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.