Did neurons evolve twice?

Tracing the history of the first living organisms provides evidence of whether the birth of the neuron was a one-time event.

History of Neurons

The billions of neurons in the human brain represent a variety of specialized and diverse cells in our bodies. Neurons convert electrical signals into chemical signals, and in humans, their lengths can be so small that they span only the tip of a pencil or, in some cases, even stretch across the width of a door. The flexible control of movement and decision-making explains why they are so crucial for survival in the animal kingdom.

Evolution of Neurons

Most animals rely on the specialization of their neurons for survival. It may therefore make sense that the common ancestor of all these animals moved on land millions of years ago under the guidance of electrochemical signals being transmitted and received through networks of neurons. The idea that these vital cells evolved multiple times seems unreasonable because neurons are incredibly complex and very similar across animal lineages. However, a series of modern evolutionary biological studies suggest that all animal neurons have a single origin.

Modern Evolutionary Biological Studies

These findings represent a culmination of many years of research and discussion regarding the evolutionary lineages of early animals and the cells and systems present in those species. The first result came from a study of relationships between early animals, focusing on two specific types of organisms: sponges (including marine sponges and freshwater species) and ctenophores, invertebrates often known as comb jellies, although they are not related to sea jellies. For nearly 15 years, evolutionary biologists have debated whether ctenophores or sponges were the first animals to branch off from all other animals in the evolutionary tree. Hundreds of millions of years ago, the common ancestor of all living organisms branched into two types. On one side was the common ancestor of all animal groups except one. On the other side was this “one” – the “sister group” that was the first to branch off from all other animals. The ongoing question was whether the “sister group” was sponges or ctenophores.

Recent Studies

A compelling paper published last year provides strong support for the hypothesis that ctenophores are indeed the long-sought sister group. The researchers found that ctenophores branched before sponges and thus represent the most distantly related group to all other animals. However, the situation remains unsettled due to the puzzle it raises in explaining the evolution of neurons.

Scenarios for Neuronal Evolution

Neurons are absent in sponges and present in ctenophores and almost every other animal on the planet. If ctenophores branched before sponges in the tree of life, this suggests two scenarios for the evolution of neurons. In one scenario, the common ancestor of all animals, which lived about a billion years ago, had neurons, and each animal type inherited them. This means that sponges must have lost their nerve cells at some point, as they no longer have the neurons inherited from their ancestors.

The alternative posits that the common ancestor of all animals did not possess neurons, which explains the absence of neurons in early-branching animals such as sponges. Neurons in most animals must have arisen later, after the branching of sponges – except for the neurons in ctenophores. If the common ancestor lacked neurons, and neurons evolved in most animals after the branching of ctenophores and sponges, then the neurons in ctenophores must have evolved independently. Neurons evolve twice in this scenario – once in ctenophores and once again in other animals – which raises doubts about the existence of a single origin for neurons.

Conclusions

To Consensus

Currently, Detlef Arendt, a professor and senior scientist at the European Molecular Biology Laboratory specializing in the evolution of nerve cells and nervous systems, says, “At present, I am unsure which of these scenarios is more likely.” Meanwhile, Max Telford, a professor of zoology at the Origins of Life and Evolution Centre at University College London, supports the first scenario more strongly. He says, “It is entirely possible that sponges do not have a nervous system because they lost it, as they are considered potential filter feeders and do not require a complex nervous system.” He adds, “Simplification and loss occur all the time.”

Sponges are not unique in losing their nerve cells. Telford points to the example of myxozoans, which are among the smallest animals in the world and are closely related to jellyfish and sea anemones. The common ancestor of these three groups of animals almost certainly had a nervous system, but myxozoans lost theirs at some point in deep evolutionary history.

The picture is also unclear due to the fact that nerve cells in ctenophores are very strange – so strange that it might not be surprising if the nerve cells in ctenophores evolved independently from nerve cells in other animals. A recent research paper found that the bulk of the nervous system in ctenophores consists of nerve cells without synapses, a feature that has not been confirmed elsewhere in the animal kingdom. Arendt says, “There is no other example of this extreme type of nervous system,” and adds, “But there are many examples where nervous systems have been reduced and become very simple.”

Leslie Babonis, an assistant professor of ecology and evolutionary biology at Cornell University who studies the origin of new animal traits, can envision scenarios where these strange nerve cells still evolved from the same common ancestor of nerve cells in other lineages. She says, “There is a lot of evidence suggesting that nerve cells evolved once in the common ancestor of all animals and that each lineage… modified those cells in complex and different ways.” At the same time, “It also challenges our view that these animals may have abandoned these important things,” such as nerve cells, she notes.

No consensus has been reached yet. Telford says, “We need to know more, I think, about nerves and nerve cells and what the common ancestor of these cells is.” Indeed, unraveling the history of nerve cell evolution may require addressing some basics about how nerve cells appeared in the first place. Biologists have not settled on a model for how nerve cells evolved once, and the matter remains unsettled. One of the main candidates is the “chemical brain hypothesis,” sometimes known as the “transmitting nervous network hypothesis,” which suggests that the common ancestor of nerve cells was composed of cells that relied solely on chemical messages to send signals through a living organism. The chemical brain hypothesis received a significant boost last September, thanks to microscopic animals known as plasmodia. Plasmodia are invisible without the aid of a microscope. They live in the ocean, like ctenophores, but consist only of several layers of cells, and their bodies are irregularly shaped. Unlike ctenophores, plasmodia do not have nerve cells. Instead, they rely heavily on specialized peptidergic cells, which secrete or respond to short chains of amino acids, to drive their small bodies using only chemical signals.

The peptidergic cells in plasmodia are not considered nerve cells. They do not use electrical impulses, and their messages to neighboring cells are limited to sending signals to other cells – unlike nerve cells that can send and receive. However, a new analysis found that peptidergic cells have some striking genetic similarities to nerve cells and contain proteins associated with the physical structures surrounding synapses in nervous systems. This suggests a model for how nerve cells might have evolved in animals and reinforces previous work in establishing a link between cancer nerve cells and neuron cells.

On
Although this research supports the chemical brain hypothesis, it does not exclude other models for the evolution of neurons. Another framework from the mid-twentieth century is referred to by Arndt as the “contractile neural network hypothesis,” which posits that neurons were once part of hypothetical “neuro-muscle cells” that may have integrated the functions of muscles and neurons. Importantly, the chemical brain hypothesis and the contractile neural network hypothesis are not mutually exclusive.

Arndt states, “Neurons, even within a single evolutionary lineage, may have dual origins. Perhaps both are true but occur in different locations in the body.” Different types of synapses in our brains even have different origins. It may turn out that many components of our nervous system have evolved more than once – even if neurons in animals can be traced back to a single ancestor.

Source: https://www.scientificamerican.com/article/did-neurons-evolve-twice/