Ancient Fish was First to Have Two Nostrils, Leading the Way for Vertebrates with Faces

The evolution of an ancient fish offers insight into how vertebrates, including humans, came to have a face.

A team of European researchers have presented new fossil evidence on the origins of the face by using micron resolution X-ray imaging to show how a series of fossils documents the step-by-step evolution of the face during the evolutionary transition from jawless to jawed vertebrates.

The fish at the center of the research is a 410 million-year-old armored fish called Romundina. For scientists trying to better understand how vertebrates came to evolve a face, Romundina plays an important role because of its evolutionary position as a transitional species.

Today, most vertebrates are jawed; the only jawless vertebrates are hagfishes and lampreys. More than 50,000 species, including humans, are considered jawed vertebrates. But jawed vertebrates evolved from their jawless counterparts, which the researchers likened to a process that effectively turned the face inside out.

"In embryos of jawless vertebrates, blocks of tissue grow forward on either side of the brain, meeting in the midline at the front to create a big upper lip surrounding a single midline 'nostril' that lies just in front of the eyes," the researchers said in a statement. "In jawed vertebrates, this same tissue grows forward in the midline under the brain, pushing between the left and right nasal sacs which open separately to the outside. This is why our face has two nostrils rather than a single big hole in the middle. The front part of the brain is also much longer in jawed vertebrates, with the result that our nose is positioned at the front of the face rather than far back between our eyes."

The Romundina represents an intermediary phase in this facial evolution. It has separate right and left nostrils, but they sit back behind its lips, like a jawless vertebrate's.

"This skull is a mix of primitive and modern features, making it an invaluable intermediate fossil between jawless and jawed vertebrates," said Vincent Dupret of Uppsala University, one of two lead authors of the study, which is published in the journal Nature.

Romundina's brain had a short front end, much like a jawless vertebrate, the researchers found though high-resolution X-ray images of the skull.

"In effect, Romundina has the construction of a jawed vertebrate but the proportions of a jawless one," said Uppsala University's Per Ahlberg, the study's other lead author. "This shows us that the organization of the major tissue blocks was the first thing to change, and that the shape of the head caught up afterwards," he said, referring to the evolution of vertebrates with a face.