The Eyes Have It: Why Nature (Sometimes) Stops at Just Two
In 2021, a veterinarian in the UK made a shocking discovery: a 4-month old cow with a third eye smack center on its forehead. “I have never seen anything like this before,” said the vet — and a lot of us Internet readers must have agreed, given how rapidly the story spread across the web.
It’s an uncanny, unsettling freak occurrence of nature… or is it?
Two eyes seems to just be the natural order of things. Humans are born with two eyes, dogs and cats are born with two eyes, and it’s often hard to visualize what the alternative might entail — although, not in the realm of fiction and fantasy. Many-eyed creatures have been a staple of mythology for thousands of years, so clearly we imaginative two-eyed humans have the capacity to think beyond our biological limits at least to some degree.
But in reality? It seems just too alien, at least at first glance—until we look deeper into the rich diversity of the natural world and realize that evolution isn’t biased against the many-eyed amongst us—be that three, eight, or far into the hundreds. What seems like an uncanny work of straight fiction is actually deeply precedented in actual biology. In many cases, it’s even more bizarre than we’re conditioned to believe possible.
Eyes: Fact or Fiction?
Let’s first get my rhetorical title out of the way. Why does biology — at least, human biology — prefer the two-eyed template?
It’s not rocket science, although the math involved can get pretty sophisticated if you’re into that sort of thing. Binocular (two-eyed) vision grants us the ability to perceive depth in addition to the color and detail that we’d be able to process through a single point alone.
When two eyes are employed, two additional factors play a role, the one not very important — namely, the act of convergence or divergence of the eyes — and the other very important — namely, the stereoscopic perception of depth by virtue of the dissimilarity of the images presented by a three-dimensional object, or array of objects, to the separate eyes. (Britannica)
But why stop there?
Eyes are an expensive organ — meaning that the ‘metabolic cost’ of eyes is significant-enough for the evolutionary process to (in simple terms) only “justify” the investment if there it will be outweighed by the benefits to our wellbeing throughout life. If we didn’t benefit significantly from having two eyes, it probably wouldn’t have happened — and over enough time if three eyes (or more) was even more valuable and outweighed our body’s investment in more complex organs, it’s likely we would have seen that adaptation arise in our lineage.
‘Two eyes’ really is the most optimal strategy for the way that we live — and that’s the case for all mammals. There’s still, of course, a lot of variety in the class. Primates (like us) have two eyes facing forward for a better perception of depth but a narrower field of vision, around 120°. Other mammal species, especially those that are often hunted as prey (with the goat as an extreme) have fields of vision upwards of 320° with very little overlap.
And in even more extreme cases, why have eyes at all? Contrary to popular belief, mammals like the star-nosed mole do have eyes, but they’re severely underdeveloped and connected to the brain by “tiny optic nerves” far smaller than would even be expected for creatures of their size.
This gets us back to our mutant cow story with some new, valuable intel. Three eyes (and possibly more) are clearly possible in mammalian evolution, but even if the calf’s third eye were to be fully equipped to collect light signals, it needs to be wired into a visual cortex capable of processing that information. In the mammal kingdom, two eyes will always reign supreme. It’s just how we’re wired.
But it’s not impossible to imagine the alternatives.
Some such creations of science-fiction are nice and straightforward. Nibbler from Futurama presents us with a three-eyed creature that might be able to see over the tall grass of planet Vergon 6, or to provide a sort of camouflage among the foliage. That is, until the Nibblonian origin story goes off the rails.
Deeper in lore, we stumble onto beings like Argus the many-eyed Greek giant (above left) or the archetypal Dungeons & Dragons baddie The Beholder (above right). Now we’re getting somewhere. As is often the case in many-eyed myth such as this, a multitude of eyes grants exceptional, spectacular abilities. The Beholder can slow, petrify, or disintegrate foes with beams originating from each eye stalk. For Argus, the eyes grant sight that is unwavering and omniscient — all-seeing and all-knowing, by some accounts.
It’s a trope in fiction that surfaces time and time again — an abundance of eyes that grant vision beyond our normal realm. Sometimes it’s the “Third Eye” that offers “perception beyond ordinary sight.” In Christian symbolism, the peacock’s “multitude of ‘eyes’ on its tail suggested God’s omniscience.” Looking broadly at the body of fiction and myth, creature’s with more eyes generally don’t just see in richer quality or depth—they see far beyond the limits of human capacity. They see through space, time, or privacy. It’s a line drawn between our real, limited, two-eyed world and the bounds of imagination.
Right?
Extreme Eyes of Nature
Humans often fall into the pitfalls of the ‘mammal-centric’ mold that we sometimes use to shape our perceptions of biology writ large. In truth, the rules that dictate our anatomy and the definitive building blocks of our charismatic, fur-covered, lactating cousins are severely limiting in understanding the very real extremes of nature all around us. While it’s comforting to draw a fine line between “two eyes = normal” and “more eyes = freaky”, that’s definitely not how the world works.
And even the “sight beyond sight” that we like to delegate to strictly fantasy and mythology is, to be quite frank, a real phenomenon of nature that usually isn’t internalized when we think about evolution.
For starters, the parietal eye.
In a myriad of amphibians, sharks, lizards, and famously a non-lizard reptile called the tuatara, the parietal eye is a light-sensitive organ placed at the top of the head. It’s usually found right between the two “normal” eyes—which I know is a shamefully mammal-centric phasing that I should probably try to avoid.
While it’s functional existence isn’t consistent across all animal groups and has often baffled animal behavioralists, some studies indicate that the organ’s sensitivity to light can be calibrated to act like a compass and allow for more accurate navigation against the path of the sun.
A real “third eye” allowing animals to see beyond what humans can see? Egad!
And we haven’t even discussed arthropods yet.
Not all spiders have eight eyes, but for the ones that do, those extra sensory organs offer some pretty amazing abilities:
The secondary eyes serve a variety of functions. In some cases, the lateral eyes expand the range of the primary eyes, giving the arachnid a wide angle image. The secondary eyes act as motion detectors and provide depth perception information, helping the spider locate the distance as well as direction of prey or threats. In nocturnal species, the eyes have a tapetum lucidum, which reflects light and helps the spider see in dim light. (ThoughtCo)
And we can’t forget (again) the mantis shrimp! These bad boys have up to 16 flavors of photoreceptor cells, compared to the measly three in mammals, and can see colors and polarization that humans would never be able to naturally detect without enhanced equipment. Moreover, they’re able to process depth within each eye without the need to synthesize multiple images in the brain like us — something scientists call “trinocular vision.”
The list goes on to include more and more extreme, mind-bending instances of spectacular sight in the depths of the animal kingdom. The final creature on our list is reserved for the chiton, the small flat mollusk that frequent intertidal zones all across the planet — and that possesses hundreds of pinhole eyes scattered across their shells. In 2015, further research refined our understanding of chiton vision and deemed that the eyes aren’t as primitive as we once may have thought — in fact they’re capable of forming focused images and help the chiton detect motion coming from nearly angle surface angle.
The hundred-eyed Argus or omniscient ‘Third Eye’ of myth seem far less bizarre in contrast with a slideshow of nature’s extremes.
It’s a bit of a fallacy to see the world — literally — through just our mammalian two-eyed perspective. While we’d like to think that our perspective is the norm, it’s actually far from it. For centuries, storytellers were postulating about impossible scenarios with vision capable of things far alien to our anatomy — when in reality, nature had already figured out a good degree of extremes that top even our wildest imaginations.
All this is to say that if you’re a cow in Wales who is teased because of your third eye, you’re actually in great company — fact and fiction alike.
