Researchers have been trying to work out why humans appear to be the only mammals with chins and now think it may be down to the fact that humans started cooking food.

Dr James Pampush, from the Department of Anthropology at the University of Florida, says he couldn't link it with any natural selection and thinks it is connected to teeth shrinking as our early ancestors started cooking food and large teeth were no longer needed.

"It's very strange that not only does every human have a chin, but, we're also the only things that do," he said.