A 3D reconstruction of a 410-million-year-old fish skull has shed light into how vertebrates, including humans, came to have a face, scientists say.
A team of French and Swedish researchers used micron resolution X-ray imaging to show how a series of fossils documents the step-by-step assembly of the face during the evolutionary transition from jawless to jawed vertebrates.
Vertebrates, or backboned animals, come in two basic models: jawless and jawed. Today, the only jawless vertebrates are lampreys and hagfishes, whereas jawed vertebrates number more than 50,000 species, including humans.
It is known that jawed vertebrates evolved from jawless ones, a dramatic anatomical transformation 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.
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, researchers said.
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.
Until now, very little has been known about the intermediate steps of this strange transformation. The scientists studied the skull of Romundina, an early armoured fish with jaws, or placoderm, from arctic Canada.
The skull is part of a collection of the French National Natural History Museum in Paris.
Romundina has separate left and right nostrils, but they sit far back, behind an upper lip like that of a jawless vertebrate.
"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.
By imaging the internal structure of the skull using high-energy X-rays at the European Synchrotron (ESRF) in Grenoble, France, researchers showed that the skull housed a brain with a short front end, very similar to that of a jawless vertebrate.
"In effect, Romundina has the construction of a jawed vertebrate but the proportions of a jawless one," said Per Ahlberg, of Uppsala University and the other lead author of the study.
"This shows us that the organisation of the major tissue blocks was the first thing to change, and that the shape of the head caught up afterwards," he added.
The research is published in journal Nature.
