Ever wondered how a nervous system evolved, and how we evolved to get around to thinking? If you do think about it, at first sight it is not obvious how it evolved; what caused it? The point about evolution is that it progresses in tiny steps, so what could possibly be a step towards a nervous system? It has to be something really simple that a minor change from the first simple organism that feeds and reproduces, BUT it has to do something that gives it an advantage, so the question comes down to what could that be?
The first thing to note is that there would be little point in a single-cell creature developing such a system. The point of a nervous system is to coordinate the activities of different parts of the whole, but a single cell is sufficiently small that coordination is unnecessary. Notwithstanding that, there may be an advantage for a single cell to sense whether there are nutrients nearby. The first such cells would simply absorb, but if it could sense when there were nutrients or not, it would have a better way of knowing whether to reproduce. That could arise initially with nothing more than having two activities. Microalgae show such an extremely primitive sensing. If a microalga has a good supply of nitrogen, it makes nucleic acid as fast as it can, together with some protein, and these are just what it needs to reproduce. If it is nitrogen starved, it cannot turn off its photosynthesis mechanism so it takes CO2 from the air and makes lipids. It just swells up with fat! If it cannot get nitrogen nutrients for a prolonged time, it bloats and dies.
According to Musser et al. 2021 (Science 374: 717 – 723) a clue to how the nervous system evolved comes from sponges. Sponges are an animal clade that lack neurons, muscles or a gut, so they are rather simple. They have canals for filter feeding and waste removal and they have cilia that drive water flow. Yet despite this simple structure, they perform whole-body contractions that can expel debris, and while they have no integrated signalling functions, nevertheless they have genetic material usually found in nerves and muscles. Apparently sponges can use an intricate cell communication system to regulate their feeding and potentially eliminate invading bacteria. They do not have neurons, but they have genes that encode proteins to help transmit chemical signals, which could be regarded as an initial move towards a nervous system.
The sponge that was studied has 18 distinct cell types and synaptic genes (i.e. potentially capable of transmitting a signal) were active in some of the cells that were clustered around the digestive chambers.
They then showed that some such cells send out long arms to contact the cells with hair-like protrusions that drive the water flow systems. In other words, there is something there made of protein that starts where food is digested and stretches to the cells that control the flow of water, thus either telling these cells to send more food or alternatively to clear out the debris from previous digestion. It is important to note that these connectors are not nerves and it is not a rapid communication. Nevertheless, a system that could tell when it was time to get rid of debris from the region where it digests would be an evolutionary advantage over those that could not, and would hence take a greater percentage of the food and reproduce faster. Eventually it would predominate, especially those specimens that could do it a little better than the others. Over the generations the system would gradually predominate. It should also be noted that this does not mean we evolved from a sponge. This sort of behaviour could have started many times in different families. The point is, there is a distinct advantage when developing multi-celled creatures for one end to let another end know that it would like more food, or that it is flooded with debris. Obviously, this is a long way from a nervous system. The next evolutionary step would probably be to do it faster in larger multi-celled species. However, the means of sensing food would be the first prerequisite for sending messages to help digestion; it is not just the ability to send messages, but the message must have some sensible relevance. Food (or nutrient acquisition) would be the first reason to communicate across cells. Whether this was really how a nervous system started is debatable, but at least it makes sense.