How Humans Became (Mostly) Right-Handed

Thanks to Great Courses Plus for Supporting PBS.  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 chimpanzees:  namely, 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 other.  But it’s actually more complicated than that.    Handedness seems to be more of a spectrum, with   some people being strongly right or left handed,  and 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 lateralization,  and 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 animals,  hunting 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 player,  Neanderthals 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 specimen,  they 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? And,  what’s so special about the right hand anyway?   Many studies have turned to genetics to   try to find the elusive “handedness” gene.     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 effects,  and 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 preference,  which 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 species,  and that there’s no right…way to be human. Thanks to Great Courses Plus  for Supporting PBS. The Great   Courses Plus is a subscription on-demand  video learning service with lectures and   courses from professors from top  universities and institutions.  Through your subscription, you get access to a  library of lectures about anything that interests   you...science, math, history, literature, or even  how to cook or become a better photographer.  For example, you should check out The  Scientific Wonder of Birds. In this course,   Dr. Bruce E. Fleury covers fascinating topics like  the origin of flight, how birds navigate over long   distances,, and of course, the connections between  birds and their extinct dinosaur ancestors. To learn more click on the link in the  description below to start your trial today. And we’ve gotta hand it to  this month’s Eontologists:   Sean Dennis, Jake Hart, Annie & Eric Higgins,  John Davison Ng, and Patrick Seifert!   By becoming an Eonite at patreon.com/eons, you   can get fun perks like submitting a joke  for us to read, which I’m gonna do now...    This one is from Betsy.   Why couldn't the Tyrannosaurus get  a driver's license? Because T. rex The w is silent and invisible, I guess. And as always thanks for joining  me in the Konstantin Haase studio. Subscribe at youtube.com/eons  for more creature features.

Channel: PBS Eons

Views: 1,482,367

Rating: 4.9157233 out of 5

Keywords: dinosaurs, dinos, paleo, paleontology, scishow, eons, pbs, pbs digital studios, hank green, john green, complexly, fossils, natural history, neanderthal, handedness, righties, lefties, right handed, left handed, teeth, lateralization, brain, Homo heidelbergensis, bipedalism, tool use, brain hemisphere

Id: vb11oOHYNXM

Length: 10min 0sec (600 seconds)

Published: Wed Mar 24 2021

Reddit Comments

[u/pjokinen]

My grandpa was born in Finland and immigrated to the US as a boy. Finnish was his native language, and he was a natural lefty, so he wrote with his left hand by the time he started school. There, he learned English and the teachers forced him to write with his right hand. In the end, he was only able to write in Finnish with his left hand and English with his right, he could never switch

[u/Augen76]

Born a lefty, trained to be a righty (couldn't afford left handed equipment) in sport.

So I throw right and write left.

[u/YouKnowWhatToDo80085]

Can't watch at work, can someone give me a TLDW?

[u/cptbeard]

1:31 talking of precise, one of my favorite trivia about human fingers is their sensitivity

commonly accepted single disturbance on surface detectable by human finger is about 9 microns, or 9/1000th of a millimeter, barely bigger than the wavelength of visible light, or about 10 covid viruses stacked on top of each other.

but then they did a study comparing continuous patterns on a surface:

The smallest pattern that could be distinguished from the non-patterned surface had grooves with a wavelength of 760 nanometres and an amplitude of only 13 nanometres.

https://www.sciencedaily.com/releases/2013/09/130916110853.htm

[u/dcdttu]

I love Eons!

[u/Daimakku1]

Being left handed is a genetic mutation? Does that mean I'm an X-Men?

[u/Diablo_Unmasked]

And here i was thinking it was all those years where left handed people were considered devil worshippers and burned at the stake.

[u/Ru4pigsizedelephants]

I'm left handed so to hell with this film.

[u/BillHicksScream]

Before the Progressive educational reforms after WW2, many left handed kids were forced to learn how to write with their right hand in the United States of America, my Dad included.

These are things to remember when people talk about "the good old days" or bring up dumb concepts like common sense.