Extinct Four-Eyed Monitor Lizard Busts Myth of a Congruent Nested Hierarchy

One of the most essential doctrines of Darwinian evolution, apart from universal common descent with modification, is the notion that complex similarities indicate homology and are ordered in a congruent nested pattern that facilitates the hierarchical classification of life. When this pattern is disrupted by incongruent evidence, such conflicting evidence is readily explained away as homoplasies with ad hoc explanations like underlying apomorphies (parallelisms), secondary reductions, evolutionary convergences, long branch attraction, and incomplete lineage sorting.

When I studied in the 1980s at the University of Tübingen, where the founder of phylogenetic systematics, Professor Willi Hennig, was teaching a first generation of cladists, we still all thought that such homoplasies are the exceptions to the rule, usually restricted to simple or poorly known characters. Since then the situation has profoundly changed. Homoplasy is now recognized as a ubiquitous phenomenon (e.g., eyes evolved 45 times independently, and bioluminiscence 27 times; hundreds of more examples can be found at Cambridge University’s “Map of Life” website).

Life’s Solution

This state of affairs compelled George McGhee, a paleobiology professor at Rutgers University, to write a book, Convergent Evolution: Limited Forms Most Beautiful (2011). He suggests that convergence is so common because viable forms are so limited. However, he fails to explain how evolution manages to find these limited solutions over and over again through a random search process. After all, selection only explains the survival of the fittest but not the arrival of the fittest.

Likewise, paleontologist Simon Conway Morris wrote two books, Life’s Solution (2003) and The Runes of Evolution (2015), in which he concluded that the incredible number of convergences came to be because evolution is not the contingent process postulated by Stephen Jay Gould (1989) in his book Wonderful Life. Gould thought that if we could somehow rewind the tape of evolution, everything would develop very differently. According to Conway Morris, the same novelties occur so often in unrelated groups that this suggests these novelties are not products of mere contingency but instead are so constrained by external factors that rewinding the tape of evolution would lead to very similar results (also see Conway Morris 2009).

Of course, Darwinists are not comfortable with the deeper implications of a non-contingent process of evolution (Ruse 2004, Coyne 2012, 2015), which smacks of being designed for a purpose. Apart from that, most biologists do not even read between the lines that this is basically a surrender of a fundamental paradigm of Darwinism, which claimed that similar biological novelties suggest phylogenetic relationship (common ancestry).

The problem gets worse the more we learn about the fossil record, the distribution of characters among recent organisms, and the genetic and developmental underpinnings of many characters. Some taxonomists had hoped that genomics might save biosystematics from the evil of homoplasy, since the sheer amount of data would flood the “minor” noise of homoplasies. But this turned out to be a pipe dream as the numerous conflicting molecular phylogenies easily document. Even those genetic characters that were believed to be impossible to suffer from convergence, like transposable elements, turned out to be incongruent (e.g., in the case of the gorilla, chimp, and human trichotomy), which required a whole new epicycle of ad hoc explanation in terms of incomplete lineage sorting.

The Third Eye

The third eye of vertebrates provides a perfect illustration of this point, topped by a very surprising recent discovery reviewed below. What, a third eye? I am neither talking about New Age spiritualism, nor about the cyclops of ancient Greek mythology, but about a little known light-sensing organ.

Unpaired median dorsal eyes, along with a lateral pair of more efficient eyes, are known from crustaceans (nauplius eye), arthropods (e.g., 3 ocelli in insects), and vertebrates (third eye, pineal eye, or parietal eye on the top of the skull). The latter is always smaller than the paired lateral lens eyes, and in living species very inconspicuous. Evolutionists believe this organ to be possibly homologous to the light-sensitive spot on top of the head of lancelets, and the median eye of tunicate larvae, because phylogenetic studies suggested that tunicates are the closest relatives of vertebrates, which are sometimes supposed to have originated from a kind of neotenic tunicate larva.


All vertebrate eyes, paired lateral as well as unpaired median ones, develop from an evagination of the brain (diencephalon). The posterior part of the diencephalon (epithalamus) develops an initially single dorsal evagination (pineal complex), which then divides into two roughly bilaterally symmetric organs that rotate their location to become a caudal pineal organ (pineal gland) and a rostral parapineal organ (Kolb et al. 1998). These often retain a slight asymmetry with the pineal organ originating more right and the parapineal organ more left of the brain midline.

This corresponds with the fact that modern lampreys possess two median eyes that are either oriented on top of each other or behind each other, while some Devonian fish (e.g., arthrodira, stegoselachians, and very early lungfish) had two pineal/parietal foramina in the skull beside each other (Eakin 1973: 16-17). In modern aquatic jawed vertebrates (“fish” in everyday language), the third eye, if developed at all, is formed by the pineal organ, while the parapineal organ is more or less reduced.

In tetrapods, the caudal pineal organ is atrophied as a still photoreceptive pineal gland (epiphysis), while the rostral parapineal organ forms the third eye called the parietal eye. Among recent vertebrates the parietal eye is absent in salamanders, turtles, crocs, birds, and mammals, but very well developed in lepidosaurs (juvenile tuataras and many lizards) with a lens, cornea, and an everted retina, with the latter being more similar to that of an octopus rather than to the inverted retina of the lateral lens eyes.



In juvenile frogs and toads a similar, but less sophisticated third eye develops as a terminal vesicle of the parapineal organ.


These third eyes in vertebrates do not allow image-like vision but only differentiate light from dark. They may help in detecting shadows from predators attacking from above, as suggested by the behavior of some lizards. More importantly they are crucial for circadian and seasonal rhythms. This also happens to be the function of the pineal gland in humans, which produces the sleep hormone melatonin. Many other vertebrate species have an intracranial pineal organ as deep-brain photoreceptor. In fossil vertebrates, the possession of an extracranial third eye can be inferred from the presence of a parietal foramen between the parietal bones of the skull.

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Source: Evolution News