Have you ever pondered why our teeth are so sensitive to pain, especially when consuming cold drinks? Recent research suggests that the answer lies in the evolutionary history of teeth, which were originally developed for a purpose vastly different from chewing. A groundbreaking study published on Wednesday sheds light on this fascinating aspect of dental evolution.
The exact origin of teeth has long been a puzzle for scientists. It is believed that the evolutionary precursors to modern teeth, known as odontodes, first emerged not in mouths but on the external armor of primitive fish approximately 500 million years ago. These ancient structures can still be seen today in the form of microscopic teeth on the skin of sharks, stingrays, and catfish, which give these animals a rough texture akin to sandpaper.
Several theories attempt to explain the initial appearance of odontodes. Some suggest that they provided protection against predators, facilitated movement through water, or served as storage for minerals. However, the new study supports a compelling hypothesis: that these structures were originally used as sensors that communicated sensations to surrounding nerves.
Yara Haridy, the lead author of the study and a postdoctoral researcher at the University of Chicago, initially set out to answer a different question in the field of paleontology: identifying the oldest fossil of a vertebrate animal. To achieve this, Haridy requested hundreds of vertebrate specimens from museums across the United States, some so minuscule they could fit on the tip of a toothpick. Her analysis focused on dentin, the inner layer of teeth that transmits sensory information to nerves.
During her research, Haridy examined a Cambrian period fossil known as Anatolepis, which seemed to hold the answer she was seeking. Its exoskeleton featured pores beneath the odontodes called tubules, suggesting they may have once contained dentin. This led some paleontologists to classify Anatolepis as the first known fish. However, upon comparing it with other scanned specimens, Haridy discovered that the tubules bore a closer resemblance to sensory organs called sensilla, found in arthropods like crustaceans and insects.
This revelation demoted Anatolepis to the status of an invertebrate. For contemporary arthropods such as crabs, scorpions, and spiders, sensilla are crucial for detecting temperature, vibration, and even smell. The remarkable stability of these features over time indicates that they have served the same functions for nearly half a billion years.
Researchers noted striking similarities between the features of Anatolepis and vertebrate fish dating back around 465 million years, as well as more familiar fish species. Haridy conducted experiments on modern fish, confirming the presence of nerves within the external teeth of catfish, sharks, and skates. This demonstrates that the tooth tissues of odontodes outside the mouth can possess sensitivity, implying that the earliest odontodes likely had similar capabilities.
The study's senior author, Neil Shubin, also from the University of Chicago, emphasized that these primitive animals evolved in a highly predatory environment. The ability to sense their surroundings was crucial for survival. Haridy elaborated that over millions of years, fish developed jaws, making it advantageous for some fish to have pointed structures near their mouths. Gradually, certain fish species began to possess pointed odontodes at the mouth's edge, eventually leading to their incorporation within the mouth itself.
A toothache, it seems, is not merely a modern affliction but an ancient sensory feature that may have played a vital role in the survival of our fish ancestors. This research not only enhances our understanding of tooth sensitivity but also provides a fascinating glimpse into the evolutionary history of vertebrates. As we continue to explore the origins of our teeth, we uncover the intricate connections between ancient life forms and the biological functions we experience today.
