An article published in the journal “Cell” describes a research on the origin of the types of neurons needed to coordinate walking on the mainland. A team of researchers from the New York University School of Medicine focused in particular on the species little skate (Leucoraja erinacea), part of the most primitive vertebrate group, studying its neuromuscular and genetic development. The conclusion is that some fish started walking on the sea floor about 420 million years ago, about 20 million years before the early tetrapods started colonizing the mainland.
Neurobiologist Jeremy Dasen, one of the authors of this research, explained that generally the idea existing about the development of the ability to walk was associated with the passage from the sea to the mainland. However, there are also species of fish that can walk and use neural and genetic structures that are almost identical to those used by mainland vertebrates, including humans.
To try to reconstruct the origins of that ability, the researchers decided to study the skate of the species Leucoraja erinacea. These fish are related to sharks and other primitive cartilaginous fish. They’re considered among the most primitive vertebrates, therefore suitable for a study of the origin of a very ancient ability such as walking.
The researchers used an RNA sequencing technique to assess which genes are expressed in the motor neurons of the little skate, the ones that manage its walking. The result is that many of those genes are the same between this skate and mammals. They also found that the subtypes of neurons crucial for the muscle control that manage limbs bending are present in the motor neurons of this skate.
The research has gone further, studying the circuits that connect the motor neurons and determine the activation of the muscles. Again, there are almost a dozen types of neural structures that control walking in both the little skate and terrestrial mammals.
The common ancestors of these species started using certain specific proteins to control genes connected to the types of movement related to walking. Subsequently, tetrapods evolved with the development of limbs and walking on land rather than on the sea floor.
Those common ancestors of skates and tetrapods date back about 420 million years ago so this research indicates that the ability to walk already started developing at the time. In essence, tetrapods developed an already existing skill by adapting to the land.
Jeremy Dasen stated that his team intends to use little skates to study the connection between motor neurons and other types of neurons and how they are regulated. Vertebrates have many types of neurons and muscles, making this type of research much more complex. The little skate is much more primitive and therefore simple so it allows to better study the systems that control walking.