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.