Phylogenetic relics are organs or species that have lost their original function but still exist due to lack of selective pressure. Examples include the human appendix and the pineal gland, which was once thought to be a “seat of the soul.” Some species, like the cycads and coelacanth, are also considered phylogenetic relics. The vampire squid and tuatara are examples of organisms with unique features that suggest they are phylogenetic relics. Many other organisms have been claimed as phylogenetic relics, including Neoglyphea neocaledonica.
A phylogenetic relic is a bodily organ that has lost its true use and still exists only because there is no particularly strong selective pressure against its existence. A common example in humans is the appendix. The appendix is thought to be a phylogenetic relic of a bodily organ found in many herbivores called the caecum, which contains special bacteria for digesting cellulose, the hard material in plants. The human appendix lacks these bacteria, so people cannot digest cellulose.
The pineal gland was once thought to be a phylogenetic relic, until the 1960s when it was discovered that it produces melatonin, a hormone that modulates circadian rhythms. The uniqueness and mystery of the pineal gland led Rene Descartes to call it the “seat of the soul,” a bodily organ that connected humans to God, although we haven’t seen this theory advanced in biology journals lately.
The term phylogenetic relic is also sometimes used to describe species that have no contemporary cousin species and have more in common with species that lived hundreds of millions of years ago than with those today. These include cycads, the horseshoe crab and the famous coelacanth, originally thought to be extinct for tens of millions of years, caught off the coast of Africa in 1938. These species have changed little over hundreds of millions of years. Phylogenetic relics are sometimes called living fossils.
The vampire squid, or Vampyroteuthis infernalis, which means “vampire squid from hell,” is a small, velvety black organism that lives in the depths of the ocean. It is neither a squid nor an octopus, but a phylogenetic relic of a largely extinct phylum. The tuatara, a type of reptile, has a highly developed pineal gland, with a median pineal eye, paraphyses and a pineal sac, in great contrast to our homogeneous pineal gland, suggesting that the pineal gland is indeed a relic-part despite the its small hormonal role in humans. Evidence such as these allows us to evaluate whether or not a particular organ or body species is a phylogenetic relic.
At one point, biologists claimed various sharks, rodents, crustaceans, plants, and many others as phylogenetic relics. One crustacean, Neoglyphea neocaledonica, looks like a bizarre cross between a shrimp and a lobster. There are many other interesting living fossils you can find if you do an internet search for the term.
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