Where does our head come from?

A research group at the Sars Centre in Bergen has shed new light on the evolutionary origin of the head. In a study published in the journal PLoS Biology they show that in a simple, brainless sea anemone, the same genes that control head development in higher animals regulate the development of the front end of the swimming larvae.

In many animals, the brain is located in a specific structure, the head, together with sensory organs and often together with the mouth. However, there are even more distantly related animals, which have a nervous system, but no brain, like sea anemones and corals.
In this study a research group led by Fabian Rentzsch used the sea anemone Nematostella vectensis to find out if one of the ends of the sea anemone corresponds to the head of higher animals. To do this they studied the function of genes that control head development in higher animals during the embryonic development of the starlet sea anemone.

“Despite looking completely different, it has become clear over the last decade, that all animals have a similar repertoire of genes, including those that are required to make the head of higher animals”, says first author and PhD-student Chiara Sinigaglia.

Stands on its head
When the sea anemone is in the larval stage it swims. As adults, the sea anemone stands with one end on the sea floor and uses long tentacles on its upper end to catch small animals which they stuff into the only body opening in the middle of the ring of tentacles.

“Based on the appearance of the adult animals, the lower end of these animals has traditionally been called the foot and the upper end the head”, explains Rentzsch.
What the research group found out was that in the sea anemone the “head gene” function is located at the end that corresponds to the “foot” of the adult animals. The key was to study the larvae of the sea anemones when theystill move around.

“The larvae swims with the “foot” end forward and this end carries their main sense organ, so at this stage it looks more like this might be their head”, says Rentzsch. And indeed, the “head genes” function on this side of the animals.
Sea anemones and all higher animals, including humans, share a common brainless ancestor which lived between 600 and 700 million years ago.

“By revealing the function of “head genes” in Nematostella, we now understand better how and from where the head and brain of higher animals evolved”, Sinigaglia and Rentzsch explain.

In Search of the Origin of Our Brain

While searching for the origin of our brain, biologists at Heidelberg University have gained new insights into the evolution of the central nervous system (CNS) and its highly developed biological structures. The researchers analysed neurogenesis at the molecular level in the model organism Nematostella vectensis. Using certain genes and signal factors, the team led by Prof. Dr. Thomas Holstein of the Centre for Organismal Studies demonstrated how the origin of nerve cell centralisation can be traced back to the diffuse nerve net of simple and original lower animals like the sea anemone. The results of their research will be published in the journal “Nature Communications”.

Like corals and jellyfish, the sea anemone – Nematostella vectensis – is a member of the Cnidaria family, which is over 700 million years old. It has a simple sack-like body, with no skeleton and just one body orifice. The nervous system of this original multicellular animal is organised in an elementary nerve net that is already capable of simple behaviour patterns. Researchers previously assumed that this net did not evidence centralisation, that is, no local concentration of nerve cells. In the course of their research, however, the scientists discovered that the nerve net of the embryonic sea anemone is formed by a set of neuronal genes and signal factors that are also found in vertebrates.

According to Prof. Holstein, the origin of the first nerve cells depends on the Wnt signal pathway, named for its signal protein, Wnt. It plays a pivotal role in the orderly evolution of different types of animal cells. The Heidelberg researchers also uncovered an initial indication that another signal path is active in the neurogenesis of sea anemones – the BMP pathway, which is instrumental for the centralisation of nerve cells in vertebrates.

Named after the BMP signal protein, this pathway controls the evolution of various cell types depending on the protein concentration, similar to the Wnt pathway, but in a different direction. The BMP pathway runs at a right angle to the Wnt pathway, thereby creating an asymmetrical pattern of neuronal cell types in the widely diffuse neuronal net of the sea anemone. “This can be considered as the birth of centralisation of the neuronal network on the path to the complex brains of vertebrates,” underscores Prof. Holstein.

While the Wnt signal path triggers the formation of the primary body axis of all animals, from sponges to vertebrates, the BMP signal pathway is also involved in the formation of the secondary body axis (back and abdomen) in advanced vertebrates. “Our research results indicate that the origin of a central nervous system is closely linked to the evolution of the body axes,” explains Prof. Holstein.