Lecture 5.1: Early Fossil Hominin Sites in Africa PDF

Lecture 5. 1 An Introduction to Early Fossil Hominin Sites in Africa We learned about the ape lineage in Lecture 4. 2. The hominins, our lineage, is a special branch of the ape lineage. And so far, all early hominin fossils, older than 2 million years, have been found in Africa. This is the place w...

Lecture 5. 1 An Introduction to Early Fossil Hominin Sites in Africa We learned about the ape lineage in Lecture 4. 2. The hominins, our lineage, is a special branch of the ape lineage. And so far, all early hominin fossils, older than 2 million years, have been found in Africa. This is the place where our earliest ancestors first evolved from some distant ape ancestor. This should have been obvious a long time ago, considering how much the continent of Africa resembles a hominin cranium. Clearly a cosmic clue, pointing us to the oldest hominin fossils. While there are a few things you should learn in this lecture, it's mainly meant to give you an idea of what it's like to hunt for fossils, and what the places are like where these early hominin fossils are found. In Africa, there are two main regions where researchers have the most luck finding the remains of early hominins. Parts of East Africa. And South Africa in these regions, there are specific areas that are particularly rich in hominins, skeletal remains, perhaps this is partly because larger populations of hominins lived in these areas compared to other areas over the last several million years. But it is more likely that there are specific features of the geology of these regions that tends to better preserve. And so more frequently preserved fossils here. We will briefly examine some of the more important fossil sites and fossil hunting areas beginning in South Africa. So far in South Africa, all the hominin remains have been discovered in caves. Let's look at some of the most famous ones. Famous because they were discovered a long time ago, and many fossils have been found there over the years. Tong, or the Tong Child Cranium, we were introduced to in the first lecture, was discovered in 1925. Sterkfontein, Kromdrei, Swartkrans. Archaeologists started working at these sites in the 1930s and 1940s. Malapa is actually a very recent discovery. It was found in 2008. These last four sites, along with many others, are in a region so rich in fossil bearing deposits that it has been made a UNESCO World Heritage Site, called the Cradle of Humankind. There are dozens of other important sites, but these are among the best known, and you should memorize them. But don't worry about the dates when work started there. These are all cave sites, but many are not really the types of caves you might imagine. They are actually vertically oriented caves called dolinas. While there are several different ways that dolinas can form, The ones in South Africa formed by the slow dissolving of the limestone or dolomite bedrock. These are both relatively soft sedimentary rock, which is easily eroded by rain and running water, but are also composed of calcium carbonate, which will react with rainwater, and is easily dissolved away because rainwater is often just slightly acidic. Small crevices form on the surface through mechanical erosion and chemical dissolution, and very slowly, over hundreds of thousands or millions of years, these grow larger, and deeper, and deeper. Until after millions of years, they often become large cavities with only a small opening visible at the ground surface. These dolinas make natural sediment traps and slowly fill up with dirt and rocks that fall in, or are blown in, or get washed into the hole. Occasionally animals fall in and die, and their bones become part of the deposits. This likely happened to hominids as well from time to time. Some dolinas were also used as dens by hyenas, who would drag their meals back to these places. This would also include the occasional unlucky hominid. Very slowly the sediments built up and the bones of animals and hominins became incorporated into the deposits. Some dolinas were also being used as campsites by hominin groups. They must have climbed inside and used the caves as protection from the elements and from predators. We know this because we find their stone tools there. Then, for hundreds of thousands of years, water percolating through the deposits cements them together with calcium carbonate and turns them into a new type of rock called breccia. This makes getting the bones out of them very difficult. While the dolinas in South Africa are not especially large. They can be perhaps 10 to 40 meters deep. In some parts of the world, dolinas are huge. Like this one in Mexico, called the Cave of Swallows. It's 350 meters deep. That's 150 meters taller than the tallest building in Vancouver. Let's look at just a couple of examples of hominin fossil sites in South Africa. This is an aerial view of Sterkfontein. You can see that there are several openings that join together beneath the ground. It has 20 meters of breccia deposits that date to 3. 5 million years ago at the bottom, and 200, 000 years ago at the top. Rock quarrying began here in the 1890s and archaeological excavations in 1936. And so far, researchers have recovered over 700 hominin bones and bone fragments. Here's another view from the ground. And another view. Here you can see the ladders sitting on the layers of breccia full of small rocks and bones. In the 1990s, an almost complete Australopithecus skeleton was found. This is a hominin species related to the Tongue Child. Because these fossils are cemented in breccia, Researchers have to use small dental tools and tiny electric drills to slowly remove them. It took years to remove this particular fossil. Malapa was only just discovered in 2008. You can see the trees growing out of the breccia in the delina. Here the researchers are clearing out the vegetation, preparing the site for excavation. And here's the first year of excavation. These excavators are students taking a field school and learning how to dig archaeological sites. This is one of several fossils recovered from Malapa that we'll look at in more detail later. It's a mostly complete hominid skull dated to 2 million years ago. You can see the tiny tool marks in the breccia left from the dental tools used to remove the breccia from around the bone to expose it. Let's jump to East Africa where the geology is quite different. Caves are not as common as in South Africa and so fossils are found in very different circumstances. Here fossils are found buried in deposits along the edges of ancient rivers and lakes that had formed inside the East African Rift Valley. Rivers have been flowing and lakes forming in the Rift Valley for millions of years. We were briefly introduced to the African Rift Valley in lecture 4. 2. This is a huge geologic fault, 6, 000 kilometers long, formed by the movement of two tectonic plates. The Nubian plate on the west side and the Somalian plate on the east side are slowly moving apart from each other, resulting in the slumping of the thick deposits between them. This exposes huge sections of sediments that have been filling the fault and building up for many millions of years. In many places, these are hundreds of meters thick and date more than 20 million years ago. The fault is a logical place for rivers to form, and the rivers cut down into these ancient deposits and expose fossils from various time periods. As we saw in Lecture 4. 2, the fossils of very early ape species like akimbo are found in the rift. But we also find the remains of early hominid species that only date to the last 5 million years. These early hominids lived along the rivers and lakes and hunted and gathered wild animals and plants. When a hominid died, its remains were sometimes rapidly covered by river or lake sediments. There are many more hominin sites in East Africa than South Africa, and some are much older as well. We'll look at a few of the more famous ones. In Tanzania, there's Olduvai Gorge and Laetoli. In Kenya, there's Kubifora and West Turkana. And in Ethiopia, there's Hadar and Katagona. These are not individual sites, but large areas on the landscape where fossils are regularly found. We'll start from the south, in northern Tanzania, and move north. Olduvai Gorge, in northern Tanzania, is one of the most famous sites. This is a small side branch coming off the Rift Valley, just north of the Ngorongoro Crater, an ancient massive volcanic crater 20 kilometers in diameter. Two million years ago, Olduvai Gorge was a lake, and hominins and other animals lived along its shore. We know that hominins were living along the edge of the lake because, besides their bones, we also find their stone tools and butchered animal bones. Analysis of the sediments at Olduvai identified ancient plant remains called phytoliths. These particular phytoliths were from palm trees and other tropical plants. In fact, excavations here have even uncovered the remains of fossilized palm trees. This is more or less what Olduvai Gorge looked like when the hominids lived there 2 million years ago. However, as with much of East Africa, today the environment around Olduvai Gorge is very dry, with lots of erosion that tends to expose ancient deposits and the fossils they contain. In 1931, Lewis and Mary Leakey began hunting for early hominids here. They were convinced that East Africa was the best place to look for our ancient ancestors, and they hunted with almost no luck for almost 30 years. It wasn't until 1959 that they started finding important fossils. In places like Olduvai, where regular erosion can expose fossils, researchers hunt for hominin bones by simply walking along the bottoms of these erosional slopes, where exposed fossils will end up. When they locate a fossil, they then figure out which layer it came from, and then begin a formal excavation into that layer. Excavations carried out here since the 1960s have recovered lots of hominin remains and early stone tools. Also in northern Tanzania, and not too far from Olduvai Gorge, is Laetoli. While skeletal remains have been recovered here, the site is most famous for Mary Leakey's 1978 discovery of hominin footprints preserved in volcanic ash, dated to 3. 7 million years ago. Shortly after a volcanic eruption, a small group of hominins walked across the new ash. The ash then hardened and perfectly preserved the footprints as if it had happened just yesterday. There are over 70 footprints from five different individuals, and they provide information about foot morphology and walking gait of these early hominins. Here's an artist's reconstruction of what Laetoli might have looked like at the time. Footprints of many different animals were found along with the hominins. Moving up into northern Kenya, on the east side of Lake Turkana, is an area called Kubifora, where river sediments accumulated between 4 million and 1 million years ago. As at Laetoli, researchers have uncovered animal and hominin footprints, in this case, preserved in mud along the bank of an ancient river. While these footprints are more than 2 million years younger than the ones at Laetoli, they're still 1. 5 million years old. Lots of bones of early members of our genus Homo are recovered here as well, including some of the most complete and most important ones. On the west side of Lake Turkana, across from Kubi 4, is a region called West Turkana. These are also mainly river deposits, but much older. They extend back to 4. 5 million years ago. Fossils of five different hominid species have been recovered here over the years. And in 2015, researchers announced the discovery of possible stone tools, dating to 3. 3 million years ago. If these turn out to be actual stone tools, they'll be the oldest found so far. These will be the oldest artifacts ever found. Moving yet further north, we end up in the Afar Depression, or Afar Triangle in Ethiopia. This is a major geological feature that is part of the Rift Valley, but it is also an ancient river delta where the, where sediments picked up and carried by rivers running through the Rift Valley were deposited over many, many millions of years. In the past, this region would have been covered by thick forest and wet, marshy areas common in river deltas. It would have looked something like this, and it would have been attractive to animals and hominins alike. Hadar is an area along the Awash River in Ethiopia. It's particularly rich in hominin remains. Thousands of bones have been recovered here that date from four and a half to three million years ago. Fossils have also been recovered from the western side of the Afar Triangle, at an area called Katagona, where the previously oldest stone tools were recovered. The Katagoda stone tools date to 2. 6 million years ago. I know this stone tool does not look like much, but these very early stone tools were, not surprisingly, very simple. However, they represent the start of a technology that humans used right up until modern times, and we'll get into this more, starting in Lecture 7. These last several slides are from a colleague of mine, who has been fossil hunting in the Hadar region for decades. Here, the crew is arriving at the start of the 2015 field season, driving down into the Awash River Valley at Hadar, where they set up their camp. The crew is composed of several professors, students from various universities around the world, and local people interested in archaeology and human evolution. Each day, the entire crew heads out to do systematic searches along exposed sections of deposits, where fossils might be eroding out of the four million year old sediments. Because of ongoing erosion, every year the landscape changes a little bit and new fossils can be exposed in areas that had already been searched over in previous years. When someone finds a hominid fossil, maybe just bone fragments or a couple of teeth as in the case here, they then try to figure out what layer the fossil eroded out of. Usually they can search up the slope just above where the fossil was found and see where more of the fossil is still buried. Then they begin a formal excavation, opening up a larger area to hopefully find more parts of that bone or maybe even more bones from an entire skeleton. The sedums are screened through fine mesh to make sure small teeth and bone fragments are not missed. They try to recover everything. Most fossils are not found complete and intact. Mainly, researchers are lucky to find small fragments of broken bones. So, the next step is to try to reconstruct them. Like a puzzle, try to put all the fragments back together as best they can. If they find enough of the skeleton, then they might have a good chance of figuring out what species it belonged to.