Comb Jellies Brain Reveals the Evolution of Ancestral Nervous System

A recent study on the brains of combo jellies reveals the type of neural messenger that likely functioned in the ancestral nervous system. The findings also suggest the evolution of the nervous system.

Neurons are specialized cells of the body's nervous system. It is one of the most complicated cell types that have evolved. For humans, neurons are capable of processing and transmitting sums of information. However, how the complicated cell came about remains debated.

Unraveling the Evolution of Neurons

Comb jellies and neuron evolution — TOPSHOT - This picture taken on August 21, 2019 in Paris, shows jellyfish displayed in a tank at the Aquarium of Paris. - At the Aquarium of Paris, the excrement of roaches, goldfish and sturgeons feed tomatoes and peppers, to the surprise of visitors. Aquaponic systems are closed circuits, where the fish, via their excrement rich in nitrogen, create fertilizer for the plants, which absorb these nitrates and filter the clean water back to the fish. ERTRAND GUAY/AFP via Getty Images

Japanese scientists have revealed the molecular messenger that carries signals from one cell to another, likely functioning in the most ancient nervous systems. The study published in the journal Nature Ecology and Evolution, titled "Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons," reveals vital similarities between the nervous systems of two diverging animal lineages; the lineage of jellyfish and anemones and that comb jellies, which reignites the earlier debates that neurons only evolved once.

Despite the theorized simplicity, little is known about the nervous system of early animals. Out of the four animal lineages that branched before the rise of complex animals, only comb jellies and cnidarians are known to have neurons.

However, the unique properties of the nervous systems of comb jellies compared to the cnidarians and more complex animals, the absence of neurons in the two that diverged in between led researchers to hypothesize that neurons, In fact, evolved twice, reports Neuroscience.

Professor Watanabe, the leader of the Evolutionary Neurobiology Unit at the Okinawa Institute of Science and Technology, says that although com jellies lack numerous natural proteins that are seen in more evolved animal lineages, the lack of the proteins isn't sufficient evidence for two independent neuron origins.

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Studying Comb Jellies for Hints of Neuron Evolution

In his study, he focused on ancient and diverse groups of neural messengers. Neuropeptides are short peptide chains first synthesized in neurons as long chains of peptides before being cleaved by digestive enzymes into shorter peptides. These molecules are the major messenger in cnidarians and play a vital role in neuronal communications in humans and complex animals.

On the other hand, past researchers that attempted to find similarities between neuropeptides in comb jellies have been unsuccessful. The main problem for the professor is that the mature shorter peptides are encoded by a short sequence of DNA and frequently mutate in ancient lineages, making DNA comparisons too challenging. While artificial intelligence has identified potential peptides, these studies have yet to be experimentally validated.

Hence, a team of researchers approached the problem in a new direction. The team extracted peptides from cnidarians, comb jellies, and sponges and used mass spectrometry to search for short peptides. The team found 28 short peptide chains in comb jellies and cnidarians and determined their amino acid sequences.

The researchers also compared what genes were expressed by the neurons of cnidarians and comb jelly. The team found that as well as having short neuropeptides in both lineages, both neurons expressed a similar array of other proteins essential for neural function.