字幕表 動画を再生する 英語字幕をプリント Usually, I'd start an episode of Eons by telling you about the discovery of some strange fossil or setting the scene for a world-changing event like an ice age or an extinction. But today, I want to try something a little different. Instead of thinking about a story, I want you to stop and think about the thing I'm doing here - literally, that I'm standing here and telling you a story. Because, the evolution of our ability to speak is its own epic saga - and it's worth pausing to appreciate that. It's taken several million years to get to this moment where I can tell you about how it took several million years for us to get here. And, yes, there are other animals alive today that communicate in sophisticated ways - like whales, elephants, and crows, to name a few. Still, our vocal abilities as a species are pretty unique. They're part of what makes us human. From the anatomy of one particular bone in our throats and the proportions of our vocal tract, to the morphology of our ears, paleoanthropologists are piecing together the puzzle of when and how this adaptation arose. And while speech itself doesn't fossilize, the fossil record of our ancestors and relatives can still give us important clues about the time when we first talked. Now, this story could start between 400 and 360 million years ago, when the first ancient tetrapods transitioned from life in the water to life on land, and evolved lungs and a movable tongue. Controlling both of these with precision is important for creating the many different sounds that make up human speech. Or it could start between about 8 and 6 million years ago, when our lineage - the hominins - split off from the ancestors of our closest living relatives, chimpanzees and bonobos. While there's no question that they can communicate with vocalizations, gestures, and expressions, they can't speak like us - despite decades of effort to teach them how. And our earliest hominin relatives probably couldn't talk like us, either. Because! The first piece of fossil evidence that can be used to reconstruct hominin vocalization comes from the skeleton of a juvenile Australopithecus afarensis dated to 3.3 million years ago. This special bone is called the hyoid. It's a U-shaped bone that sits in your neck just below the level of your jaw and it doesn't connect to any other bone. Instead, it's held in place by muscles and ligaments. In humans, the hyoid is an important attachment point for the muscles of the tongue. It does that in chimps, too. But, in chimps - and in most other living apes - it also helps support structures called laryngeal air sacs. Now, we don't know exactly what these things do. Some research has suggested that they might help make vocalizations louder and help primates call for longer or more often without hyperventilating. But they also seem to introduce new, lower resonances to vocalizations and reduce the differences between higher-pitched sounds. Both of which would make human speech sounds harder to understand. And, here's the thing, the hyoid of Australopithecus afarensis looked more like those of chimps and gorillas than it does like ours. Which means that this hominin likely had air sacs attached to their hyoid. So, her species probably couldn't speak like we do. And that one hyoid is the only one we have from any species of australopithecine. The hyoid is a small, fragile bone, so it's one of the least well-known bones in the hominin fossil record. In fact, the next oldest hyoids we've ever found come from a site that's almost three million years younger. In that span of time, the australopithecines disappeared, and our own genus, Homo, evolved, with some populations even making their way out of Africa to Asia and Europe. And a cave site in northern Spain called the Sima de los Huesos - or the Pit of Bones - is where those next-oldest hyoids were found. They've been dated to around 450,000 years old and belonged to members of the species Homo heidelbergensis. This species may be the common ancestor of Neandertals and our own species, Homo sapiens, or it might just be a close relative of both. And while both of the hyoids from this site are incomplete, what is preserved looks a lot more like our hyoid than like the hyoid of Australopithecus afarensis or a chimp. So, these hominins probably didn't have laryngeal air sacs. But that doesn't necessarily mean they could talk like us - at least, not based on their hyoid bones alone. See, along with a lack of laryngeal air sacs, members of our species also have unique vocal tract proportions. You can split the vocal tract above the larynx, or voice box, into two basic parts. The mouth makes up the horizontal part and the pharynx makes up the vertical part - the bit between the mouth and the voice box. In adult humans, these two parts are about the same length. This allows us to make three of the different vowel sounds - a, i, and u - and to make them sound really distinct from each other. And almost all human languages have at least three vowels - and they tend to be these three. This might be because they sound the most different from each other, so they're less likely to be misheard. But some anthropologists don't think that other hominins had these same proportions. For example, researchers previously suggested that Neandertals had much shorter vertical parts of their vocal tracts and longer horizontal parts. Human infants and chimpanzees also have short vertical segments of their vocal tracts, and can't make those distinct vowel sounds as a result. But that fossil site in Spain can also give us clues about this piece of the puzzle. Because, there's one individual with a nearly-complete skull and all seven neck vertebrae that researchers have used to estimate the lengths of the two parts of the upper vocal tract. They found that the horizontal part was actually only a little bit shorter than the vertical part. This makes the fossil's proportions more like those of a 10 year old human child than an adult. And 10 year olds can still make those same distinct vowel sounds, meaning this almost half-a-million-year-old hominin probably would've been able to do it, too. Some of our more recent relatives also seem to've had vocal tract proportions similar to that individual. When anthropologists reconstructed the vocal tract length of one adult Neandertal from France dated to between 50,000 and 70,000 years ago, they found that the horizontal section of his upper vocal tract was slightly shorter than the vertical section. So, he, too, could probably make the full range of sounds found in human speech. And there's more anatomical evidence for speech in Neandertals than just one reconstructed vocal tract. Bringing it back to the hyoid bone, we've found two from these extinct relatives of ours. One is from another cave site in Spain and dates to around 43,000 years ago. It's incomplete, but was described as being “almost indistinguishable” from a modern human hyoid. The second is a little older, dating to around 60,000 years ago, and comes from a site in Israel called Kebara Cave. It also looks a lot like our hyoid bones. The scientists who worked on this bone even CT-scanned it to see if its internal structure was a match for a human hyoid. See, bone can actually change or remodel its microscopic architecture over time, based on how it's being used. So if a Neandertal was using the muscles and ligaments that attach to its hyoid the same way we do, CT scans of our hyoids should look similar, too. And they did! Which means that both the outside and the inside of the hyoid of this Neandertal suggest that he was capable of making human-like speech sounds. But! Being able to make human-like speech sounds is only half the story - the other half is being able to hear them. While paleoanthropologists haven't found very many fossil hyoids, they have found a lot of skulls. These allow them to study things like the size and shape of the ear canals - and sometimes even the tiny bones of the ear themselves - because they're contained within the temporal bones of the skull. And by comparing the fossils to the anatomy of living primates and humans, they can model the hearing ranges of our extinct relatives. Early hominins, like Australopithecus africanus and Paranthropus robustus, have some features of their ears that look more like ours than like a chimpanzee. For example, they have a slightly shorter and wider passage leading from the outside of the skull to the membrane of the eardrum. And they have a malleus - one of the bones of the middle ear - that looks human-like. But the other two bones of the middle ear - the incus and stapes - are more chimp-like in size and shape. And when their hearing abilities are modeled, they're not quite like ours or like those of a chimp. These early hominins seem to have been more sensitive to mid-range frequencies than modern humans or chimps are. More human-like ear anatomy and hearing abilities seem to originate in our genus, Homo. Homo erectus fossils from Asia have some human-like features, and the hominins from Sima de los Huesos and the Neandertals are even more similar in ear anatomy to Homo sapiens. The same hearing model used for the early hominins predicts that the later members of our genus probably had hearing abilities like ours. They lost a little bit of the mid-range frequency sensitivity seen in early hominins, but expanded their range of maximum sensitivity to include higher frequencies. It's been suggested that greater sensitivity to those higher frequencies is important for hearing consonants, especially t, k, f, and s. And the use of consonants is a key feature that distinguishes human language from most animal communication. These changes in ear anatomy and hearing ability go along with the changes in the hyoid bone over time - early hominins were more similar to our closest living relatives and the members of our genus Homo are more similar to us. And by the time we get to the Sima de los Huesos hominins and the Neandertals, we seem to have all the anatomy in place for distinct vowel and consonant sounds -- which is pretty incredible! Now, we don't know if this means these extinct relatives had language. This is still a big topic of debate and we don't have enough evidence to say either way. It might just come down to how we define language. But what we can say is that there's no anatomical reason they couldn't make and hear human-like speech sounds. And we know the Neandertals were capable of very human-like behavior, like caring for injured members of their groups and using objects for personal ornamentation. So maybe they were storytellers, too. As the only hominins left, it's up to us to piece together the puzzle of our ancestors' speech abilities — and use our own to tell this evolutionary story. Hey if you wanna learn more about all the ways in which Neandertals were similar to us...as well as what happened to them, be sure to check out our episode, “When We Met Other Human Species”. Now I have to say this month's Eontologists are : Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert! By becoming an Eonite at patreon.com/eons you'll get fun perks including submitting a joke for us to read like this one... from my friend Matty Dahman. Why did the T Rex need a nap? Because he was wiped out. I need a nap after this script! Thanks Matty for submitting your joke. And as always thank you for joining me in the Konstantin Haase studio. Subscribe at youtube.com/eons for more adventures in deep time.