How Did You Get Your Face? Scientists Use Fish Fossils To Explain How Jaws Evolved
The human face, as we see it today, has evolved quite a bit. Scientists in France and Sweden used fossilized fish to explain how humans and other vertebrates evolved from being jaw-less to jawed, according to a press release released Wednesday.
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Vertebrates, which are essentially animals with backbones, can be classified as Gnathostomes and Ostracoderms. About 99 percent of all living vertebrates, including humans, are Gnathostomes, meaning they have jaws. Ostracoderms are jawless fish, and presently comprise of only two species, lampreys and hagfishes.
So, how does being a jawed vertebrate change your facial structure? In jawless vertebrates, the brain is surrounded on either side by blocks of tissue. These tissues meet in front of the brain to create a big upper lip, which surrounds a single midline nostril between the eyes.
Cranial structure in jawed vertebrates is different because the tissues that surround the brain grow forward in the midline under the brain. The tissue pushes between the left and right nasal sacs that open separately to the outside, thus forming two nostrils. Furthermore, the front part of the brain is much longer in jawed vertebrates, which causes the nose to be positioned at the front of the face rather than between the eyes.
Jawless vertebrates pre-dated jawed ones by about 420 million years. Somewhere along the evolutionary cycle, these jawless fish developed jaws and appeared as the placoderm fish (that are now extinct), like the Romundina. The fossilized remains of this species, found in arctic Canada, now lies in the French National Natural History Museum in Paris.
Scientists examined the skull of the Romundina at the European Synchrotron (ESRF) in Grenoble, France and used high energy X-rays to create a virtual representation of the internal structures of the skull. They found separate left and right nostrils, a characteristic of jawed vertebrates, but they were positioned far back behind an upper lip, which is reminiscent of a jawless vertebrate. Researchers also concluded that the Romundina's brain would have had 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, one of the main contributors to this study.
By studying Romundina and other fossilized fishes in the order of their evolution, the scientists concluded that organization of the major tissue blocks was the first step in the evolution to jawed vertebrates followed by changes to the head and other structures.
Source: Dupret, V. Sanchez, S. A primitive placoderm sheds light on the origin of the jawed vertebrate face. Macmillan Publishers Limited. 2014.
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