The Evolution of Human Neurons

Author: Okinawa Institute of Science and Technology (OIST) Graduate University
Published: 2022/08/08 - Updated: 2023/01/04 - Peer-Reviewed: Yes
Contents: Summary - Main - Related Publications

Synopsis: A new study into the neurons found in the earliest-diverging animal lineages reveals vital clues about the form of the most ancestral nervous system and how it first evolved. 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 and cnidarians are known to possess neurons. 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.

Neurons

Neurons (neurones or nerve cells) are the fundamental units of the human brain and nervous system, the cells responsible for receiving sensory input from the external world, sending motor commands to our muscles, and transforming and relaying the electrical signals at every step in between. Neurons are information messengers. They use electrical impulses and chemical signals to transmit information between different areas of the brain and between the brain and the rest of the nervous system. Everything we think, feel, and do would be impossible without the work of neurons and their support cells, the glial cells called astrocytes and oligodendrocytes.

Main Digest

Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons. Neurons, the specialized nervous system cells, are possibly the most complicated cell type ever to have evolved. In humans, these cells can process and transmit vast amounts 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.

Continued below image.
A species of comb jelly housed in the Evolutionary Neurobiology Unit, OIST - Image Credit: Soumen Jana/ OIST.
A species of comb jelly housed in the Evolutionary Neurobiology Unit, OIST - Image Credit: Soumen Jana/ OIST.
Continued...

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.

Complete neuron cell diagram showing dendrites, neurotransmitter and receptacle. Neurons (also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. In vertebrate animals, neurons are the core components of the brain, spinal cord and peripheral nerves.
Complete neuron cell diagram showing dendrites, neurotransmitter, and receptacle. Neurons (also known as neurones and nerve cells) are electrically excitable cells in the nervous system that process and transmit information. In vertebrate animals, neurons are the core components of the brain, spinal cord and peripheral nerves.

However, past research that has attempted to find similar neuropeptides in comb jellies have been unsuccessful. The main problem, explained by Prof. Watanabe, is that the mature short peptides are encoded by only short sequences of DNA and frequently mutate in these ancient lineages, making DNA comparisons too difficult. While artificial intelligence has identified potential peptides, these have not yet been experimentally validated.

So, Prof. Watanabe's research team approached the problem in 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.

Now knowing their structures, the researchers visualized the short peptides under a fluorescent microscope, allowing them to see which cells were produced in both cnidarians and comb jellies.

In comb jellies, they found that one type of neuropeptide-expressing cell looked similar to classic neurons, with thin projections called neurites extending out from the cell.

But the short peptides were also produced in the second type of cell that lacked neurites. The researchers suspect these could be an early version of neuroendocrine cells - cells that receive signals from neurons and then release signals, like hormones, to other organs in the body.

The researchers also compared the genes expressed in cnidarian and comb jelly neurons. They found that having some sort of 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 Prof. Watanabe. "In other words, it's most likely that neurons only evolved once."

This means, added Prof. Watanabe, that peptide-expressing neurons are probably the most traditional form, with chemical neurotransmitters arising later. For Prof. Watanabe, these findings bring new, exciting questions to the forefront of his research.

"If this is true, I'm most interested to know - 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."

Attribution/Source(s):

This peer reviewed publication pertaining to our Anthropology and Disability section was selected for circulation by the editors of Disabled World due to its likely interest to our disability community readers. Though the content may have been edited for style, clarity, or length, the article "The Evolution of Human Neurons" was originally written by Okinawa Institute of Science and Technology (OIST) Graduate University, and submitted for publishing on 2022/08/08 (Edit Update: 2023/01/04). Should you require further information or clarification, Okinawa Institute of Science and Technology (OIST) Graduate University can be contacted at the oist.jp website. Disabled World makes no warranties or representations in connection therewith.

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