The lowly hydra may hold the explanation. Their epithelial cells can have a neuron-like function:-http://phys.org/news/2015-11-hydra-genetic.html-"Champion of regeneration, the freshwater polyp Hydra is capable of reforming a complete individual from any fragment of its body. It is even able to remain alive when all its neurons have disappeared. Researcher the University of Geneva (UNIGE), Switzerland, have discovered how: cells of the epithelial type modify their genetic program by overexpressing a series of genes, among which some are involved in diverse nervous functions. Studying Hydra cellular plasticity may thus influence research in the context of neurodegenerative diseases. - ***-"Epithelial cells do not possess typical neuronal functions. However, Hydra's loss of neurogenesis induces epithelial cells to modify their genetic program accordingly, indicating that they are ready to assume some of these functions. These "naturally" genetically modified epithelial cells are thus likely to enhance their sensitivity and response to environmental signals, to partially compensate for the lack of nervous system", explains Wanda Buzgariu, co-first author of the article. The detail of these new functions remains to be discovered, as well as how epithelial cells proceed to overexpress these genes and thus adapt their genetic program.-***-"This study also allows to go back to the origins of nervous systems. Epithelial cells most probably preceded nerve cells, performing some of their functions, although in a much slower way. «The loss of neurogenesis in Hydra may provide an opportunity to observe a reverse evolutive process, because it sheds light on a repressed ancestral genetic toolkit. An atavism of epithelial cells, when they most probably also possessed proto-neuronal functions", concludes Brigitte Galliot. "-Comment: As shown in the preceding articles in this series, neurons appeared about 600 million years ago, the most complex of the cell types to develop. They allowed the very complex Cambrian animals to appear, and this all led finally to our brains.

Where id they come from? possible sponge evidence:

https://www.newscientist.com/article/2296329-brainless-sponges-have-cells-that-might-be...

"Sponges lack anything resembling brains, but they nevertheless may have played a key role in the early evolution of the nervous system. A new study finds that sponges contain cells that have some of the capabilities of neurons – and these may be the evolutionary precursors of true brain cells.

“'The nervous system came about very early in animals and this transition is completely enigmatic so far,” says Detlev Arendt at the European Molecular Biology Laboratory in Heidelberg, Germany.

"Most animals have brains, or at least neurons, the cells that are their building blocks. Neurons carry electrical signals along their length and can communicate with each other by releasing chemicals called neurotransmitters, often at specialised junctions known as synapses.

"However, sponges are the exception. They are one of the oldest animal groups still extant – possibly the very oldest. And they don’t have a nervous system.

***

"Musser, Arendt and their colleagues studied a freshwater sponge called Spongilla lacustris. They broke apart sponges and tracked individual cells to see which genes were active.

"This revealed that the sponges were made up of 18 distinct cell types, each with a different pattern of gene activity. The team then stained the different cells to figure out where they were within the body.

"One cell type stood out. The team calls them “neuroid” because they had long tendrils, resembling those of neurons. They were found in the sponge’s digestive chamber and made contact with many of the other cells within. Their gene activity pattern suggested they were secreting signalling chemicals, similar to those that neurons release at synapses to communicate with their neighbours.

"Arendt emphasises that the sponges’ neuroid cells aren’t neurons. “We still think they don’t have a nervous system,” he says. But these cells may be coordinating the activities of the digestive cells. “We see a lot of vesicles in those neuroid cells that would indicate that they secrete something, which is a very strong indication for communication,” he says. “And we also know the kind of molecules they might produce.”

***

"Arendt points out that many of the genes and chemicals used by neurons are actually ancient: they predate the evolution of multicellular animals and can be found in our single-celled relatives. Later, the genes were duplicated and some versions became altered, ultimately leading to neurons that are highly specialised for fast communication.

"That step happened early in animal evolution. The other candidate for the oldest animal group, the comb jellies, do have neurons – and they are organised into a net. “I would be fine with calling that a brain,” says Arendt. Arguments have raged over the past decade over whether sponges or comb jellies are the older group. Arendt is inclined to believe that sponges, being simpler, are older."

Comment: Darwinist scientists abhor gaps in form, whether in cell types or body forms, since Darwin proposed evolution was in tiny steps. If evolution is formed by design there is no need for precursors.