Turtles, birds and crocodylians have genetic remnants of a ‘third eye’

Many lizards have a so-called ‘third eye’, more formerly known as a parietal or pineal eye, which is located smack dab in the middle of their heads.

 

If you dissect a parietal eye, you’ll see that it very much resembles a normal eye, with structures similar to a cornea, lens and retina, as well as a nerve projecting from the light-sensitive retinal cells.

tuatara-parietal-eye-e1407353354308

Just like our normal (‘lateral’) eyes, these structures absorb light and translate it into electrical signals, which are then sent to the brain, communicating information about the abundance and composition of light in the environment. Though the function of the parietal eye is not fully agreed upon, it appears that it at least helps lizards to know how long they need to stay in the sun to warm up, since removing it or blocking it from sunlight leads to lizards staying out the sun longer than normal.

basking_lizard_by_raeyenirael_stock
Basking monitor lizard

While this third eye is only found in lizards and the closely-related tuatara, there is evidence in the fossil record that it was formerly much more widespread. If you look at the skulls of many types of fossil reptiles, you’ll find a little hole at the top of their skulls, just like in modern day lizards that have a parietal eye. What’s also interesting is that some fossil species that show similarities to birds, crocodylians and turtles also have this hole in their skulls, despite the fact that these modern birds, crocs and turtles do not have a parietal eye.

image_3197_2e-Eunotosaurus-africanus
Sequence of fossils from primitive reptiles to the earliest turtles

For example, the image above is a phylogenetic hypothesis showing a sequence of early reptiles that all have a hole at the top of their skulls (parietal foramen) until the appearance of some of the earliest turtles (Proganochelys).

Recent research has isolated a couple of proteins that are expressed in the parietal eyes of lizards. These proteins, parietopsin and parapinopsin, function as pigments that absorb light in the parietal eye [1].

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Iguana parietal eye with parapinopsin (magenta) and parietopsin (green) staining [1]
In a recent study [2], I found both the parapinopsin and parietopsin genes in birds, crocodylians and turtles, but they are full of mutations that prevent the formation of functional proteins. This is consistent with the idea that the ancestors of birds, crocodylians and turtles had parietal eyes with functional parietopsin and parapinopsin, but ultimately lost the need for them. The reasons for the loss of this organ are a bit mysterious, but the consistent story told by both genetics and the fossil record makes a convincing argument for these birds, crocs and turtles having evolved from animals with a third eye.

Questions for Creationists

Why did the Creator create birds, crocodylians, and turtles with remnants of genes that are found in the ‘third eyes’ of lizards? Is it just a coincidence that fossils of animals that look similar to these species had holes in their skulls that correspond with the parietal eye of lizards?

References

1. Wada, S., Kawano-Yamashita, E., Koyanagi, M., & Terakita, A. (2012). Expression of UV-sensitive parapinopsin in the iguana parietal eyes and its implication in UV-sensitivity in vertebrate pineal-related organs. PLoS One7(6), e39003.

2. Emerling, C. A. (2017). Archelosaurian Color Vision, Parietal Eye Loss, and the Crocodylian Nocturnal Bottleneck. Molecular biology and evolution34(3), 666-676.

Photo credit

Parietal eye 1, parietal eye 2, parietal eye 3, pre-turtle skullsbasking lizard

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