Teeth started outside.

Then, Australia’s ABC Science tells us, they gradually moved inside our mouths:

The study, published in the Journal of Vertebrate Paleontology, supports what is known as the “outside-in” hypothesis of tooth evolution.

“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,” says researcher Dr Mark Wilson, a professor in the Department of Biological Sciences at the University of Alberta.

For the study, Wilson and colleagues studied animals called ischnacanthid acanthodians, an extinct group of fish that resembled sharks. They lived during the Early Devonian period, which lasted from 416 to 397 million years ago.

Must’ve been quite a lovely face.