At the origins of the nervous system: the different ways in which a neuron can be formed

In 1906, two neuroscientists shared the Nobel Prize in Physiology or Medicine for their work on the structure of the nervous system. Surprisingly, the two diverged precisely on one fundamental aspect of this structure: while Camillo Golgi proposed that animal nerve cells form a continuous and joint network, i.e. technically a syncytium, Santiago Ramón y Cajal maintained that neurons are distinct cells connected via synapses. Cajal’s theory was later confirmed by electron microscopy, ending the debate. Since then, the architecture of every known nervous system has been found to be the one Cajal first described, with neurons of different types specifically contacting other neurons via dendrites and shorter or longer axons, connected by the synapses through which they occur. the transmission of an impulse from one neuron to another. Over 100 years later, however, an unexpected discovery has shown that Golgi’s model of nerve network does exist in nature after all: in a work just published in Science, it was found that in marine animals similar to jellyfish, sea anemones and corals, called ctenophores, there is a widespread nervous network, part of the nervous system, which has precisely the structure hypothesized by the Italian doctor. That is, it is a single syncytium, with the neuronal cells directly fused with each other, rather than connected via synapses.

The fused neurons that make up the syncytium are specific to the diffuse nervous network: they are not found elsewhere in a ctenophore’s nervous system, which includes sensory cells and tentacle nerves. Now, the behavior of ctenophores includes rather complex activities, such as the escape from predatory species through the modulation of swimming so that this is faster and more erratic and the search for a refuge among the corals, not to mention sophisticated feeding behaviours. That is, we have in this group of transparent pelagic animals, which is evolutionarily basal with respect to the majority of other types of animals, complex behaviours, controlled by a nervous system which is alien and structurally different, so as to make it probable that it evolved independently from that of other animals. Moreover, the absence of synapses and the fusion in a single syncytial structure of the neurons are only the latest and most definitive evidence of this difference: the ctenophores lack most of the neurotransmitters that are observed in the other nervous systems, just as they lack or most of the enzymes that are used to produce them are inactive. This, in essence, implies that, even if we don’t know how, the alien nervous system that these animals possess not only has a profoundly different architecture, but works by transmitting electrochemical stimuli in a totally different way from what has been known up to now.

The Golgian nervous network of the ctenophores therefore leads to a very interesting observation: in the history of animal life on our planet, a nervous system has evolved at least twice with the same function, but based on different cells, architectures, chemistry and involved genes. How there are wings, fins, eyes and other physiological systems that have emerged several times as the result of convergent evolution, i.e. a selection process caused by the same forces and in the presence of the same constraints that has used different genetic materials to eventually produce morphological adaptations similar, so it must have happened very early for nervous systems: in ctenophores, the neural network that governs many behaviors evolved from scratch, using a different set of molecules and genes than in any other known animal on Earth. Not only that: looking at the details, even after the separation between ctenophores and all the other animals with a nervous system, for some neuroscientists such as Leonid Moroz there are nine to twelve independent evolutionary origins of the nervous systemincluding at least one in cnidarians (the group that includes jellyfish and anemones), three in echinoderms (the group that includes starfish, sea lilies, urchins, and sand dollars), one in arthropods (the group that includes insects, spiders, and crustaceans), one in molluscs (the group that includes clams, snails, squid and octopuses), one in vertebrates and one, most different of all, in ctenophores.

The conclusion, even if we want to separate only the ctenophores from the others, is that there are more than one way to create a neuron and more than one way to create a nervous system, starting from different subsets of genes, proteins and molecules, through duplication and random gene mutations, on which natural selection then acts. The similarity in what ultimately emerges testifies to the stringency of the selective constraints and their constancy over the time in which the tree of life has evolved, while the variety of the starting material used is proof of the plasticity of life itself. Even with regard to the nervous system, Dawkins’ blind watchmaker assembled what he had at his disposal at random, but surprisingly many watches emerged from the process, all similar even if made of different pieces: in this specific case, these watches are the system of control and interaction with the external environment that we call the nervous system, of which we, a Darwinian sprig among the latest to appear, are so proud that we call ourselves “sapiens”.