A new study has surprisingly said that the world's first teeth grew outside of the mouth before eventually moving into the oral cavity.
Scales on the exterior of the prehistoric shark-resembling fish appear to have evolved into teeth, and these were retained among most vertebrates and were passed down to multiple species, including humans.
The study supports what is known as the "outside-in" hypothesis of tooth evolution, but negates "inside-out" theory, which holds that the first teeth emerged from structures in the pharynx progressing into the mouth.
The first teeth and smile were likely flashed by small and spiny shark-like fishes rather than the fictional Cheshire cat, who's smile seemed to have a life of its own.
"The first smile would probably have been a prickly one, with many tiny teeth that looked like pointy cheek scales, and other small tooth-like scales wrapping around the lips onto the outside of the head," Discovery News quoted Mark Wilson, co-author of the study as saying.
Stephanie Blais and her colleagues from the Department of Biological Sciences at the University of Alberta studied animals called ischnacanthid acanthodians, an extinct group of fish that resembled sharks.
The ischnacanthid acanthodians lived during the Early Devonian period, which lasted from 416 to 397 million years ago.
The researchers determined that head scales from these fish were in transition, evolving from scales to teeth, and the pointy structures were identified on the lips of the fish.
"Our findings support the idea that teeth evolved from modified pointed scales on the mouth margins lips as we see in Obtusacanthus," Blais said.
The study has been published in the Journal of Vertebrate Paleontology.