By Will Dunham
WASHINGTON, Feb 12 (Reuters) - Let's face it. It's easy to take for granted that mammals, birds, reptiles, amphibians and fish - vertebrates just like people - have a face. But it has not always been the case.
The first creatures with a backbone - jawless fish from hundreds of millions of years ago - did not. Scientists have been eager to learn how the evolution of the face unfolded.
A small, primitive armored fish known as Romundina that swam the seas 415 million years ago and whose fossilized remains were unearthed in the Canadian Arctic is providing some revealing answers.
With Romundina at the center of their work, Swedish and French researchers described in a study published in the journal Nature on Wednesday the step-by-step development of the face as jawless vertebrates evolved into creatures with jaws.
The evolution of the jaw led to development of the face.
The researchers scanned the internal structures of Romundina's skull using high-energy X-rays at the European Synchrotron (ESRF) in France, then digitally reconstructed the anatomy in three dimensions.
Romundina, one of the earliest jawed fish, was found to boast a mix of primitive features seen in jawless fish and more modern ones that appear in fish with jaws. Its head had a distinctive anatomy, with a very short forebrain and an odd "upper lip" extending forward in front of the nose, they said.
Romundina was a type of fish called a placoderm that thrived during the Silurian and Devonian periods in Earth's history but disappeared about 360 million years ago. It was small, but some placoderms like the fearsome Dunkleosteus became apex predators bigger than a great white shark.
Per Ahlberg, an expert in vertebrate evolution at Uppsala University in Sweden, said Romundina was roughly 8 inches (20 cm) long, had a small defensive spine on its back and had jaws without real teeth but with flat crushing plates.
Its front end was encased in armor, while its back end was flexible, with fins and a shark-like tail, Ahlberg said. It may have hunted small invertebrates like worms and crustaceans.
While the very first vertebrates were jawless, the only ones left are lampreys and hagfishes.
"The face is one of the most important and emotionally significant parts of our anatomy, so it is interesting to understand how it came into being," Ahlberg said by email.
By comparing Romundina with other creatures, the researchers determined that the transition from jawless to jawed vertebrate with a face occurred in three major steps.
In the first step represented by Romundina, jaws evolved while a single nostril structure that reached under the brain in jawless vertebrates was replaced by a solid floor under the brain and separate left and right nostrils opening on the face.
The forebrain remained extremely short in Romundina with the nose located right between the eyes and the skull extended in front of it like a big bony "upper lip."
In the second step represented by more advanced armored fish, the "upper lip" shrank to nothing, leaving the nose at the front of the face immediately above the upper jaw, but with the forebrain remaining short. In the final step as seen in modern jawed vertebrates, the forebrain and face lengthen.
"When you look at Romundina, it's like looking at yourself in the mirror, but with a 415 million-year-old image," Vincent Dupret of Uppsala, another of the researchers, said in a telephone interview. "It's like in a science-fiction movie. You look at the mirror, but it's not you. It's your ancestor."
The study was a collaboration among researchers at Uppsala, the National Museum of Natural History in Paris, and the European Synchrotron Radiation Facility in Grenoble.
(Reporting by Will Dunham; Editing by Cynthia Osterman)