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For hundreds of millions of years, pancake-shaped animals the size of the tip of a needle have been roaming the seas with an appetite for tasty microbes and algae. They are called placozoans and are among the simplest of the main animal lineages.
Simple as they are, a team of researchers has found compelling evidence for the existence of neuron-like cells in placozoans. And given how long these animals have been around, it’s possible that placozoans served as a model for nervous systems in more complex animals, including humans. He The work was published in the journal Cell on Tuesday..
If you look under a microscope, at first glance you might think that placozoans are amoebas. But organisms are animals. On the tree of life, they are more closely related to cnidarians (which include sea anemones and corals) or bilaterians (the supergroup containing vertebrates) than to lineages such as ctenophorans or poriferans. While these other animal lineages have nervous systems governed by nerve cells known as neurons, placozoans were thought to be different.
“No one would have thought that these organisms had anything like neurons,” he said. Xavier Grau-Bovéresearcher of Center for Genomic Regulation in Barcelona, Spain.
The bodies of placozoans are simple, only three cell layers thick. But that’s enough to glide through, absorb and digest food, and respond to the environment around them. Instead of being controlled by neurons, some of these behaviors are regulated by peptidergic cells, which release short chains of amino acids that activate surrounding cells.
Because the activity of peptidergic cells is reminiscent of more complex nervous systems, such as that of humans, Dr. Grau-Bové and his colleagues were intrigued by the possibility that these cells and their connections could represent the nervous system of a human. ancient animal ancestor.
The research team began by analyzing gene expression (which fragments of DNA are converted into RNA that are used to make cellular proteins) in more than 65,000 individual cells from four species of placozoans. They discovered that placozoans have 14 types of peptidergic cells that are also important for the construction of neurons in cnidarians and bilaterians. However, they also discovered that peptidergic cells were not true neurons given their lack of electrical activity and their inability to receive messages.
The researchers then created a map showing possible interactions between peptidergic cells and other placozoan cells. They identified a complex signaling network, as well as specific pairs of neuropeptides and receptors. These cellular relationships support what scientists call chemical brain hypothesisthe idea that early nervous systems evolved as networks of cells connected through chemical signals that would diffuse through an animal and bind to specific protein receptors.
The scientists then compared the peptidergic cells with neurons or neuron-like cells from other animal species. They confirmed important similarities between the way genes are used in placozoan peptidergic cells and the way neurons function in cnidarians and bilaterians. That indicated that early nervous systems were once similar to what is seen today in placozoans, before evolving over hundreds of millions of years into complex cells that send electrical signals.
“Their result showing the genetic similarity of these peptidergic cells to neurons was very surprising,” he said. Jacob Musser, a molecular evolutionary biologist at Yale University who was not involved in the study. “This suggests that some of the neuronal machinery was being packaged into cells and used for some form of communication before the arrival of the nervous system.”
Dr. Musser said he hoped to see similar approaches applied to investigations of nervous system evolution in older animal lineages such as ctenophores, which include comb jellyfish. The place of their distinctive nervous systems in evolution is still unclear.
Michael Paulin, A computational evolutionary neuroscientist at the University of Otago in New Zealand who was also not involved in the research, said placozoans were the best living model of early animal nervous systems. He suggested it was possible “that the ancestors of animals with nervous systems were placozoans,” adding that studying them “can help us understand what all those neurons in our brains do.”
Although placozoans are simple compared to humans, “the complexity of the entire system is much greater than we anticipated,” he said. Arnau Sebé-Pedrós, author of the study from the Center for Genomic Regulation. But, he said, “evolutionary biology is a historical science” and additional research can always lead to new understandings of how life came to be what it is today.