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  • Imagine yourself in Tanzania, in eastern Africa, more than three and a half million years ago.

  • A strange, unfamiliar creature walks past you -- on two legs -- across a bed of wet

  • volcanic ash, leaving footprints behind.

  • It’s small, with an ape-like face and long arms.

  • But the tracks it leaves are unmistakably human-like, a lot like the prints you'd make

  • in the wet sand on a beach.

  • Now, fast forward to 1978.

  • Thanks to a lucky confluence of events, those footprints have fossilized, at a site called

  • Laetoli.

  • And when theyre discovered by scientists, they revolutionize the way we think about

  • the history of our species.

  • The researchers who found these tracks, led by anthropologist Mary Leakey, dated them

  • to 3.6 million years ago, which was important, because it contradicted a longstanding idea

  • about human evolution.

  • The traditional view of our evolutionary history was that we got smart before we stood up.

  • In other words, the first thing that made us human was thought to be evolving a large

  • brain, with the rest of our human-like features, like walking on two legs, coming later.

  • But judging by the presence of their fossils in that same sediment layer, those tracks

  • were probably made by a species of human ancestor called Australopithecus afarensis.

  • And it had a really small brain, about the size of a chimpanzee's.

  • So, these footprints were evidence that, maybe what first distinguished our lineage wasn't

  • being brainy.

  • Maybe it was walking upright.

  • But if Australopithecus was already striding across the landscape 3.6 million years ago,

  • who started them on that path?

  • What species pioneered this style of locomotion?

  • Who was the first to walk?

  • To get to the origins of how we came to walk on two feet, you first have to be able to

  • recognize the evolutionary hallmarks of bipedalism in your own body.

  • And they can be found literally from head to toe!

  • Think about your skeletal anatomy: Your head is balanced on top of your spine, with the

  • spinal cord connecting to the brainstem at the base of your skull, rather than at the

  • back.

  • Your spine has a series of curves in it that position your torso above your hips.

  • Your pelvis is shaped kind of like a bowl, with your hip bones curving around your sides.

  • Your thigh bones, meanwhile, angle inward from your hips, putting your knees closer

  • to the midline of your body.

  • And your feet are very different from those of your living ape cousins.

  • You've got a big heel bone called a calcaneus, your feet have arches in them, and you've

  • got short toes with a big toe, or hallux, that's in line with the rest of them.

  • You're definitely not going to be grasping any tree branches with it.

  • All of these features are adaptations to bipedalism, especially those in your pelvis and your feet.

  • They allow you to get around efficiently, using less energy than a chimp does walking

  • on all fours.

  • But there isn't a single species of human ancestor that suddenly appears with all of

  • these features.

  • And fossils are often fragmentary.

  • So anthropologists are still studying what this transition looked like and which features

  • likely appeared first.

  • Adding to the puzzle, it can be hard to tell whether a species still climbed trees but

  • could walk on two feet if it needed to -- or if it was fully committed to life on the ground.

  • Now that we know what we're looking for, we can ask again: who was the first human ancestor

  • to walk upright?

  • Well, if we walk back in time from Australopithecus afarensis at Laetoli, the next possible contender

  • for that title is Australopithecus anamensis

  • It lived in Eastern Africa between 4.2 and 3.9 million years ago.

  • And there are two leg bones from this species that tell paleoanthropologists that it was

  • a biped.

  • One of these is a partial femur, or thigh bone.

  • It looks a lot like the femur of Australopithecus afarensis, but it’s bigger.

  • And because we have a lot more of the skeleton of Australopithecus afarensis, we can tell

  • that it walked bipedally, so we're pretty sure that means that the femur of anamensis

  • also came from a biped.

  • Even better evidence for bipedalism in this species comes from the shin bone, the tibia.

  • In 1994, paleoanthropologists excavating a site in northern Kenya found both the upper

  • and lower ends of a tibia, the parts that form the knee and ankle joints.

  • They then compared these bones to those of humans and chimpanzees.

  • Because, in humans, the tibia comes straight up from the ankle, better for standing upright.

  • But in chimps, the tibia slants diagonally outward, which suits their preferred mode

  • of locomotion, called knucklewalking.

  • And the bones from Kenya turned out to be straight, like a human’s.

  • So experts think Australopithecus anamensis was probably a biped, too.

  • Now, let's take yet another step back in time, to 4.4 mya in Ethiopia.

  • Enter another ancient human ancestor, Ardipithecus ramidus.

  • This species is best known from a partial female skeleton that got a lot of buzz in

  • 2009; she was nicknamedArdi.”

  • Ardi was small, just under 1.2 meters tall, and, based on the fossils of animals found

  • around her, she lived in a wooded environment.

  • And her skeleton told a surprising story.

  • Ardi had an opposable big toe, like modern apes have, but the rest of the preserved bones

  • of her foot suggest that her foot didn’t work like a grasping hand, as it does in living

  • apes.

  • Instead, her foot seems to have acted like a rigid lever, propelling her forward the

  • way that feet of humans and Old World monkeys do.

  • Along with her strange feet, Ardi's skeleton also included a pelvisand this part of

  • her anatomy had another unique combination of features.

  • The top of Ardi's pelvis looked a bit like that of Australopithecus afarensisshort

  • and broad, with the beginnings of a bowl shape.

  • But, the bottom of Ardi's pelvis -- the bones you're probably sitting on right now -- resembled

  • the pelvis of a climbing ape, like a chimp, with an angled muscle-attachment point for

  • powerful hamstrings!

  • So what did this mosaic of traits mean for Ardi?

  • Was she a climber or a biped?

  • Her discoverers think that she was bothan adept climber in the trees, but also an effective

  • biped on the ground.

  • Itll probably take finding a more complete, less fragmentary pelvis to tell for sure.

  • Now, when you look at our ancestors before Ardi, the picture starts to get murkier.

  • But there are three other species that some paleoanthropologists think might have been

  • bipeds.

  • And, given their ages, and that bipedalism is traditionally considered one of the defining

  • traits of our lineage, these three creatures are also contenders for the title of the first

  • known hominin.

  • A hominin is anything that’s more closely related to us than it is to chimpanzees.

  • So, this includes us and all of our extinct relatives that existed after the last common

  • ancestor of chimps and humans.

  • Fewer fossils of these three potential first hominins have been found, so paleoanthropologists

  • know less about their anatomy and are less certain about whether they were bipeds.

  • The first of these is an older species of Ardipithecus called Ardipithecus kadabba.

  • It was found in the Middle Awash region of Ethiopia and dates to between 5.8 and 5.2

  • million years ago.

  • Evidence that it might have been a biped comes from the shape of the joint of one of the

  • bones of its big toeand that's it.

  • The second species is called Orrorin tugenensis.

  • Discovered in the Tugen Hills in central Kenya, it lived around 6 million years ago.

  • There are 13 fossils of this species, and two of them are partial femora, or thigh bones.

  • And the latest research on these bones has found that their shape was part-way between

  • that of later australopithecines and those of earlier apes.

  • So, some experts think Orrorin was a biped, but that it didn't walk quite like we do

  • it might have had its own kind of swagger.

  • Finally, the oldest contender for the earliest biped is Sahelanthropus tchadensis.

  • Its partial skull was excavated in Chad and dated to around 7 million years ago.

  • And its discoverers think that it was bipedal based on the position of the hole at the base

  • of the skull, where the spinal cord connects to the brain.

  • Because, remember, in bipeds, this hole is underneath the skull, rather than on the back.

  • But there's still some debate about whether this is good proof of bipedalism or not, and

  • no other fossils of this species have been found.

  • So who was the first biped?

  • Most paleoanthropologists would agree that Australopithecus anamensis is currently the

  • the safest bet.

  • But others would be willing to go with Ardi or Orrorin.

  • It all comes down to how neat the transition from tree climber to upright walker wasbecause

  • it didn't happen all at once.

  • But this raises another really important question: Why did bipedalism happen at all?

  • For a long time, the go-to explanation for why we became bipeds was the known as the

  • savannah hypothesis.”

  • It proposed that changes in Africa's climate caused forests to shrink and grasslands to

  • grow, and moving between the remaining patches of trees was easier on two legs.

  • This idea spawned a number of spin-offs theories: like, that walking upright exposed less of

  • our bodies to the sun than walking on all fours did, making it easier for us to regulate

  • our body temperature;

  • And that freeing up our hands to carry things was key to our development, which is what

  • Charles Darwin thought.

  • In fact, chimpanzees do sometimes walk on two legs when they need to carry something,

  • like highly-prized foods.

  • Also, walking on two legs is more energetically efficient than walking on four, when you're

  • a primate on the ground.

  • But waitremember when I said that Ardi was found in a wooded environment?

  • That throws a bit of a wrench into the savannah hypothesis.

  • The geochemistry of the Orrorin site also suggests that it lived in a forested area.

  • So, maybe early hominins didn't have to leave the trees to become bipedal.

  • Maybe life in the trees was actually pushing them in that direction.

  • In contrast to the savannah hypothesis, some researchers have suggested that our upright

  • body plan comes from there being an advantage to standing on branches while feeding.

  • This idea is based largely on the behavior of some living apes that stand on branches

  • to feed.

  • Also since finding a lot more fossil apeswe see that many of their skeletons indicate

  • they held their torsos upright.

  • So, we don't know for sure why bipedalism evolved.

  • Answering these kinds of questions about the deep past is always difficult.

  • In this case, it's made even harder by that fact that we're totally unique in this type

  • of locomotion among mammalswe have a living sample size of one.

  • What we can say is that the earliest potential hominins, one of whom mightve been our

  • direct ancestor, had some bipedal features.

  • And it could probably move around on the ground and in the trees in a lot of different ways

  • – a lot like apes today.

  • To truly solve this puzzle, all we can do to keep looking for more fossils and more

  • footprints.

  • Even Laetoli still has more to tell us.

  • Researchers digging in 2015 found the tracks of two more individuals, moving in the same

  • direction and at the same speed as the ones Mary Leakey’s team found in 1978.

  • So with more study, and more time, we may yet understand how that fascinating, human-like

  • creature managed to walk past you on an African bed of ash, more than 3 million years ago.

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当我们第一次走路时(When We First Walked)

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    joey joey 發佈於 2021 年 05 月 03 日
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