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  • Once there was a Neanderthal  who was cleaning an animal skin.

  • They were holding the skin between their teeth  while pulling it tight and scraping it with a  

  • stone tool.   

  • And every now and then, the scraper would  slip and accidentally scratch their front  

  • teeth.   

  • This individual lived about 130,000 years ago  in what's now Croatia. And the scratches that  

  • these random accidents left on their teeth reveal  important clues about the hands that made them

  •   Based on the  

  • orientation of these scratches, and those on  teeth found from other sites, anthropologists  

  • have figured out that most Neanderthals were  right-handed, just like most of us Homo sapiens

  •   Seventy to 95 percent of us,  

  • to be exact. Including the dude here.   

  • But today, no other placental mammal that we know  of prefers one side of the body so consistently,  

  • not even our closest primate relatives.   

  • And not only that, but no human population has  ever been recorded as being mostly left-handed

  •   It turns out that our preference for one hand over  

  • another might be tangled up with some of the other  unique traits that we inherited from our ancestors  

  • after our lineage split with chimpanzeesnamely, walking upright and making stone tools.

  • In fact, being right handed may have  deep evolutionary roots in our lineage.  

  • And yet, being a leftie does seem to  come with some unexpected advantages

  •   We generally think of being right or left handed  

  • to mean preferring to use one hand over the otherBut it's actually more complicated than that

  •   Handedness seems to be more of a spectrum, with  

  • some people being strongly right or left handedand others being somewhere between the two

  •   And we can  also do a lot of different  

  • things with our hands, from simply holding  an object to precise, delicate manipulation,  

  • like threading a needle.   

  • People who generally prefer their right hand  for tasks that require fine motor skills could  

  • be said to be right handed, but they still use  their left hands a lot in their daily lives

  • Take something like cutting up a carrot - I'd  hold the knife in my right hand and do most  

  • of the work with it, but I'd still hold the  carrot on the cutting board with my left.   

  • Now, like in all vertebrates, each hand is  controlled by a different side of our brain.  

  • The left hemisphere controls the right  side of our bodies, and vice versa

  •   Also, the two hemispheres of our  

  • brain aren't perfectly symmetrical, which leads  to different cognitive processes taking place in  

  • different parts of the brain.   

  • This separation is known as lateralizationand it's found in all vertebrates and some  

  • invertebrates too, and allows us to simultaneously  process different types of information

  •   But like many other traits,  

  • asymmetry and lateralization are pretty extreme  in us humans, especially compared to other  

  • primates.   

  • And this  may be part of the reason that we  eventually came to prefer one hand over another

  •   Ninety-nine percent of  

  • people have a dominant hand.   

  • And there's lots of evidence that  it's been this way for a long time

  •   Cave paintings all over the world  

  • from the Late Pleistocene depict wild animalshunting events, and notably, a ton of human hands

  •   To make these hands, an artist probably  

  • placed one hand on the rock and then sprayed  pigment over it by blowing into a straw-like tube,  

  • leaving an outline of the hand.   

  • And interestingly, the vast majority of  the hands on the walls are left hands

  •   So, experiments that recreated this method have  

  • shown that these artists were predominantly right handed.   

  • Hand preference in our species is often so  strong that you can even see it in the skeleton,  

  • especially among athletes like tennis players  who use one arm a lot more than the other

  • The bone of their dominant upper arm becomes  thicker in certain places compared to the bone  

  • of their non-dominant arm, because they're  subjecting it to more force, more often

  •   In fact, although we're the only species on Earth  

  • today that's so strongly handed, there's a lot of  evidence that our extinct hominin relatives - that  

  • is, the members of our lineage after the split  with chimpanzees - were mostly right handed, too

  •   And weirdly enough, some of the oldest  

  • unambiguous evidence comes from teeth.   

  • Scientists had already suspected that Neanderthals  were mostly right handed based on their upper  

  • arm bones. Like a right-handed tennis playerNeanderthals had thicker upper arm bones in their  

  • right arms than their left arms.    

  • But it's rare to find fossils of earlier  hominins with bones from both arms preserved  

  • in order to compare them.   

  • So when scientists found microscopic scratches  on Neanderthal teeth that were caused by tools,  

  • like those found on that Croatian specimenthey began to wonder about two things.

  • First, how far back in the fossil record could  this evidence for handedness be recognized,  

  • and, second, what other behaviors  could handedness be  

  • associated with?   

  • As it turns out, hominins have been using their  teeth basically as a third hand for quite a while.

  • So pretend you're pulling a piece of animal hide  tight, between your front teeth and your left  

  • hand, stretching it out in front of you, and  holding a sharp stone tool in your right hand.  

  • To clean the hide, you scrape the  tool across it from left to right

  • If you slip and scratch your teeth, those  scratches go from the upper left corner to  

  • the lower right corner of your incisors. If you  were holding the stone tool in your left hand,  

  • they would go the opposite direction  - from upper right to lower left

  • Similar scratches like these were found on 500,000 year  old teeth from Spain belonging to a large group  

  • of Homo heidelbergensis, the species that might  be our last common ancestor with Neanderthals

  •   And those scratches have  

  • even been detected on the teeth of a Homo habilis from Tanzania that was 1.8 million years old

  •   Now, one right-handed Homo habilis doesn't  

  • mean the whole species was right-handed, it's  clear that handedness itself is pretty old

  •   And because no other primate species  

  • has extreme hand dominance, this trait must  have emerged after our split from chimpanzees

  •   But, why did more than one hominin species  

  • start preferring one hand in the first place? Andwhat's so special about the right hand anyway?

  •   Many studies have turned to genetics to  

  • try to find the elusivehandednessgene.    

  • Observations of families and genetic  analyses have shown that handedness does  

  • appear to be somewhat heritable, and that  men are left handed more often than women

  •   But, many searches through our  

  • genome haven't found the gene that's responsible  for left or right handedness. Instead, it seems  

  • like several  genes may have some minor effectsand that other factors might also be at play

  •   So, other scientists have focused on the  

  • importance of brain lateralization and tool use.   

  • Brain scans of people performing a variety  of tasks have shown that a specific region  

  • of our left hemisphere, called BA44, plays  an important role in manipulating objects,  

  • including making and using tools.   

  • Since the left hemisphere controls the right  hand, it's possible that the development  

  • of tools millions of years ago led to this  hand eventually being favored across hominin  

  • species.   

  • And having a species-wide hand preference at all  may be linked to an even older trait: bipedalism

  •   Some other bipedal mammals,  

  • like kangaroos, seem to have a hand preferencewhich suggests that not moving on all fours may  

  • have something to do with it.   

  • What's interesting is that these kangaroos are  mostly left handed, and they don't use tools  

  • like we do.   

  • So, if us being right handed is somehow  related to tool use and our left hemisphere,  

  • then why are there any lefties today at all?   

  •  While we still don't know for sure, it's possible  that at some point after the development of stone  

  • tools, everyone became right handed.   

  • In this scenario, left handedness may have  emerged later, as a result of one or several  

  • genetic mutations.   

  • And since lefties make up a pretty consistently  small portion of the population in our largely  

  • right-handed world, there must be  some kind of evolutionary advantage

  •   And this, too,  

  • might all go back to lateralization.   

  • Experiments and brain-scan studies have shown that  lefties tend to have less lateralized brains than  

  • righties.   

  • This means that they process information  more evenly across their brains,  

  • and this may be associated with better  coordination, memory, and verbal skills

  •   Plus, according to several studies, it also  

  • gives them an unexpected edge in physical combat.   

  • Imagine you're in a boxing match. If you're right  handed, and you've only encountered right-handed  

  • opponents, you'll probably be expecting hits  to come from your opponent's right hand

  •   But if your opponent is left-handed,  

  • they'll have the advantage of surprise, because  they'd be striking from an unexpected angle

  •   This left handed advantage is seen in a  

  • range of interactive sports today and is absent in  non-interactive sports, like darts. And it seems  

  • to be stronger in men than in women.   

  • One study from 2019 even found that left  handed boxers and mixed martial artists  

  • win matches significantly more often  than their right handed opponents

  •   This benefit could've directly led to  

  • increased survival among left handed people.   

  • And it might also explain why the frequency of  left-handers is so low: if too many people are  

  • left handed, the advantage disappears.    

  • Like tool use, bipedalism, and being relatively  hairless and sweaty, being right handed seems to  

  • have a deep evolutionary history in our lineage.   

  • Thanks to our highly lateralized brains, many of  the mental  processes that we use to make tools  

  • are concentrated in one area, which in most  of us, happens to be in the left hemisphere

  •   And since this  

  • hemisphere also controls the right side of our  bodies, we tend to favor this hand more often.

  • While having hand dominance is found in a few  other bipedal mammals, no other primate shows the  

  • degree of favoritism for one hand that we do.   

  • And even though the exact origin of  right hand preference isn't yet clear,  

  • lefties might have enough important evolutionary  advantages for them to still be around

  •   It just goes  

  • to show how variable we are as a speciesand that there's no rightway to be human.

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  • And we've gotta hand it to  this month's Eontologists:  

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How Humans Became (Mostly) Right-Handed

  • 13 1
    joey joey に公開 2021 年 05 月 02 日
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