In 2016, researchers at Nagoya University in Japan performed a simple experiment to
test a pretty weird idea.
They recruited 20 students and showed them blurry photos of four different animals - cats,
birds, fish, and snakes.
Now, at first, all the photos were too blurry to tell what the animals were.
But then, they gradually unblurred the photos and asked the students to say when they could
identify the animal in each image.
And the results were pretty striking: The students were consistently able to identify
the snakes before any of the other animals.
And this wasn't a surprise to the researchers.
Because the idea they were testing is something called The Snake Detection Hypothesis.
something tells me that would be a good band name, don't you think?
It proposes that the ability to quickly spot and avoid snakes is deeply embedded in primates,
including us - an evolutionary consequence of the danger snakes have posed to us over
millions of years.
And this experiment is only one of many that have tested this hypothesis - generally finding
similar results.
This has led some researchers to argue that elements of the primate brain and visual system
evolved, in part, as anti-snake adaptations.
And more recently, the hypothesis has taken on a new dimension: while snakes might have
shaped aspects of our evolution, it seems that, in some cases, we might’ve shaped
aspects of theirs, too.
Now, I want to be clear about something: the Snake Detection Hypothesis is just that - a hypothesis.
And a hypothesis is an idea that’s been proposed as an explanation for the way things
are -- one that can be tested through observation and experiments, and is open to being proven wrong
More importantly, I think we all know that we can’t explain our evolution in the context
of a single potential predator, like snakes.
Obviously, all of the various adaptations that make us Homo sapiens have been shaped
by all sorts of evolutionary pressures.
Plus, I love snakes.
I think they’re fascinating and beautiful, and so does my son, who has a pet rosy boa.
But researchers have found patterns that seem to point to the same conclusion: We tend
to be really, innately … touchy about snakes.
We’re really good at detecting them, sizing them up, and figuring out how much
of a threat they pose, if any.
And this might be for good evolutionary reasons.
Around 60 million years ago, the planet was gradually recovering from the Cretaceous-Paleogene
mass extinction, yknow with the asteroid and the volcanoes and all of it.
don't get me started
The non-avian dinosaurs were gone, and mammals were on the way up, finally free from the
dinos’ domination of the planet.
But our early primate ancestors and mammal relatives still faced some surviving predators
from the age of reptiles: namely, snakes.
Snakes had been on the rise through the Cretaceous Period, starting out small, but growing ever
larger to target bigger vertebrates, mostly lizards.
We even have fossil evidence of them eating dinosaur hatchlings.
Small mammals, including the earliest primates, were almost certainly on the menu, too.
So snakes represent a very ancient threat - one that primates have had continuous exposure
to for essentially as long as we’ve been around.
And the snake lineages that survived that extinction event - the ancestors of the 4000
or so species we know today - would only become more of a threat to primates as time went on.
After the extinction event, snakes became venomous and started occupying all sorts of
ecological niches left vacant by the demise of the dinosaurs.
And the Snake Detection Hypothesis proposes that primates’ continuous exposure to this
danger throughout our evolution has shaped us in key ways.
Take our visual system, for example.
Compared to most other mammals, primates have extremely good vision.
And we rely on this sense more than others, including smell, which is often the main sense
in other mammal groups.
And we have a suite of adaptations that make us so vision-oriented, many of which are rare
or even unique among mammals.
We have front-facing eyes that give us great depth perception, and we have large regions
of the brain devoted to processing visual information.
Some primates, like us apes, can even see the full spectrum of color - which actually
isn’t that common for mammals.
And as primates moved from being nocturnal to being more active during the day, these
adaptations would have certainly helped us spot and avoid dangerous snakes before it
was too late.
Now, these adaptations would’ve been useful for other things, too - like foraging for
fruit and other food, or better visual signaling among each other.
But, there are also some specific differences between the visual abilities of different
types of primates that are consistent with the Snake Detection Hypothesis.
The primates with some of the comparatively worst eyesight are the lemurs of Madagascar.
And they’re also the only primates that haven’t really had to watch out for venomous snakes over the course of their evolution.
Because the snake families with the most potent venom - like vipers and elapids
- never made it to their island.
And at the other extreme, the monkeys of Africa and Asia, which have been around predatory
and venomous snakes the longest, have the best vision, and the most advanced color vision
AND, along with potentially shaping our eyes and visual system, it’s been suggested that
the danger posed by snakes may have also shaped parts of our brain...
Specifically, a region of the brain called the pulvinar, which is associated
with processing visual information and is bigger in primates than in other mammals.
In experiments on macaques that had never seen snakes before, scientists showed them
a series of images while measuring the activity of the neurons in the pulvinar.
The images included snakes, geometric shapes, and the faces and hands of other monkeys.
The pulvinar neurons fired faster and stronger in response to the snakes than any of the
other images.
And they even found that if they showed images of snakes in a threatening posture, the neurons
fired even more strongly.
This could indicate that there are elements of the primate brain that are hardwired to
be especially good at detecting snakes, assessing their threat, and directing attention to them.
So next time you spot a snake out of the corner of your eye, know that you may have just flexed
a primate superpower.
And this proposed ability to detect snakes may have shifted the balance of power enough
that some snakes started to become better able to defend themselves from us.
After all, the danger goes both ways: many primates have been known to mob, harass, and
kill snakes on sight.
And after our lineage split from our common ancestor with chimps, hominins started to
develop things like bipedalism and tool use, which made us able to attack predators from
a distance by using clubs and throwing stuff.
In a paper published in 2021, scientists proposed that in response to this danger from hominins,
a new defensive ability developed in a few groups of cobras - one that worked at a distance.
They acquired the ability to spit venom!
Now, many snakes use venom to kill or paralyze their prey up close by injecting it through
their fangs.
But turning it into a projectile weapon?
That’s a much rarer and more recent adaptation.
The researchers found that the ability to spit venom evolved a few times independently
in closely related lineages of cobras.
And, in each case, they converged on the same general approach.
Their fangs became modified to have new front-facing openings that allow for venom to be sprayed
up to 2.5 meters away, targeting the eyes.
And the cocktail of toxins that makes up the venom changed to cause greater pain, and even
blindness.
Now, for this to have emerged independently multiple times in cobras in almost exactly
the same way, there must have been a pretty strong selective pressure.
And, the researchers found that the timing of the evolution of each spitting cobra lineage
matched up pretty closely with the arrival of hominins where the snakes lived.
Venom spitting originated in African cobras, for example, as recently as 6.7 million years
ago - soon after our lineage diverged from the ancestor of chimps and bonobos around
7 million years ago.
And the same thing happened in Asian spitting cobras around 2.5 million years ago - around
the time that we think Homo erectus may have first arrived in Asia.
Now sure, this could have helped the cobras avoid being attacked or killed by other animals
too, of course, but there are reasons to think it evolved specifically against us.
After all, we’re pretty unusual in our ability to defend ourselves by using tools or throwing
things accurately.
Plus, our forward-facing eyes would be especially vulnerable to venom, more so than, say, big
ungulates, which have eyes on the sides of their heads.
It’s yet another tantalizing pattern that hints at an ongoing arms race of “snakes
vs primates” over evolutionary time.
And the thing is, we don’t often think about the ways predators have shaped our evolution,
especially in ways that we can still experience today.
Maybe it’ll happen to you the next time you're out hiking somewhere and you see a
long, squiggly thing on the trail in front of you.
Is it a stick?
Or a danger noodle?
The Snake Detection Hypothesis -- in addition to being a pretty good band name -- is a fascinating
idea.
And while it’s hard to conclusively prove, its underlying point rings true: the features
of our species today reflect the dangers and challenges our ancestors faced, both as predators
and as prey.
and hey if you need more snake content - because who doesn’t - check out our episode “How
a Hot Planet Created the World's Biggest Snake”.
And snakes a lot to this month’s Eontologists...
By becoming an Eonite at patreon.com/eons you can get fun perks like submitting a joke
for us to read, like this one from Drew Brammer
How much room did ancient fungi need to evolve into the varieties we see today? As mushroom as possible.
and as always thanks for joining me in the Konstantin Haase studio.
Subscribe at youtube.com/eons for more evolutionary escapades.
Before you go! -Wait! We think you would love Fate & Fabled, a brand new show over on Storied.
Co-hosts Emily Zarka and Moiya McTier unpack ancient myths and legends to find out why these tales were crafted,
and investigate mythology’s influence on humanity.
Their first episode on the Fates is live right now… so, maybe your destiny is to go check it out! And let them know Eons sent you.