In evolutional developmental biology, inversion is a hypothesis that states that during the course of animal evolution, the structures along the dorsoventral axis have taken on an orientation opposite that of the ancestral form, moving to the belly to the back of the animals. More than 100 years ago, German zoologist Anton Dohrn suggested that both insects and mammals must have one common ancestor with a nervous system similar to the human spinal cord.
In the past years, researchers have debated how the nervous system evolved in mammals, and the hypothesis of inversion has been disputed, but has arisen time and time again. Yet it has remained unclear when and how nerve cords originated during the evolution.
Now, a paper published in Nature, puts this issue to rest once and for all: studying several several different animals (Xenacoelomorpha, Rotifera, Nemertea, Brachiopoda, and Annelida) show that they have developed nerve cords independent of each other. The research project has been headed by Andreas Hejnol, researcher at the Sars International Centre for Marine Molecular Biology at the University of Bergen, and a team of international researchers.
TINY BUT IMPORTANT: "This is a motile, swimming stage of a brachiopod (lampshell common english name). It swims in the plankton and uses the spines for defense. This one is only 150 microns long - pretty tiny for providing so great insights into animal evolution" - Andreas Hejnol.
"A feature of the inversion hypothesis is that one of the ancestors of humans must have flipped, an inversion of the entire body. In 1994, scientists found that a common gene cascade is responsible for the condensation of the nervous system, which is used as evidence that nerve cords in animals as diverse as flies and humans have a common ancestry. We wanted to look closer at this," says Andreas Hejnol, researcher at the Sars International Centre for Marine Molecular Biology at the University of Bergen. Hejnol has headed the investigation, which concludes seven years of research.
THE TEAM: Kevin Pang, Jose-Maria Martin Duran, Andreas Hejnol, Anlaug Furu and Aina Børve. Photo: Wei Deng.
By collecting or cultivating nine species of animals, the scientists were able to build a very strong case against the older hypothesis of inversion.
"We found that the molecular patterns could not explain how the nervous system has developed. They have been used before as evidence that both humans and flies had a common ancestor with a prominent nerve cord. Our findings prove this hypothesis to be invalid, as well as the hypothesis of inversion" Hejnol says.
SPECIMEN SAMPLES: A scanning electron image of a young acoel worm. "It looks like a tribble from Star Trek but is much smaller, only 150 micrometers. We look at the frontend - the face - where at the most anterior tip the gland and sensory cells are positioned," Hejnol explains.
Until now, it has been believed that the early origin of the vertebrate nervous system was about 600 million years ago.
"Our finds place the origin at a much later time, around 100 to 200 million years later. Where this happened in the tree of life is important for researchers to know," the research group leader says.
He explains that the article in Nature and the finds published there will make it easier to explain how the nervous systems has evolved in animals.
PREDATOR WORM: "The Lineus ruber: Its an adult ribbon worm, Nemertean in latin. Its length is about 7 cm and from the rocky shore close to Bergen. They are dangerous predators - not for humans, but for other worms," Hejnol explains.
He believes the finds will change the field of evolutional biology.
"The researchers who have believed in the hypothesis of inversion will be shocked, and sad, perhaps. The majority of the researchers in the field will be happy, on the other hand. The hypothesis has been disputed, but our article is strong enough to put it to rest," Hejnol says.
Hejnol, who received an ERC Consolidator Grant in 2015, has been main author of several articles and papers shedding light on evolution, all of them published in highly rated scientific journals.
"My approach is to look closer at one organ system after another, we want to learn more and check ruling hypotheses. My next project is blood, and how it has been evolved," Hejnol concludes.
COLLECTING SAMPLES: The research published in Nature examined nine diffferent species, collected from all over the world.