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The Body, et cetera

“Blink and you’ll miss it”: A Third Eyelid?

By Rachel Ko

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Evolution is built on logic - so where does a Third Eyelid fit in? This article unpacks the fascinating piece of evidence for evolution reflected within our very own eyes, known as the plica semilunaris, that connects us to our reptilian ancestors.

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Edited by Ashleigh Hallinan and Yvette Marris

Illustrated by Rachel Ko

The creature snarls a deep, thundering growl, tensing its protruding muscles that are covered in layers of thick, green, armour-like scales, individually rattling by the sheer force of its stance. Clenching its claws, the lizard glares with a bizarrely human expression, a villain trapped in a peculiar hybrid humanoid form. As the screams of terrified students fill the air, the camera zooms into the mutant’s glistening yellow eye, and it blinks; a slimy, translucent covering flickers across its eyeball, leaving a trail of moisture - grotesque proof of its reptilian form.

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A charm of the cinematic world is that aliens, radioactive spider superheroes and giant mutant lizards can exist in the same universe as the regular person. On a recent movie night, watching The Amazing Spiderman, the villain Lizard caught my eye. The creature is a metamorphosed version of human scientist Dr Curt Connors, who had attempted cross-species genetic regeneration on himself. Largely CGI, the Lizard’s primitive no-frills characterisation makes him an unconventional superhero antagonist. However, upon focus, these exaggerated reptilian characteristics are wha become staples of the Lizard’s uniquely villainous appeal: the alien-green colouring, the razor-sharp claws, the terrifying teeth and, of course, the glistening yellow eyes.  

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Figure 1: Spiderman's 'The Lizard'

In reference to the creation of these eerie eyeballs, animation supervisor David Schaub confirmed the purposeful inclusion of a nictitating membrane (1). This membrane is a slimy skin-like covering more commonly known as the Third Eyelid. In animals such as birds, reptiles, fish, amphibians, and some mammals (2), it acts as a bizarre protective mechanism that maintains moisture while retaining vision (3) - and also gives the Lizard’s glare that extra kick.

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Acting like a windscreen wiper, the membrane ‘nictitates’, meaning it blinks, to keep debris and dust out of the eye while simultaneously hydrating it. Its transparency also allows vision underground or underwater (4).

Figure 2: A bird blinking!

There is just one primate species known to have a prominent nictitating membrane: the Calabar angwantibo, also known as the golden potto, which is a rare African prosimian primate found only in Cameroon and Nigeria (5).

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Figure 3: Look at the Calabar's nictating membrane!

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The membrane is a major characterising feature of The Amazing Spiderman’s creepy mutant reptilian aura. However, this Third Eyelid actually has a homologous counterpart in Dr Connors’ eyes too. In fact, it is found in all humans, and is known as our plica semilunaris, a vertical fold of conjunctiva in the inner corner of the eye (6).

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Although it plays a minor role in eye movement and tear drainage (7), the plica semilunaris has nowhere near as great a function in humans as the nictitating membrane does in animals (8). The plica semilunaris and its associated muscles are merely an evolutionary remnant of the nictitating membrane that existed in our reptilian ancestors millions of years ago (9).

Evolution is driven by selective advantage: the traits that allow organisms to survive and reproduce are the ones that are selected for and thrive within the population, passed down from one generation to the next (10). Traits that are disadvantageous to organisms decrease their chance of survival and reproduction, meaning fewer offspring will inherit the trait, causing it to eventually disappear from the population (11). The mystery remains as to why human ancestors lost the nictitating membrane in the first place, but it is likely that changes in habitat and lifestyle regarding eye physiology made it selectively advantageous to lose the Third Eyelid, rather than wasting precious energy on maintaining a no-longer-vital mechanism (12).

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For some reason, though, once the nictitating membrane had evolved into nothing more than a miniscule pink fold in the corner of the eye, it still persisted.

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Some argue that this is because humans have had no evolutionary incentive to completely lose them (13) – the plica semilunaris is just harmless enough that it has flown under the radar of evolution’s cut. Having suggested that, however, the primary clinical significance of the plica semilunaris has been connected to allergies of the eye, in which release of inflammatory molecules like histamine causes the tissue to become swollen and itchy (14).

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Thus, it is worth considering another argument: that the persistence of the plica semilunaris may be indicative of some beneficial function, particularly in its role in human eye protection. It has been found that the tissue observed in early intrauterine (within the uterus) development has a dense infiltration of immune cells like macrophages and granulocytes that serve to engulf and destroy foreign invaders of the tissue (15). Along with the abundance of blood vessels and immune chemical signalling, this has suggested a specialised role in eye protection, a benefit that may have very well ensured the plica semilunaris’ survival within human populations until this day (16).

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One fascinating clinical case, which showcases the outlandish capabilities of this vestigial feature, is of a child for whom it was not a question of why the plica semilunaris persisted, but an actual nictitating membrane. This peculiar instance was presented on a 9 year-old girl whose left eye had a non-progressive translucent membrane covering it horizontally. The globe of the eye was able to move freely beneath the membrane, suggesting that there was no attachment. However, it was causing amblyopia (also known as a lazy eye), and poor vision, so the nictitating membrane was successfully removed by simple excision (17). 

Figure 4: The plica semilunaris

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Figure 5: A clinical case of a human nictating membrane

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The only other recorded case of persisting nictitating membrane was an infant boy born prematurely with Edwards syndrome, who had nictitating membranes in both eyes (18). However, due to the baby’s infancy and condition, membrane imaging was unobtainable. Thus, arguably, the most striking aspect of the 9 year-old girl’s case was the pre-procedure imaging of her eye: an intriguing, almost alien-like fusion of the human eye and that of our reptilian ancestors.

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This case study can be interpreted as an exaggerated example of an existing link between the nictitating membranes we see in animals today, and the plica semilunaris that exists, tucked away, in the corner of our very own eyes.

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So, next time you find yourself staring into your partner’s baby blues, or putting on eyeliner in the mirror, keep an eye out for this fascinating evolutionary remnant; but be quick because
- blink and you’ll miss it.

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References

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  1. Sarto D. 'Spider-Man'’s Lizard Part 1: The Animation [Internet]. Animation World Network. 2012 [cited 4 May 2022]. Available from: https://www.awn.com/vfxworld/spider-mans-lizard-part-1-animation
     

  2. Butler A, Hodos W. Comparative vertebrate neuroanatomy. Hoboken (New Jersey): Wiley-Interscience; 2005.
     

  3. Why do cats have an inner eyelid as well as outer ones? [Internet]. Scientific American. 2006 [cited 4 May 2022]. Available from: https://www.scientificamerican.com/article/why-do-cats-have-an-inner/
     

  4. The Equine Manual [Internet]. Elsevier; 2006. Available from: http://dx.doi.org/10.1016/B978-0-7020-2769-7.X5001-1
     

  5. Montagna W, Machida H, Perkins EM. The skin of primates. XXXIII. The skin of the angwantibo (Arctocebus calabarensis) [Internet]. Vol. 25, American Journal of Physical Anthropology. Wiley; 1966. p. 277–90. Available from: http://dx.doi.org/10.1002/ajpa.1330250307
     

  6. Plica semilunaris [Internet]. Merriam-Webster.com medical dictionary. [cited 4 May 2022].
    Available from: https://www.merriam-webster.com/medical/plica%20semilunaris
     

  7. LaFee S. Body and Whole [Internet]. UC Health - UC San Diego. 2016 [cited 4 May 2022]. Available from: https://health.ucsd.edu/news/features/pages/2016-06-30-listicle-body-and-whole.aspx
     

  8. Dartt D. Foundation Volume2, Chapter 2. The Conjunctiva–Structure and Function [Internet]. Oculist.net. 2006 [cited 4 May 2022]. Available from: http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v8/v8c002.html
     

  9. Gonzalez R. 10 Vestigial Traits You Didn't Know You Had [Internet]. Gizmodo. 2011 [cited 4 May 2022]. Available from: https://gizmodo.com/10-vestigial-traits-you-didnt-know-you-had-5829687
     

  10. Sukhodolets V. V. (1986). K voprosu o roli estestvennogo otbora v évoliutsii [The role of natural selection in evolution]. Genetika, 22(2), 181–193.
     

  11. Sukhodolets V. V. (1986). K voprosu o roli estestvennogo otbora v évoliutsii [The role of natural selection in evolution]. Genetika, 22(2), 181–193.
     

  12. Gonzalez R. 10 Vestigial Traits You Didn't Know You Had [Internet]. Gizmodo. 2011 [cited 4 May 2022]. Available from: https://gizmodo.com/10-vestigial-traits-you-didnt-know-you-had-5829687
     

  13. Kotecki P, Olito F. We No Longer Need These 9 Body Parts [Internet]. ScienceAlert. 2019 [cited 4 May 2022]. Available from: https://www.sciencealert.com/we-no-longer-need-these-9-body-parts
     

  14. Bielory L, Friedlaender MH. Allergic Conjunctivitis [Internet]. Vol. 28, Immunology and Allergy Clinics of North America. Elsevier BV; 2008. p. 43–58. Available from: http://dx.doi.org/10.1016/j.iac.2007.12.005
     

  15. Arends G, Schramm U. The structure of the human semilunar plica at different stages of its development a morphological and morphometric study [Internet]. Vol. 186, Annals of Anatomy - Anatomischer Anzeiger. Elsevier BV; 2004. p. 195–207. Available from: http://dx.doi.org/10.1016/S0940-9602(04)80002-5
     

  16. Arends G, Schramm U. The structure of the human semilunar plica at different stages of its development a morphological and morphometric study [Internet]. Vol. 186, Annals of Anatomy - Anatomischer Anzeiger. Elsevier BV; 2004. p. 195–207. Available from: http://dx.doi.org/10.1016/S0940-9602(04)80002-5
     

  17. Vokuda H, Heralgi M, Thallangady A, Venkatachalam K. Persistent unilateral nictitating membrane in a 9-year-old girl: A rare case report [Internet]. Vol. 65, Indian Journal of Ophthalmology. Medknow; 2017. p. 253. Available from: http://dx.doi.org/10.4103/ijo.IJO_436_15
     

  18. García-Castro JM, Carlota Reyes de Torres L. Nictitating Membrane in Trisomy 18 Syndrome [Internet]. Vol. 80, American Journal of Ophthalmology. Elsevier BV; 1975. p. 550–1. Available from: http://dx.doi.org/10.1016/0002-9394(75)90228-7

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Images

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Figure 1: Galloway, R. (2022, January 25). Lizard originally had a different look in 'Spider-Man: No way home'. We Got This Covered. Retrieved August 9, 2022, from https://wegotthiscovered.com/movies/lizard-originally-had-a-different-look-in-spider-man-no-way-home/

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Figure 2: Hudson T. (2010, July) Retrieved Sep 13, 2022, from https://en.wikipedia.org/wiki/Nictitating_membrane#/media/File:Bir

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Figure 3: Sharma R. Calabar angwantibo - Alchetron, The Free Social Encyclopedia [Internet]. Alchetron.com. 2018 [cited 7 May 2022]. Available from: https://alchetron.com/Calabar-angwantibo

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Figure 4: Amir, D. (2019, January 16). Twitter. Retrieved August 9, 2022, from https://twitter.com/dorsaamir/status/1085557444196

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