字幕列表 影片播放 列印英文字幕 We'd like to thank Raycon wireless earphones for supporting PBS. In October 2004, our understanding of the human family tree was turned upside down. That’s when anthropologists reported that they’d discovered the bones of a tiny, unknown hominin, on the Indonesian island of Flores. This little creature stood only about a meter tall and had a brain about the size of a chimpanzee’s. But that wasn’t what was so shocking. We already knew about small-bodied, small-brained hominins in the human fossil record, like the australopithecines. What was really striking about this skeleton is that, at first at least, it appeared not to have been very old, and yet it had features of much older, more basal hominins. The original description of the bones dated them to between about 35,000 and 14,000 years ago, which means that they would have lived at the same time as us. But the last time a hominin with a brain that small was around, was millions of years ago - not thousands - well before modern Homo sapiens was on the scene. And the rest of its skeleton was equally baffling. Its shoulder joint wasn’t like that of any modern human. It had short collarbones, which meant that the shoulders were rotated forward in a way that hadn’t been seen since early specimens of Homo erectus, from around 1.6 million years ago. The three bones of its wrist that were found were also strangely archaic-looking. They were shaped like the bones of African apes and australopithecines - not like ours. And its feet were really long compared to the length of its legs. We’re talking ape-like, not human-like, proportions. And yet, it had short big toes and was probably an effective bipedal walker. Its discoverers named this puzzling hominin Homo floresiensis, but it’s often called “the hobbit” for its short stature and oddly proportioned feet. And it’s been at the center of a major controversy in the field ever since. It’s been fifteen years now since its discovery was first announced, and we’re still exploring what this little hobbit can tell us about the shape of the human family tree, and what it means for our own evolutionary history. And the questions that this little creature raises are big: Was it its own species? Or was it really just one of us? Or, could it even have descended from a whole lineage of hominins that we don’t even know about? The partial skeleton that started the whole controversy is called LB1. And it belonged to an adult female Homo floresiensis, based on the shape of her pelvis and the fact that her wisdom teeth had come in. But she’s not the only hobbit that anthropologists have found. Since her discovery, the remains of as many as 11 other members of her species have been recovered, although LB1 is still the only one with a skull. And all of these hobbits come from a single site: a limestone cave called Liang Bua, located in the western part of the island of Flores. So, when LB1 was discovered in 2003, one of the first things scientists had to figure out was how to explain her tiny size and strange mix of features. What branch of our family tree could’ve produced such an odd hominin - a hominin, by the way, being a primate that’s more closely related to us than to chimps? The team suggested that her species had evolved from a population of Homo erectus that became isolated on the island of Flores. And then, thanks to evolutionary pressures, their bodies got smaller over time. Now, there are fossils of Homo erectus from other Indonesian islands, and they cover a wide range of time -- from 1.6 million to just 143,000 years ago. So we know that Homo erectus made it to that part of the world. And we know that mammals on islands can change dramatically in body size over time, thanks to a phenomenon known as Foster’s rule, which we’ve talked about before. This rule says that big mammals on islands often get smaller, and small mammals tend to get bigger, as they adapt to limited resources and fewer predators. So in the original paper about the hobbit’s discovery, the researchers proposed that a group of Homo erectus somehow got stuck on Flores, and they eventually evolved into a new species: the smaller Homo floresiensis. And, Flores wouldn’t have been a terrible place to be stranded. Today, it’s a forested tropical island, and it seems to have been pretty similar, if more variable, while Homo floresiensis was around. Now, it’s not clear whether the hobbits actually lived in the cave where their remains were found. But, based on the number of animal bones and stone tools found in different layers of the site, they seem to have used the cave more when the environment was wetter, and less when it was drier and less forested. And the hobbits seem to have had a rather … exotic diet, at least by our standards. The animal remains found in the cave include a lot of very young pygmy Stegodon, an elephant relative that, like the hobbits, seems to have evolved into a smaller version of its mainland cousins. A number of these bones have cut-marks on them, and some are even burned, so Homo floresiensis seems to have been able to use fire. And there were also lots of bones of Komodo dragons, which are still formidable predators today. It’s not clear whether the hobbits were hunting the Komodo dragons or just scavenging them - but it’s possible that the hobbits might have been hunted by the dragons! Interestingly enough, there don’t seem to be any bones of adult pygmy Stegodon in the cave, which suggests that the hobbits might not have been able to take down a full-grown elephant. The hobbits also left behind a number of simple stone tools, like cores, flakes, and points, some of which seem to have been used not only for hunting, but also for processing plant materials. But other experts weren’t convinced that these artifacts had been made by a new species of hominin. Instead, they thought the so-called hobbits were actually modern Homo sapiens, but from a population of very small-bodied people, like some groups of tropical hunter-gatherers today. And, they argued, LB1 likely had some kind of pathological condition. To support their case, these researchers compared the bones from LB1 with the skeletons of various indigenous peoples of Indonesia and Australia. Their thinking was that the best populations to compare them to would be ones that lived in the same region and environments as the hobbit. And, they said, more than 140 features of LB1’s skull matched those of modern humans from the area. But the researchers who thought Homo floresiensis was a new species countered this argument. They said that there are a number of groups of people from around the world who have adapted to their environments by becoming smaller - but none of them ended up with the same tiny brain and odd limb proportions as the hobbit. So that just left the claim that LB1 was an individual with some kind of pathological condition. In a series of papers, the supporters of this hypothesis proposed a number of different conditions that might explain LB1’s very small skull, short stature, and other features. These included Laron Syndrome, which is caused by an insensitivity to certain growth hormones, as well as microcephaly, or having a much smaller-than-average head circumference, and Down Syndrome. And every time, the scientists who thought Homo floresiensis was a new species pointed out that none of the proposed disorders quite matched LB1’s anatomy. Plus they didn’t explain all of the hobbit’s features that resembled those of older hominins, like its archaic-looking wrist bones. The two camps went back and forth, publishing paper after paper, questioning each other’s arguments - and not always in civil terms. And while one group of researchers still thinks LB1 is a pathological modern human, some recent work has suggested a third theory of where the hobbits came from. The Shire! No, just kidding Instead of being a dwarfed version of Homo erectus, or a modern human with a developmental disorder, maybe the hobbit actually evolved from another, earlier hominin species -- one we don’t know about yet. In this scenario, Homo floresiensis is still its own, new species, but its ancestor wasn’t Homo erectus. And in 2017, some experts tested this hypothesis. They collected a whole lot of skeletal data from 11 different hominin species, and built two kinds of evolutionary trees that showed how the species might be related. One tree was designed around the notion of parsimony, the idea that the simplest path for one species to diverge into another is the one with the fewest changes in features. The other model was built using statistics, analyzing how likely a path might be, based on different models of evolutionary change. And both methods came up with pretty similar results. In one scenario, Homo floresiensis shared a common ancestor with Homo habilis, a hominin that lived in Africa between 2.4 million and 1.4 million years ago. In the other scenario, the hobbits are part of the sister group to the branch that includes Homo habilis, Homo erectus, and us. What all this suggests is that Homo erectus might not have been the first hominin to leave Africa - even though that’s what the current fossil record shows. It also suggests that there’s probably a lot of ancestors on the hobbit’s branch of the family tree that are still out there, waiting to be found. Which is amazing even to think about, there’s so much we don’t know! Now where does the debate about the hobbit stand today? Well, the consensus among most experts is that Homo floresiensis is probably its own, unique species. This was helped along by the publication of revised dates for the bones and stone artifacts that were found in the cave. Instead of the dating to between 35,000 and 14,000 years ago, as we first thought, the deposit that the skeleton came from was more like 100,000 to 60,000 years old. And the stone artifacts were between 190,000 and 50,000 years old. So the odd little skeleton was older than we originally thought, which made its archaic-looking anatomy somewhat easier to understand. And it looks like there were changes in the island’s climate and volcanic eruptions around 50,000 years ago, which might explain why the species disappeared. But who it’s descended from is still a wide-open question. And the more digging we do in Southeast Asia, the more complicated our evolutionary story seems to be. For example, in 2019, scientists working in the Philippines announced their discovery of teeth and bones from a new species of hominin dated to about 50,000 to 67,000 years old. It was named Homo luzonensis, and it overlapped in time with the hobbits and us, along with some of our other extinct relatives. And it had a different mix of ancient and modern features than the hobbit, like very small molars but with larger premolars, and curved finger bones. This discovery -- of another new hominin on a remote island in Southeast Asia-- just reinforces how much more we have to learn about our family tree. Excavations at the cave where the original hobbit was found are on-going. And anthropologists are trying all the latest genetic techniques to try to unravel the mystery of the hobbit at the molecular level. But so far, our attempts to extract DNA from the hobbit’s bones have failed, because hot, humid caves are just terrible for preserving DNA. But even newer methods, like extracting ancient proteins, like collagen, from the bones have yet to be tried. So maybe there’s still some hope for figuring out where Homo floresiensis fits into our family tree. And maybe that will help us better understand this particular chapter of human evolution, back in the time when hobbits were real. 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