Placozoans might just have the blueprint for our nervous system. A team of researchers have made this riveting discovery in which they found that neuron-like cells exist within these creatures and those cells could be the basis for the neuron cells that make up the nervous systems of more complex beings such as humans today.
Having been around for hundreds of millions of years, placozoans are extremely small animals that enjoy floating about in the oceans and consuming algae and microbes. They are closely related to cnidarians (a group that relates to sea anemones and corals) and bilaterians (a group that includes vertebrates). Studying placozoans is always of interest to scientists as they have existed for a very long time and are very simple creatures—only having three cell layers. Taking a closer look at these animals could reveal many answers and, in this case, could help us learn more about our programming and how our body’s nervous system came to be.
While studying placozoans, scientists were not expecting these creatures to have neurons, despite many of the placozoans’ related groups having neurons. Many of the organism’s behaviors are controlled by peptidergic cells, which are cells that release short streams of amino acids that alert nearby cells. However, because this activity of the peptidergic cells is also found in more complex systems such as the human nervous system. Dr. Xavier Grau-Bové and his colleagues at the Center for Genomic Regulation (CGR) in Barcelona, Spain, felt compelled to explore further.
The research team started by analyzing gene expression in over 65,000 cells across four different placozoan species. Gene expression is the process by which bits of DNA are transformed into the RNA used to make proteins. In doing this, the researchers found that placozoans have 14 different types of peptidergic cells and that these cells also exist in cnidarians and bilaterians, groups that utilize these cells to create neurons. Additionally, the scientists were also able to determine that peptidergic cells were still quite different from neurons as neurons also have the ability to receive messages and possess electrical pulses.
Using their data and findings, the group of scientists mapped out the potential interactions between peptidergic cells and other cells within placozoans. The cellular relationships they were able to identify supported the chemical brain hypothesis. The hypothesis presents the notion that early nervous systems evolved cells connected by certain chemical signals would diffuse and bind to specific protein receptors. This allowed the group of researchers to essentially theorize that the early versions of the nervous system is similar to what currently exists within placozoans. The early versions of the nervous system then evolved into much more complicated structures that have the ability to relay messages and electrical signals.
Dr. Jacob Musser, a molecular evolutionary biologist at Yale University and not affiliated with this study, stated “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 also reflected how he hopes that these findings could be used to determine the status of nervous systems in ancient animal lineages. This would present a much more clear timeline of the evolution of the nervous system. Michael Paulin, a computational evolutionary neuroscientist at the University of Otago in New Zealand and not involved in this study, explained that by studying placozoans we are one step closer to understanding what neurons are doing in our brains and how they got there.
All in all, placozoans, although very different from us humans, might just have the building blocks for what is our nervous system now. This study is very valuable as it brings up one step closer to learning more about our evolution. However, as stated by Arnau Sebé-Pedrós, an author of this study at the CGR, “evolutionary biology is a historical science,” and there is still a lot left to be explored.
