Humans can, in exceedingly rare cases, be born with three eyes. The condition arises not from some special “third eye gene” but from a developmental accident called diprosopus, in which parts of the face duplicate during embryonic growth. Fewer than a handful of documented cases exist in the medical literature, and the biology behind them reveals just how precisely orchestrated normal facial development has to be.
What Diprosopus Looks Like in Practice
Diprosopus is a form of conjoined twinning in which the face partially or fully duplicates while the rest of the body remains single. The duplication can range from something as small as an extra nose to a complete second face, and anything in between.1PubMed Central. Partial facial duplication (diprosopus): a case report and review of the literature It accounts for roughly 0.4% of all congenital anomalies, which already puts it into the territory of conditions most obstetricians will never encounter in a career.2PubMed Central. Rare Case of “Diprosopus Bicephalous Triophthalmus” and Review of Literature Within that tiny fraction, the subset where exactly three eyes form is even rarer still.
One well-documented case involved an infant born with two normally positioned eyes and a third eye located in the left temporal region. That extra eye had its own eyelids and lashes, which blinked spontaneously along with the other two eyes. However, the cornea was hazy, and clinicians could not determine whether any vision was present.3West African Journal of Radiology. A “three-eyed” infant: A case of partial facial duplication (diprosopus monocephalus triophthalmos) Other diprosopus cases involve different configurations. One report described a baby with two full sets of noses, eyes, ears, mouths, and tongues, along with bilateral cleft lips and palates.4PubMed. Diprosopus (partially duplicated head) associated with anencephaly: a case report The pattern depends on how much of the facial structure duplicates and at what stage of development the process goes awry.
Why Parts of the Face Sometimes Duplicate
During normal embryonic development, the face assembles from several tissue blocks that migrate toward the midline and fuse together. A complex web of signaling molecules tells each block where to go, when to stop, and what structures to build. One of the most important signals involves a pathway called Sonic Hedgehog, which helps establish the midline of the face and brain. When that signaling is disrupted, the consequences are dramatic.
If the Sonic Hedgehog pathway is underactive, the face can fail to divide properly, leading to cyclopia, where the two eyes merge into one. The gene encoding Sonic Hedgehog was identified as the first known gene to cause holoprosencephaly, the brain malformation underlying cyclopia.5PubMed. Mutations in the human Sonic Hedgehog gene cause holoprosencephaly If, on the other hand, the organizing signal is duplicated or the facial tissue receives conflicting instructions, structures can form twice instead of once. Diprosopus sits at the opposite end of this spectrum from cyclopia: too much facial organization rather than too little.
The exact trigger for facial duplication is not pinned down in most cases. Complete duplications tend to come alongside severe anomalies elsewhere in the body, including brain malformations like anencephaly and spinal defects.2PubMed Central. Rare Case of “Diprosopus Bicephalous Triophthalmus” and Review of Literature This overlap suggests the problem begins very early, when the entire body plan is being laid out, not just during the fine-tuning of facial features.
Can a Third Eye Actually See?
Having the physical structure of an eye does not guarantee it works. For vision to function, the eye needs a clear optical pathway (cornea, lens, vitreous humor), a functioning retina, and a nerve connection to the brain’s visual processing areas. In the documented three-eyed infant, corneal haze alone was enough to make visual assessment impossible.3West African Journal of Radiology. A “three-eyed” infant: A case of partial facial duplication (diprosopus monocephalus triophthalmos) Even if the globe itself were perfectly formed, there is no guarantee the optic nerve would reach the brain’s visual cortex in a way the brain could interpret.
The brain’s visual system is wired for two eyes. It uses the slight difference in angle between left and right images to construct depth perception, and its visual cortex is organized into columns that correspond to input from each eye. A third eye would need its own neural real estate and its own set of connections. In the rare cases where supernumerary eyes form, there is no evidence that the brain reorganizes itself to accommodate extra visual input. The third eye may blink, it may respond reflexively to light, but functional sight from it is a different matter entirely, and no clinical documentation confirms it.
Other Growths That Can Resemble a Third Eye
Not every lump near the eye socket that looks vaguely eye-like is an actual supernumerary eye. Ocular choristomas are congenital growths made of normal tissue that ended up in the wrong place during development. They can contain skin, fat, cartilage, or even bone, and they appear on or near the eye.6PubMed Central. Epibulbar osseous choristoma A dermoid cyst sitting on the surface of the eye or near the orbit might alarm a parent, but it is not a third eye forming. These growths are classified by which tissue types are present and how many embryonic layers contributed to them. They are far more common than true diprosopus and are usually treatable with surgery.
The distinction matters because the underlying biology is different. A choristoma is misplaced normal tissue, like finding a patch of skin cells where cartilage should be. Diprosopus involves actual duplication of an entire developmental field, a fundamentally different kind of error in the body’s construction plan.
Cyclopia and the Opposite Extreme
If diprosopus represents an excess of facial organization, cyclopia represents its absence. In cyclopia, the developing brain fails to split into two hemispheres, and the face condenses into a single midline structure, often with one central eye socket containing a fused eye or two partially merged eyes. Cyclopia occurs in roughly 1 in 13,000 to 20,000 live births and is almost always fatal.
Cyclopia can result from genetic mutations, particularly in the Sonic Hedgehog pathway, but it can also be triggered by environmental exposures. Research on teratogenic chemicals that cause cyclopia in animal models found that while different toxins all suppressed a key gene in the Hedgehog signaling cascade, they did so through different mechanisms and at different speeds, suggesting the pathway can be disrupted at multiple points with the same devastating outcome.7PubMed. Molecular changes associated with teratogen-induced cyclopia One well-known natural cause is a plant alkaloid found in corn lilies, which historically caused cyclopia outbreaks in lambs born to ewes that grazed on the plant. This chemical was later shown to directly inhibit Hedgehog signaling.
The cyclopia-diprosopus spectrum is a useful way to think about facial development: the same signaling systems that, when underactive, fuse the face into one can, when overactive or duplicated, split it into two. The vast majority of births fall in the healthy middle of this range, where the signals balance correctly.
Nature’s Actual Third Eye in Lizards and the Tuatara
While a functional third eye in humans is essentially impossible, some animals have one as standard equipment. Many lizards and the tuatara, a reptile native to New Zealand, possess a parietal eye, a small photosensitive organ on top of the head. It sits beneath a translucent scale on the skull and connects to the brain through a structure called the parietal foramen, a hole in the skull’s roof.
The parietal eye does not form images. Instead, it detects light levels and helps regulate body temperature, circadian rhythms, and seasonal behavior. Experiments in which the parietal eye was covered or removed showed changes in how long lizards basked in sunlight, their body temperature, and their daily activity patterns.8PubMed Central. Evolution of Pineal Nonvisual Opsins in Lizards and the Tuatara and Identification of Lepidopsin: A New Opsin Gene The organ uses a unique two-channel light-detection system. Its photoreceptor cells contain two different opsins: one sensitive to blue light that causes the cell to decrease its electrical activity, and another sensitive to green light that increases it. This means a single cell can distinguish between blue-dominated and green-dominated light, providing information about the time of day and sky conditions.9PubMed. Parietal-eye phototransduction components and their potential evolutionary implications
The parietal eye is not some evolutionary oddity limited to a few species. It was widespread among ancient vertebrates, and the parietal foramen through which it connects to the brain appears in the skulls of many fossil reptiles, amphibians, and even early relatives of mammals.
How Mammals Lost the Parietal Eye
If the parietal eye was once so common, why don’t mammals have one? The fossil record offers a compelling answer. Studies of skulls from Permo-Triassic therapsids, the group of animals that eventually gave rise to mammals, show that the parietal foramen became progressively smaller and eventually closed entirely in the lineages leading toward mammals and archosaurs (the group including dinosaurs, crocodilians, and birds).10Royal Society Open Science. The evolution of the ‘third eye’ illuminates shifting amniote ecologies through the Late Paleozoic and Triassic The disappearance of the parietal foramen correlates with the evolution of higher metabolic rates and possibly endothermy, the ability to generate internal body heat. An animal that controls its own body temperature has less need for a light-sensing organ dedicated to thermoregulation.
As the parietal foramen closed, the underlying pineal organ shifted from being a photoreceptor to functioning purely as a hormone gland. In modern mammals, the pineal gland produces melatonin and still helps regulate circadian rhythms, but it does so by receiving light information indirectly through the eyes and neural circuits rather than by sensing light itself.11Acta Palaeontologica Polonica. The Sixth Sense in Mammalian Forerunners: Variability of the Parietal Foramen and the Evolution of the Pineal Eye in South African Permo-Triassic Eutheriodont Therapsids The human pineal gland is, in a sense, the evolutionary remnant of what was once a third eye. It still responds to light cycles, still affects sleep and seasonal biology, but it no longer sees anything.
The size trends in the fossil record are not uniform across all lineages. While the ancestors of mammals and archosaurs lost their parietal foramen, some groups went in the opposite direction. Dicynodonts and marine reptiles actually developed dramatically larger parietal foramina before the end-Permian mass extinction, presumably because their ecological niches still rewarded having a light-sensing organ on top of the head.10Royal Society Open Science. The evolution of the ‘third eye’ illuminates shifting amniote ecologies through the Late Paleozoic and Triassic
Growing Eyes in Unexpected Places in the Lab
Some of the most striking evidence for how eyes form comes from experiments where researchers deliberately grew them in the wrong place. A gene called Pax6 acts as a master switch for eye development across an enormous range of animals, from insects to fish to mammals. When scientists forced Pax6 to activate in tissues that would normally become wings or legs in fruit flies, those tissues grew functional eye structures instead.12PubMed Central. Transcriptional activities of the Pax6 gene eyeless regulate tissue specificity of ectopic eye formation in Drosophila
The specifics reveal how deeply conserved this machinery is. Even Pax6 genes taken from completely different species, including squid, sea squirts, and mice, could trigger eye growth when placed into fruit fly tissues.13PubMed. Zebrafish contains two pax6 genes involved in eye development Related genes in the same family, like Pax2, could not do this, which means eye induction is not just a general property of this gene family but something specific to Pax6 and its closest relatives. The implication is that the genetic toolkit for building an eye is ancient and remarkably portable, at least in a laboratory setting. In nature, layers of regulatory control keep Pax6 from activating in the wrong tissues. But the fact that a single gene can redirect a wing precursor into becoming an eye shows that the boundary between “eye tissue” and “non-eye tissue” is maintained by signals, not by some fundamental incompatibility.
These experiments do not mean that activating Pax6 in a human would sprout a working eye on someone’s arm. Fruit fly eye development is simpler than vertebrate eye development, and growing an eye in its normal socket involves a cascade of additional signals, tissue interactions, and structural support that a random patch of skin cannot provide. But the experiments illustrate why developmental errors like diprosopus can sometimes produce recognizable eye structures: the genetic program for building an eye is powerful, and when it gets activated in roughly the right context, it can produce something that at least resembles one.
Birds and Hidden Light Sensors
Mammals may have traded their third eye for a hormone gland, but birds found yet another solution to the problem of sensing light through the skull. Many bird species have specialized neurons deep inside the brain, called deep brain photoreceptors, that respond directly to light passing through the skull and feathers. These neurons have been identified in at least four distinct brain regions, and they use photopigments including melanopsin, neuropsin, and vertebrate ancient opsin.14PubMed Central. Exploring avian deep-brain photoreceptors and their role in activating the neuroendocrine regulation of gonadal development
These photoreceptors play a key role in seasonal reproduction. As day length changes, light penetrating the skull stimulates these neurons, which in turn trigger hormonal cascades that activate the reproductive system. This is why poultry farmers manipulate lighting schedules to control egg production. The birds are not “seeing” with their brains in any conscious sense, but their brains are directly measuring ambient light levels without relying on the eyes at all. It is a third-eye function accomplished without a third eye.
Extra Eyes in Other Animals
Beyond the parietal eye in reptiles and the deep brain photoreceptors in birds, other animals have evolved multi-eye systems that make the question of a “third eye” seem modest. Spiders commonly have eight eyes arranged in different configurations optimized for different tasks, from detecting motion to gauging distance. Scallops line their mantles with dozens of small eyes, each with a mirror-based focusing system. Some species of shrimp possess an accessory eye structure called a nebenauge, though its function remains uncertain. This accessory eye appears commonly in free-living shrimp species but rarely in those that live as symbionts inside other animals.15PLOS ONE. Linking Eye Design with Host Symbiont Relationships in Pontoniine Shrimps (Crustacea, Decapoda, Palaemonidae)
These examples are not developmental accidents. They are stable, evolved features shaped by millions of years of natural selection. The difference between a spider’s eight functional eyes and a human diprosopus case is the difference between a feature the body was built to support and a duplication error the body has no plan for. In spiders, the brain expects eight channels of visual input and has circuits dedicated to processing each one. In a human infant with a third eye, the brain has no such wiring, which is the fundamental reason why even a well-formed supernumerary human eye is unlikely to produce useful vision.