign to the genus Homo, but we'll just look at the better known of these, Homo habilis. The textbook talks a bit about the other species, Homo rudolfensis. I won't talk about rudolfensis here, and I haven't included it in this tree. Currently, Homo habilis appears to be the root of the Homo genus tree. Most Homo habilis fossils come from East Africa, especially Kubifora in Kenya and Olduvai Gorge in Tanzania. Although there are some possible skeletal fragments from the site of Swartkrans in South Africa. The East African fossils date to between 2. 8 million and 1. 5 million years ago, which means our human lineage overlapped with Australopithecus africanus and all the Paranthropines. The very first fossil the researchers find, upon which a new species is based, is called the holotype fossil, or just a type fossil for that species. The holotype for Homo habilis was fossil number 7 from Olduvai Gorge, discovered in 1960 by the Leakey family. It includes several skull fragments, most of a mandible and some hand bones. Assigning this specimen to the genus Homo was controversial throughout the 1970s. Many researchers argued that it was not that different from Boisei or Africanus to call it a new species. However, over the subsequent years, more fossils turned up that had similar characteristics. They clearly distinguished it from the Austral Lipic or opine. Homo hous was still small bodied like osteopathic and paranthropus around 130 centimeters tall and maybe 35 to 45 kilograms. But we cannot tell male from female fossils. The fact that it's difficult to be sure which falses are male and which are female, indicates that there's much less sexual dimorphism than we saw in the Austral or, or the opine. Other differences, like the small size of the molars, their large brain size, and the shape of the hand, make them more similar to later species in the genus Homo, and they are very likely a direct ancestor to us. Although the post cranial fossils assigned to Homo habilis are few, and generally not complete, we can get some idea of the evolution of their body proportions. One of the best measures is the intermembral index, the ratio of arm length to leg length, divided by 100. If your arms and legs are the same length, then your intermembral index is 100. And if your arms are half the length of your legs, then your intermural index is 50. Chimpanzees have an intermural index of between 105 and one and 110. Their arms are slightly longer than their legs. Osteopath and opines have interm indices of between 85 and 95. We modern humans have an INTERM index of about 70. Homo is still higher than ours, but lower than the astro or the opines, perhaps around 80, showing this consistent trend towards longer legs and shorter arms. Presumably as hominins become more and more efficient bipeds, and rely less and less on climbing trees. A number of different fossils are attributed to Homo habilis, but there are four important ones. Fossil 1813 is a complete cranium from Kubifora in northern Kenya. It's dated to 1. 9 million years ago, and has a cranial capacity of 510 cubic centimeters, well within the range of Paranthropus, Boisei, and Robustus. Fossil 24 from Olduvai Gorge in northern Tanzania, nicknamed Twiggy, is dated to 1. 8 million years ago, and has a cranial capacity of 590 cubic centimeters. Notably above the average for the Paranthropines. The third fossil is 1470, discovered in 1972 in Kobi Foro. It's also a fairly complete cranium, although it was highly fragmented and missing all its teeth. However, it has a much larger brain, at 775 cubic centimeters. The fourth fossil is a combination of two different fossils, an upper jaw on the left, and a lower jaw, or mandible, on the right. The lower jaw is not complete, but because mandibles are symmetrical, and we have half of one, it's simple enough to reconstruct the other half. To see what the whole dental arcade would look like. Both fossils are from the Hadar region in Ethiopia, but from different locations. They are not from the same individual. However, both jaws have much smaller, more modern looking teeth, and a more parabolic dental arcade. Much more like ours, than like the Australopithecines or Paranthropines. The upper jaw is 2. 3 million years old, and at 2. 8 million years old, the partial mandible is the oldest Homo fossil we've found so far. When researchers began comparing the Homo habilis fossils to other hominins, It became clear that there were three main traits that set Habilis apart from the Australopithecines and the Paranthropines, and that made it clear that Homo habilis was transitional to the later Homo species, Homo erectus and Homo ergaster. The first is a significant increase in brain size. The range in Homo habilis cranial capacity is 510 to 775 cubic centimeters, with a mean of about 650. This is more than 100 cubic centimeters larger than the Paranthropines. Who had a brain size 100 cubic centimeters larger than the australopithecines. But homo habilis increase in brain size is particularly significant because they did not increase in body size. Their brain is notably larger relative to their body. As you can see here, homo habilis degree of encephalisation puts it well above the australopithecines and the paranthropenes. Well in the path to later homo species like us. Secondly, homo habilis has significantly reduced molar size. There is no evidence of the molar megadontia of the paranthropene. A trend that first started to show up. Inus apheresis, and then africanus Homo is back in line with the more general characteristics of smaller molars that predates the osteopathic, along with a much more parabolic dental arcade. One result of this reduced tooth size is a flattening of the face or reduced pragmatism. Let's review the overall trends in tooth size and jaw size. Remember back at the start of our Homi and lineage, Arius Ramus and Aus Ensis had relatively small molars like chimps, and unlike us. With Afarensis, we see the first appearance of large molars and a big jaw. With Africanus, molars get larger and the jaw gets bigger too. With the appearance of the first Paranthropus, Aethiopicus, we see a huge jaw, and molars and premolars so big they're called Megadontia. And finally, with the dead end, Boisean Robustus, we have masses, massive jaws in Megadontia. But with the emergence of Homo habilis around 3 million years ago, or just after, we are back to relatively small molars, smaller than Afarensis, and not far off chimps and us. Thirdly, Homo habilis has very modern looking hands with a more advanced precision grip. The grip used for holding small objects and manipulating them more precisely as compared to a power grip that you'd use to hold a hammer. This has long been assumed to reflect the evolution of anatomical traits associated with tool making. In particular, the small bones at the tips of habilis's fingers, the distal phalanges, or apical tufts, are more like ours. Chimpanzees have quite pointy apical tufts, while ours are flat and wide. Homo habilis has apical tufts quite similar to ours. There are slightly expanded apical tufts in the Paranthropus line, but not so much in the Australopithecines. These wide, flat apical tufts are generally associated with increased precision in manual dexterity that may be directly associated with the manufacturing use of stone tools, a behavior that, up until very recently in human history, has been universal and a constant. Many of you are using your expanded apical tufts to hold your pens or pencils. Perhaps eventually we will evolve back to having pointed apical tufts like chimps to aid in tapping tiny keys on our iPhones. There are some other Homo habilis traits worth looking at too. For example, while the feet of the Australopithecines and Paranthropines were already similar to ours, Homo habilis did have even more modern looking feet. Fortunately, researchers recovered a fairly complete Homo habilis foot from Oldhamay Gorge. While it is missing the toe bones, the more important parts are the bones that make up the ankle and the arch of the foot. Here we can compare these bone to bone to a modern human foot. They're very similar. When we compare this fossil to chimps and to us, we can clearly see An enlarged big toe, designed for bipedal walking and carrying more weight. And a fully abducted big toe, resulting in it being parallel to the other toes. Unlike a chimp's abducted big toe, which diverges from the foot at an angle. Homo habilis also had a large heel bone, which is part of their having a fully developed double arch. They had a longitudinal arch running front to back, and a transverse arch running side to side, like in our feet. This complex arch system means that every time they took a step, their foot was mechanically set up for efficient weight transference from the toes to the ball of the foot to the heel and through to the ankle, an important mechanical aspect for obligate bipedal hominins. The name Homo habilis means handyman, and the name stems from the initial thinking that this species was the first to begin making stone tools. The oldest Homo habilis fossils roughly corresponded to what were once the oldest known stone tools, the 2. 6 million year old tools from Katagona in Ethiopia. However, in recent years, evidence has been uncovered for much older stone tools. This includes stone tool cut marks on 3. 4 million year old animal bones from the Hadar region, and a very recent claim for 3. 3 million year old stone tools from Western Kena in Kenya. Since this evidence indicates the stone tools well predate Homo habilis, it seems that the earliest stone tool makers must have been the Australopithecines. Since we are on the topic of stone tools, let's take some time to introduce this subject in more detail. Let's look at Hominin technology in general. and stone tool technology more specifically. The term that archaeologists use for stone tools is lithics. Lithic technology refers to stone tools and the technology associated with manufacturing and using them. Just note though that the term lithic technology does not include the use of stone for building structures like the pyramids. Hobbits started using stone tools at least 3. 4 million years ago and continue to use them right up to the present day. There are still a few traditional societies that make and use relatively simple stone tools. The thing about stone is that it does not biodegrade, like other raw materials people have traditionally used in their material culture. This means that essentially 100 percent of all the stone tools ever made, starting millions of years ago, are still in existence today somewhere. Either still buried in the ground, or in university or museum collections. So lithic technology is one of the longest lived technologies, along with using wood. But wood does not survive well, so we'll never find the really old examples of wooden tools. What makes stone tools so useful is that if you select the right type of stone and fracture it in the right way, you can produce very, very sharp edges that are also very hard and durable. Such hard, sharp edges work perfectly for killing and butchering animals, preparing and cutting up hides to make clothing, and carving wood into a wide range of tool types, like spears, bows and arrows, digging sticks, and handles for tools. However, only certain types of stone work well for this. Since the goal is sharp, durable edges, the stone must be hard. Yes, stones are generally hard, but not all are hard enough. Chalk is a type of stone, for example. Even relatively hard types of stone, like limestone, will still not have very durable edges, and will crumble if you try to use them to cut or scrape something. The stone must be fine grained. Granite, for example, is very hard, but it's composed of large crystals, so if you create a flake of granite, the edges will not be very sharp. And thirdly, the stone must be homogenous. This means it must be the same throughout the rock. It can't have different characteristics in different places in the rock. Or else it will be very difficult to control the creation of sharp flakes. Some of the best types of stone for making flakestone tools are flint and a natural volcanic glass called obsidian. Producing simple flake tools is not complicated, but you do need to understand some basic principles that govern how brittle materials like stone fracture, so that you know where to strike a nodule with a hammerstone to be able to remove a flake. This flint knapper is flaking a nodule of flint. You can clearly see how sharp the edges of the flakes are. The earliest and the simplest stone tool technology is called Mode 1, or the Old One industry. This is the type of stone tool technology that the Australopithecine, Pranthepine, and early Homo species were using. They were selecting nodules of suitable stone, and without any real planning, they struck these with a hammer stone to remove small, sharp, but simple flakes that they could then use in that form for whatever tasks they had at hand, perhaps butchering a wildebeest. When they were done with the flake, they simply dropped it at that location. When they needed a new flake, they'd strike it off the nodule. Once a few flakes had been removed from a nodule, it might serve as a simple chopping tool itself, maybe for breaking open animal bones to get the nutritious marrow out. Lithic technology could not really be much simpler than this, but the older one lasted for almost 2 million years without changing much. We will learn more about Mode 2 when we get to Homo erectus. As soon as our hominin ancestors started making tools, whether of stone or wood, They dramatically changed how they would evolve from then on, rather than just evolving biologically, as all other organisms do. Hominins began evolving through what we have come to call biocultural evolution. This is the combination and interaction of biological evolution and the evolution of our technology as we adapt to changing circumstances and new environments. For example, prior to the development of lithic technology and the use of sharp stone tools, our hominin and hominid ancestors We relied on their teeth to do things like kill prey animals and cut them up into small enough pieces to swallow. But once we invented sharp stone tools, these rapidly replaced our large incisors and huge canines, and they slowly became smaller and smaller. Our technology and anatomy evolved in concert to allow us to adapt. There's another very similar way to view this called dual inheritance theory. Most organisms just inherit genetically determined characteristics like eyes to see, wings to fly, or hooves to run. We certainly certainly do inherit many of our traits through our genetics. We inherit eyes, ears, noses, skin, hair, and limbs, and they have evolved to serve certain purposes that are necessary for us to survive, reach reproductive age and pass on our genes. However, unlike most other organisms, humans also inherit important adaptive traits through social learning. We are taught how to do certain things and make certain tools that also serve specific purposes and are necessary for our survival. We humans inherit important traits through two, through two different yet closely linked paths. And this dual inheritance has made us particularly good at adapting to new conditions. We've been an extraordinarily successful organism in the relatively short time that we've been around.