Neurons, the specialized cells of the nervous system, are possibly the most complicated cell type ever to have evolved. In humans, these cells are capable of processing and transmitting vast sums of information. But how such complicated cells first came about remains a long-standing debate.

Now, scientists in Japan have revealed the type of messenger — molecules that carry signals from one cell to another — that likely functioned in the most ancestral nervous system.

The study, published 8th August in Nature Ecology and Evolution, also revealed key similarities between the nervous system of two early-diverging animal lineages — the lineage of jellyfish and anemones (also called cnidarians) and that of comb jellies (ctenophores), reigniting an earlier hypothesis that neurons only evolved once.

Despite their supposed simplicity, very little is known about the nervous system of ancient animals. Out of the four animal lineages that branched off before the rise of more complex animals, only comb jellies (the first ancient lineage to diverge) and cnidarians (the last ancient lineage to diverge) are known to possess neurons. But the uniqueness of the comb jellies nervous system compared to that seen in cnidarians and more complex animals, and the absence of neurons in the two lineages that diverged in between, led some scientists to hypothesize that neurons evolved twice.

But Professor Watanabe, who leads the Evolutionary Neurobiology Unit at the Okinawa Institute of Science and Technology (OIST), remained unconvinced.

“Indeed, comb jellies lack a lot of neural proteins that we see in more evolved animal lineages,” he said. “But for me, a lack of these proteins isn’t enough evidence for two independent neuron origins.”

In his study, Prof. Watanabe focused on an ancient and diverse group of neural messengers. Called neuropeptides, these short peptide chains are first synthesized in neurons as a long peptide chain, before being cleaved by digestive enzymes into many short peptides. They are the major form of messenger found in cnidarians, and also play a role in neural communication in humans, and other complex animals.

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