Tooth enamel evolved from the skin of sharks, study finds

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A recent study suggests that the enamel coating our teeth hasn’t always been there – it evolved from the skin of ancient fish, like sharks.

Living organisms didn’t develop teeth for the first few billions of years. The enamel on the outside of our teeth protects them from damage and allows us to crush, dice, and chew a wide range of foods that would otherwise be impossible to digest on our own. But how did these structures evolve? According to a press release from Eurekalert, the origins of enamel can be traced back to some of the most prehistoric fish on the planet.

A recent study published in the journal Nature from researchers at Uppsala University in Sweden and the Institute of Vertebrate Paleontology in Beijing, China who have joined forces brings together data from both the paleontological and genomic fields. Together, the team found a surprising discovery about the evolution of teeth – they first began to develop under the skin, and made their way to the mouth millions of years later.

Enamel is the shiny white tissue that coats our teeth, which we work diligently to protect by brushing and flossing. It’s the hardest substance produced by the body, and is made up of the mineral apatite, or calcium phosphate. This is deposited onto a substrate of three different enamel matrix proteins.

Other land vertebrates only have teeth in the mouths, but some fish still have enamel elsewhere on their bodies. Many sharks have what scientists call “dermal denticles,” or tooth-like scales that coat the surface of their bodies. Other ancient fish that are still around today, like the gar from North America, are covered in scales that produce a tissue similar to enamel called ganoine. According to Tatjana Haitina, a researcher from the Department of Organismal Biology at Uppsala University, the gar’s genome contains genes for two out of the three enamel matrix proteins that make up teeth. These genes are expressed in the gar’s skin, suggesting that ganoine is very similar to enamel.

The answer to where enamel actually originated can be traced to two different fossil fish. Psarolepis from China and Andreolepis from Sweden. Both fish are more than 400 million years old, and have been studied by researchers Qingming Qu and Per Ahlberg from Uppsala University in conjunction with Min Zhu from IVPP in Beijing. Psarolepis had scales with denticles on its face that were covered in enamel, but it lacked teeth that had the substance as well. Andreolepis only had enamel in its scales.

They are among the earliest bony fishes, so researchers believe that they simply hadn’t had the evolutionary timeframe to develop enamel-covered teeth. The researchers are confident that enamel was first produced in the skin of fish and was eventually incorporated into teeth much later.

The study was unique in that it combined paleontological data with genetic analysis to solve a unique evolutionary question. The team hopes to continue its research on the development of hard tissues in vertebrates with this approach.

Sharks are some of the most fierce creatures around that still have enamel on their skin. Many shark attack survivors report being severely scraped by coming into contact with the giant fish’s denticles, which are often razor-sharp. Sharks can use these denticles to swim past prey and immobilize it with a swift scrape. Often, sharks and smaller fish engage in a symbiotic relationship, with schools of brave fish scraping off dead skin cells and scales by gently rubbing up against the skin of a shark.

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