A species of comb jelly housed in the Evolutionary Neurobiology Unit. Credit: Soumen Jana/ OIST
Key points:
- Neurons, the specialized cells of the nervous system, are possibly the most complicated cell type ever to have evolved.
- Neurons found in the earliest-diverging animal lineages reveal neurons only evolved once throughout history.
- Researchers are now going to examine the original function of neurons in ancient animals.
A new study by researchers in Japan has reignited an earlier hypothesis that neurons evolved only once throughout history. In results published in Nature Ecology and Evolution, the scientists reveal key similarities between the nervous system of two early-diverging animal lineages—jellyfish and anemones (also called cnidarians) and comb jellies (ctenophores).
Hiroshi Watanabe, who leads the Evolutionary Neurobiology Unit at the Okinawa Institute of Science and Technology (OIST), 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. However, past research that has attempted to find similar neuropeptides in comb jellies has been unsuccessful.
Watanabe’s research team approached the problem from a new direction. They extracted peptides from sponges, cnidarians and comb jellies and used mass spectrometry to search for short peptides. The team was able to find 28 short peptides in cnidarians and comb jellies and determine their amino acid sequences.
The researchers also compared what genes were expressed in cnidarian and comb jelly neurons. They found that as well as having some short neuropeptides in common, both neurons also expressed a similar array of other proteins essential for neuronal function.
“We already know that cnidarian peptide-expressing neurons are homologous to those seen in more complex animals. Now, comb jelly neurons have also been found to have a similar genetic signature” suggesting that these neurons share the same evolutionary origin,” said Watanabe. “In other words, it’s most likely that neurons only evolved once.”
These findings bring about even more questions for Watanabe’s line of research.
“Where did the peptide-expressing neurons come from? And why did the ancestral animal need to evolve neurons? Now that we have a clearer idea of what the earliest neurons looked like, research into their original function can begin,” he said.
Information provided by OIST.