Origin of Human Teeth Traced to Primitive Prehistoric Fish

A tropical reef in the Czech Republic, 409 million years ago: Radotina, one of the most primitive jawed vertebrates with teeth, emerges from its hiding place in the empty shell of a giant nautiloid to hunt for food. (Jan Sovak)

(CN) — The origin of our teeth has been traced back to over 400 million years ago when a mysterious species of fish first developed their prey-catching jaws, and in a study released Thursday, scientists report they have located and studied this creature in never-before-seen detail.

In the study, published in the journal Science, scientists from Uppsala University in Sweden led a collaborative, international effort with the European Synchrotron Radiation Facility (ESRF) in France where they “digitally dissected” the oldest known jawed fish fossil to date. The fossil was recovered near Prague and it exhibits surprisingly modern dental features.

Humans, as well as all other living jawed vertebrates like sharks, reptiles and mammals, are all descended from the first jawed fish. Today, we see distinct patterns in this area, like new teeth developing behind old teeth, or underneath inside the jawbone in humans.

The scientists noted some distinct differences between these ancient bony fish and sharks, including the shark’s lack of bones in favor of cartilage and the fact that their teeth and dentine scales sit in the skin rather than attach to the skeleton.

Bony fish on the other hand, as well as their land-dwelling ancestors, have teeth that are attached to the jaw bones. Also, the bony fish and land animals shed teeth by dissolving the bases, while sharks detach their teeth from the skin once they become worn out.

Researchers wondered what these variations meant in relation to the origin of modern teeth. Previously, there was a large focus on arthrodires, an ancient species of fish that lived 360-430 million years ago, as they were the only known stem-jawed vertebrate with teeth. This way of thought was flawed, however, because of the massive differentiation in the position of their teeth and the development of new teeth when compared to bony fish and sharks.

The team of scientists from Uppsala University as well as Charles University in the Czech Republic, the Natural History Museum in London and the National Museum in Prague then looked to another earlier fish group called the acanthothoracids. This group is much older and likely more closely related to the first jawed vertebrates, but their fossils are hard to come by — especially in adequate condition for study.

The best and most complete acanthothoracid fossils are known to come from the Prague Basin in the Czech Republic, as rocks there can be over 400 million years old. Some fossils of these origins were recently recovered from the site but have been difficult to analyze in the desired detail as they are irreparably encased in rock.

To solve this obstacle, the researchers sought to use the ESRF, the world’s brightest X-ray source, to analyze the fossils in 3-D. The high-powered X-ray beams penetrated the surface and scanned the embedded material without damaging the matter.

Further scans of the fossils showed dental growth patterns and cell spaces within the dentine of the teeth.

The fossil skull of Radotina in front view (left), left side view (right, top) and right side view (right, bottom). (Vít Lukáš / National Museum, Prague)

“The results were truly remarkable, including well-preserved dentitions that nobody expected to be there,” said Valéria Vaškaninová, lead author of the study and scientist from Uppsala University.

Similar to the arthrodires, the acanthothoracid teeth are attached to their bones, indicating that bony fish and land animals inherited this trait while sharks did not. They are dissimilar, however, in the way that acanthothoracids added new teeth on the inside and the oldest teeth sat at the jaw margin, giving them a surprisingly modern take.

“To our surprise, the teeth perfectly matched our expectations of a common ancestral dentition for cartilaginous and bony vertebrates.” Vaškaninová said.

Furthermore, the tooth-bearing bones also contained non-biting dentine elements of the skin on their surface, similar to bony fish but not arthrodires. This small detail is significant because it highlights the distinct difference that acanthothoracid jaw bones sat at the edges of the mouth and arthrodire jaw bones were further embedded.

“These findings change our whole understanding of the origin of teeth” said co-author Per Ahlberg, professor at Uppsala University.

“Even though acanthothoracids are among the most primitive of all jawed vertebrates, their teeth are in some ways far more like modern ones than arthrodire dentitions. Their jawbones resemble those of bony fish and seem to be directly ancestral to our own. When you grin at the bathroom mirror in the morning, the teeth that grin back at you can trace their origins right back to the first jawed vertebrates.”

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