For hundreds of millions of years, pancake-shaped animals about the size of a sharp pencil tip have been roving the seas with an appetite for tasty microbes and algae. They’re called placozoans, and are among the simplest of the major animal lineages.
As simple as they are, a team of researchers has found compelling evidence of neuron-like cells in placozoans. And given how long these animals have existed, it’s possible that placozoans served as the blueprint for the nervous systems in more complex animals, including humans. The work was published in the journal Cell on Tuesday.
Look under a microscope and you may think at first glance that placozoans are amoebas. But the organisms are animals. In the tree of life, they are more closely related to cnidarians (which include sea anemones and corals) or bilaterians (the supergroup that contains vertebrates) than lineages such as ctenophores or poriferans. While these other animal lineages have nervous systems governed by the nerve cells known as neurons, placozoans were thought to be different.
“No one would have thought that these organisms had anything even resembling neurons,” said Xavier Grau-Bové, a researcher at the Center for Genomic Regulation in Barcelona, Spain.
Placozoan bodies are simple, only three cell layers thick. But that’s enough to glide around, absorb and digest food, and respond to their surrounding environment. 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 — like the one in humans — Dr. Grau-Bové and his colleagues were intrigued by the possibility that these cells and their connections might represent the nervous system of an ancient animal ancestor.
The research team began by analyzing gene expression — which bits of DNA are converted into RNA used to make cell proteins — in more than 65,000 individual cells across four placozoan species. They discovered that placozoans have 14 types of peptidergic cells that are also important for building neurons in cnidarians and bilaterians. However, they also found that peptidergic cells were not true neurons given their lack of electrical activity and inability to receive messages.
The researchers then created a map showing potential interactions between peptidergic cells and other cells in placozoans. They identified a complex signaling network as well as specific pairs of neuropeptides and receptors. These cellular relationships support what scientists call the chemical brain hypothesis, the idea that early nervous systems evolved as networks of cells connected through chemical signals that would diffuse across an animal and bind to specific protein receptors.
The scientists then compared peptidergic cells to neurons or neuron-like cells in other animal species. They confirmed major similarities between the way genes are used in placozoan peptidergic cells with the way neurons work in cnidarian and bilaterian animals. That indicated that early nervous systems were once similar to what is seen in placozoans today, before evolving for 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 striking,” said Jacob Musser, a molecular evolutionary biologist at Yale University who was not involved in the study. “It suggests that some of the neuronal machinery was being packaged into cells and used for some form of communication prior to the advent of a nervous system.”
Dr. Musser said he was looking forward to seeing similar approaches applied to investigations of nervous system evolution in more ancient animal lineages like ctenophores, which include comb jellies. The place of their distinctive nervous systems in evolution remains 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 “may help us to understand what all those neurons are doing in our brains.”
Although placozoans are simple compared with humans, “the complexity of the whole system is much higher than what we anticipated,” said Arnau Sebé-Pedrós, an author of the study at 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 became what it is today.