14 DAY TRIAL // In a paper entitled “Fossil jawless fish from China foreshadows early jawed vertebrate anatomy,” which is published in the latest issue of the top scientific journal Nature, experts say that the key to the evolutionary success of vertebrates was the complex reorganization of their brains and sense organs.
According to studies of a newly-found set of fossils, it would appear that these aspects played the most significant role in the rise to dominance vertebrate animals showed over millions of years of evolution.
The path they took was not an easy one, and it took them a long time to reach the top, but eventually the survival advantages their adaptations provided them far outweighed the costs associated with getting there.
However, the rise to prominence of vertebrates remained one of the most hotly-debated and mysterious aspects of evolutionary biology. The new study solved some of the problem, by studying the missing link between living jawless and jawed vertebrates.
The evolutionary intermediate was a 400-million-years-old jawless fish. Researchers were lucky enough to find the fossil in such a good shape that they could even analyze the animal's brain structure.
Scientists from the University of Bristol, in the UK, the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP), in China, the Museum national d’Histoire naturelle, in Paris, France and the Paul Scherrer Institut, in Switzerland, were a part of the work.
The primitive fossil jawless fish is called a galeaspid. The members of the international research team were primarily interested in analyzing the animal's head. In order to do that, they used the Swiss Light Source installation, which produces high-energy X-rays.
After battering the fossil with intense radiations, the team was able to determine the shape of the animal’s brain and sense organs, and then base an entire computer model on these data. The lead author of the study was ICPP/UB investigators Gai Zhi-kun.
“We were able to see the paths of all the veins, nerves and arteries that plumbed the brain of these amazing fossils. They had brains much like living sharks – but no jaws. We’ve been able to show that the brain of vertebrates was reorganized before the evolutionary origin of jaws,” the expert says.
“In the embryology of living vertebrates, jaws develop from stem cells that migrate forwards from the hindbrain, and down between the developing nostrils,” adds UB professor and study co-author Philip Donoghue.
“This does not and cannot happen in living jawless vertebrates because they have a single nasal organ that simply gets in the way,” says the investigator, who is based at the UB School of Earth Sciences.
Evolutionary biologists have been proposing this as the logical step in vertebrate evolution, but thus far they were unable to produce any real evidence to support this. The discovery of these new fossils provided the scientific community with all the data they needed on the subject.
French researcher Philippe Janvier says that – for the most part – this type of study was held back by the lack of specialized technologies capable of imaging individual structures inside solid rocks.
“We could not have done this work without this crazy collaboration between palaeontologists and physicists,” concludes Janvier, who is a professor at the Museum national d’Histoire naturelle, in Paris.
Funds for the new investigation were provided by the Royal Society, the Natural Environment Research Council, the Chinese Academy of Sciences, the Chinese Foundation of Natural Sciences, the EU Framework Program 7, and the Paul Scherrer Institut.