How did the first nervous systems originate? Clues to this question have now been provided by particularly primitive representatives of animals: the sponges. These comparatively simple beings have neither a brain nor nerves – but possible pre-forms of these cell types, researchers report. They discovered special cells with neural characteristics that are believed to mediate cellular communication. This concept could have been at the beginning of the development of the increasingly complex nervous systems of animals, say the scientists.
As you read these lines, the most mysterious of all organs is at work: the brain. The human organ of thought is the most complex known form of a network of nerves. Brain research has shown that our physical and mental abilities are based on the interaction of around 86 billion neurons. Despite numerous scientific findings, however, many aspects of the brain and nervous system remain a mystery. This also includes the question of what was at the beginning of the evolutionary history of the concept that made complex abilities possible in animals and ultimately also produced our minds and consciousness.
Sponges in the sights
In order to obtain basic information, Detlev Arendt’s team from the European Molecular Biology Laboratory in Heidelberg (EMBL) turned to the investigation of the sponges. It is assumed that the evolutionary history of today’s animal world goes back to similar beings that once lived in the primordial seas. The sponges are assigned to the animal kingdom, but have a comparatively primitive structure: their multicellular bodies form structures that enable food to be taken from the water flow. However, the sponges do not have complex organ systems, nor do they have any nerve structures that are responsible for communication in the body in other animals.
However, genetic studies on sponges have already shown that their genetic material includes genes that are known to have functions in the animal nervous system. Above all, it is about the synapses – the connecting elements between the neurons that sit at the ends of their processes. They enable communication between neurons and thus form the basis for the functioning of the nervous system and the brain. Arendt and his colleagues have now investigated the question of what significance these “synaptic genes” could have in sponges. “We know that these genes are involved in neuronal function in higher animals. The fact that they also exist in a living being without a nervous system therefore raised questions, ”says Arendt.
In order to gain insights into the function of these genetic make-up, the researchers first examined in which cell types of the sponges the synaptic genes are active. They used the freshwater sponge Spongilla lacustris as a model. In their investigations, they used a technique that makes it possible to break down the genetic profile of individual cells. “In this way, we were able to show that certain cells in the digestive chambers of the sponge activate the synaptic genes. Even in a primitive animal without synapses, these genes are active in certain parts of the body, ”says co-author Jacob Musser from EMBL.
Tracking down neuroid sponge cells
As the researchers explain, sponges use their digestive chambers to filter food out of the water, where they also react to microbes in the environment. In order to understand what role the special cells with synaptic gene activity play there, the scientists targeted them specifically. “By combining electron microscopy and X-ray imaging, we were able to visualize the amazing behavior of these cells,” explains co-author Schwab from EMBL.
It was shown that the sponge cells move through the digestive chambers with the synaptic gene activity. In doing so, they apparently eliminate bacterial invaders, and the researchers also found a particularly exciting behavior: the cells form long processes through which they apparently connect with digestive cells. It is therefore a similarity to features of animal nervous systems. It stands to reason that the sponge cells with synaptic gene activity are used in cellular communication to regulate food intake, the scientists explain.
This also results in the possibly more far-reaching significance of the study: “Our results suggest that these cells, which regulate food intake and control the microbial environment, formed possible evolutionary precursors of the first animal nervous systems,” says Musser. In conclusion, the scientists write: “Our work puts the sponges at the center of the explanation of the evolution of the nervous system”.
Source: European Molecular Biology Laboratory, technical article: Science, doi: 10.1126 / science.abj2949