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{♫Intro♫}
Despite what you might have seen on the internets, there’s no such thing as a right-brained
or left-brained person.
But your brain is split down the middle, and the two halves have different specializations
and are even structured differently at the neuronal level.
This is what’s known in neuroscience as brain lateralization.
We used to think this was something unique to us, which led researchers to think that
we evolved asymmetry because of our unique cognitive abilities.
But lateralization has now been found in everything from chickens to spiders—and that’s allowed
us to come up with some other, more interesting ideas for why and how our hemispheres evolved.
You see, despite superficial similarities between your brain’s right and left hemisphere,
they’re quite distinct.
There are more miniature columns of neurons in the left hemisphere than the right, for
example.
Also, neurons in the left hemisphere tend to have more myelination—a fatty coating
that speeds up signal transmission.
And we’ve long known that our hemispheres are functionally different, too.
Most of your ability to process language is associated with brain activation in the left
hemisphere, while things like spatial processing or facial recognition activate the right.
How specialized these hemispheres are does vary between people.
Still, we’re all at least somewhat lateralized, and at first, we thought this was because
humans were special.
Classic.
But once we started to find asymmetry in birds and other animals, we had to come up with
other ideas.
One of those ideas is that the selective pressure for lateralization came from the need to perform
skilled tasks that require just one limb.
You see, studies have found that the more lateralized an animal’s brain is, the more
likely it is to have a preference for one side—what we often call “handedness”.
Let’s, say, a monkey is slightly better at grabbing for food with its right hand.
It benefits most by always using that hand instead of sometimes trying to use the other.
So monkeys with more lateralized brains might outcompete less lateralized ones.
Research has-- found that when chimpanzees fish for termites with just one hand, they
get more of them per minute.
And we might see the benefit of handedness in people, too.
Like, back in 1970, researchers timed 219 kids aged 3 - 15
to see how quickly the kids could move some pegs around a pegboard
using only one hand.
And those who were faster at moving the peg
also had a clear preference for one hand on
other tasks, like cutting paper and throwing.
But, it hasn’t been shown that this kind of task performance translates to real differences
in evolutionary success.
Another potential explanation for lateralization
is more basic: to speed up thinking by shortening
the connections between neurons.
In humans, neural connections between the
hemispheres have to pass through the corpus
callosum.
That can cause a signal transmission delay
of over 25 milliseconds—which is a long
time when we’re talking neuronal chit-chat.
And the bigger a brain is, the longer those signal delays can be—
especially if the neurons
that need to communicate end up on opposite
sides of the brain.
So, all animals, but especially humans and
larger mammals, could have benefited from
consolidating tasks to one side or the other.
But again, this is more of a convenient explanation
than an evolutionary smoking gun.
Lateralization could have evolved because
grouping certain neurons together is just
a better way to build a big brain, or it could
have to do with performing skills better.
Finally, there’s a chance things are even more complicated than that.
Like, maybe lateralization developed in most
animals for one thing, but human evolution
took that one step further.
In animals, scientists have seen that lateralization
helps with something called parallel processing
— basically, being able to do two things at once.
That may explain why it’s so common.
For example, we know that chicks mostly use
their left hemispheres when hunting for grain
among pebbles, but use their right hemispheres
to monitor for predators.
And the more lateralized they are, the better
they are at keeping an eye out for trouble
while searching for food.
It’s easy to see how that would be good from an evolutionary perspective, but there’s
not much evidence that this happens in humans.
Studies have found that people with more asymmetry tend to have better verbal intelligence and
visuospatial skills, though.
So some researchers have proposed that, in us, lateralization helps with a different
kind of parallel processing: thinking about the world in two different ways
The idea here is that there is no ideal kind of neuron or neuron structure that works best
for everything we might want our brains to do.
If you eat some red berries and then get sick, for example, you might need to come up with
a quick explanation for what happened.
That’s where the faster-firing, discreetly organized neurons in the left hemisphere might
jump in.
Woah!
They were poisonous and you should avoid them.
At some point, though, it’s important to know that you’re right—like if you’re
lost in the woods and those berries are the only thing you can find to eat.
That’s when the right hemisphere, with more overlapping neurons and more holistic processing,
can step up to detect if any of your explanations conflict.
Like, maybe you realize you’ve eaten those berries other times and didn’t get sick,
so they’re probably not poisonous.
In support of this idea, there does seem to be some evidence that the left hemisphere
focuses on creating explanations and drawing inferences, while the right hemisphere inhibits
your responses while checking for conflicts.
But that evidence is not super clear cut, and it’s hard to test the evolutionary relevance
of having different modes of reasoning separated into two hemispheres.
So ultimately, we’ll need to do more research to fully unlock the mystery of why brains
in humans and other animals are lateralized.
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